Advanced RISC Machines
ARM
Document Number:ARM DDI 0029E
Issued: August 1995
Copyright Advanced RISC Machines Ltd (ARM) 1995
All rights reserved
ARM 7TDMI
Data Sheet
Open Access
Proprietary Notice
ARM, the ARM Powered logo, EmbeddedICE, BlackICE and ICEbreaker are trademarks of
Advanced RISC Machines Ltd.
Neither the whole nor any part of the information contained in, or the product described in, this
datasheet may be adapted or reproduced in any material form except with the prior written
permission of the copyright holder.
The product described in this datasheet is subject to continuous developments and
improvements. All particulars of the product and its use contained in this datasheet are given by
ARM in good faith. However, all warranties implied or expressed, including but not limited to
implied warranties or merchantability, or fitness for purpose, are excluded.
This datasheet is intended only to assist the reader in the use of the product. ARM Ltd shall not
be liable for any loss or damage arising from the use of any information in this datasheet, or any
error or omission in such information, or any incorrect use of the product.
Change Log
Issue Date By Change
A (Draft 0.1) Sept 1994 EH/BJH Created.
(Draft 0.2) Oct 1994 EH First pass review comments added.
B Dec 1994 EH/AW First formal release
C Dec 1994 AW Further review comments
Mar 1995 AW Reissued with open access status.
No change to the content.
D draft1 Mar 1995 AW Changes in line with the ARM7TDM
datasheet. Further technical changes.
D Mar 1995 AW Review comments added.
E Aug 1995 AP Signals added plus minor changes.
ii ARM7TDMI Data Sheet
ARM DDI 0029E
Open Access
Key:
Open Access No confidentiality
To enable document tracking, the document number has two codes:
Major release- Pre-release
A First release
B Second release
etc etc
Draft Status Full and complete
draft1 First Draft
draft2 Second Draft
etc etc
E Embargoed (date given)
ARM7TDMI Data Sheet
ARM DDI 0029E
Contents-i
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1
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1 Introduction 1-1
1.1 Introduction 1-2
1.2 ARM7TDMI Architecture 1-2
1.3 ARM7TDMI Block Diagram 1-4
1.4 ARM7TDMI Core Diagram 1-5
1.5 ARM7TDMI Functional Diagram 1-6
2 Signal Description 2-1
2.1 Signal Description 2-2
3 Programmer’s Model 3-1
3.1 Processor Operating States 3-2
3.2 Switching State 3-2
3.3 Memory Formats 3-2
3.4 Instruction Length 3-3
3.5 Data Types 3-3
3.6 Operating Modes 3-4
3.7 Registers 3-4
3.8 The Program Status Registers 3-8
3.9 Exceptions 3-10
3.10 Interrupt Latencies 3-14
3.11 Reset 3-15
Contents
TOC
Contents
ARM7TDMI Data Sheet
ARM DDI 0029E
Contents-ii
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4 ARM Instruction Set 4-1
4.1 Instruction Set Summary 4-2
4.2 The Condition Field 4-5
4.3 Branch and Exchange (BX) 4-6
4.4 Branch and Branch with Link (B, BL) 4-8
4.5 Data Processing 4-10
4.6 PSR Transfer (MRS, MSR) 4-18
4.7 Multiply and Multiply-Accumulate (MUL, MLA) 4-23
4.8 Multiply Long and Multiply-Accumulate Long (MULL,MLAL) 4-25
4.9 Single Data Transfer (LDR, STR) 4-28
4.10 Halfword and Signed Data Transfer 4-34
4.11 Block Data Transfer (LDM, STM) 4-40
4.12 Single Data Swap (SWP) 4-47
4.13 Software Interrupt (SWI) 4-49
4.14 Coprocessor Data Operations (CDP) 4-51
4.15 Coprocessor Data Transfers (LDC, STC) 4-53
4.16 Coprocessor Register Transfers (MRC, MCR) 4-57
4.17 Undefined Instruction 4-60
4.18 Instruction Set Examples 4-61
5 THUMB Instruction Set 5-1
5.1 Format 1: move shifted register 5-5
5.2 Format 2: add/subtract 5-7
5.3 Format 3: move/compare/add/subtract immediate 5-9
5.4 Format 4: ALU operations 5-11
5.5 Format 5: Hi register operations/branch exchange 5-13
5.6 Format 6: PC-relative load 5-16
5.7 Format 7: load/store with register offset 5-18
5.8 Format 8: load/store sign-extended byte/halfword 5-20
5.9 Format 9: load/store with immediate offset 5-22
5.10 Format 10: load/store halfword 5-24
5.11 Format 11: SP-relative load/store 5-26
5.12 Format 12: load address 5-28
5.13 Format 13: add offset to Stack Pointer 5-30
5.14 Format 14: push/pop registers 5-32
5.15 Format 15: multiple load/store 5-34
5.16 Format 16: conditional branch 5-36
5.17 Format 17: software interrupt 5-38
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5.18 Format 18: unconditional branch 5-39
5.19 Format 19: long branch with link 5-40
5.20 Instruction Set Examples 5-42
6 Memory Interface 6-1
6.1 Overview 6-2
6.2 Cycle Types 6-2
6.3 Address Timing 6-4
6.4 Data Transfer Size 6-9
6.5 Instruction Fetch 6-10
6.6 Memory Management 6-12
6.7 Locked Operations 6-12
6.8 Stretching Access Times 6-12
6.9 The ARM Data Bus 6-13
6.10 The External Data Bus 6-15
7 Coprocessor Interface 7-1
7.1 Overview 7-2
7.2 Interface Signals 7-2
7.3 Register Transfer Cycle 7-3
7.4 Privileged Instructions 7-3
7.5 Idempotency 7-4
7.6 Undefined Instructions 7-4
8 Debug Interface 8-1
8.1 Overview 8-2
8.2 Debug Systems 8-2
8.3 Debug Interface Signals 8-3
8.4 Scan Chains and JTAG Interface 8-6
8.5 Reset 8-8
8.6 Pullup Resistors 8-9
8.7 Instruction Register 8-9
8.8 Public Instructions 8-9
8.9 Test Data Registers 8-12
8.10 ARM7TDMI Core Clocks 8-18
8.11 Determining the Core and System State 8-19
8.12 The PC’s Behaviour During Debug 8-23
8.13 Priorities / Exceptions 8-25
8.14 Scan Interface Timing 8-26
8.15 Debug Timing 8-30
Contents
ARM7TDMI Data Sheet
ARM DDI 0029E
Contents-iv
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9 ICEBreaker Module 9-1
9.1 Overview 9-2
9.2 The Watchpoint Registers 9-3
9.3 Programming Breakpoints 9-6
9.4 Programming Watchpoints 9-8
9.5 The Debug Control Register 9-9
9.6 Debug Status Register 9-10
9.7 Coupling Breakpoints and Watchpoints 9-11
9.8 Disabling ICEBreaker 9-13
9.9 ICEBreaker Timing 9-13
9.10 Programming Restriction 9-13
9.11 Debug Communications Channel 9-14
10 Instruction Cycle Operations 10-1
10.1 Introduction 10-2
10.2 Branch and Branch with Link 10-2
10.3 THUMB Branch with Link 10-3
10.4 Branch and Exchange (BX) 10-3
10.5 Data Operations 10-4
10.6 Multiply and Multiply Accumulate 10-6
10.7 Load Register 10-8
10.8 Store Register 10-9
10.9 Load Multiple Registers 10-9
10.10 Store Multiple Registers 10-11
10.11 Data Swap 10-11
10.12 Software Interrupt and Exception Entry 10-12
10.13 Coprocessor Data Operation 10-13
10.14 Coprocessor Data Transfer (from memory to coprocessor) 10-14
10.15 Coprocessor Data Transfer (from coprocessor to memory) 10-15
10.16 Coprocessor Register Transfer (Load from coprocessor) 10-16
10.17 Coprocessor Register Transfer (Store to coprocessor) 10-17
10.18 Undefined Instructions and Coprocessor Absent 10-18
10.19 Unexecuted Instructions 10-18
10.20 Instruction Speed Summary 10-19
11 DC Parameters 11-1
11.1 Absolute Maximum Ratings 11-2
11.2 DC Operating Conditions 11-2
Contents
ARM7TDMI Data Sheet
ARM DDI 0029E
Contents-v
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12 AC Parameters 12-1
12.1 Introduction 12-2
12.2 Notes on AC Parameters 12-11
Contents
ARM7TDMI Data Sheet
ARM DDI 0029E
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ARM7TDMI Data Sheet
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Introduction
This chapter introduces the ARM7TDMI architecture, and shows block, core, and
functional diagrams for the ARM7TDMI.
1.1 Introduction 1-2
1.2 ARM7TDMI Architecture 1-2
1.3 ARM7TDMI Block Diagram 1-4
1.4 ARM7TDMI Core Diagram 1-5
1.5 ARM7TDMI Functional Diagram 1-6
1
Introduction
ARM7TDMI Data Sheet
ARM DDI 0029E
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1.1 Introduction
The ARM7TDMI is a member of the Advanced RISC Machines (ARM) family of
general purpose 32-bit microprocessors, which offer high performance for very low
power consumption and price.
The ARM architecture is based on Reduced Instruction Set Computer (RISC)
principles, and the instruction set and related decode mechanism are much simpler
than those of microprogrammed Complex Instruction Set Computers. This simplicity
results in a high instruction throughput and impressive real-time interrupt response
from a small and cost-effective chip.
Pipelining is employed so that all parts of the processing and memory systems can
operate continuously. Typically, while one instruction is being executed, its successor
is being decoded, and a third instruction is being fetched from memory.
The ARM memory interface has been designed to allow the performance potential to
be realised without incurring high costs in the memory system. Speed-critical control
signals are pipelined to allow system control functions to be implemented in standard
low-power logic, and these control signals facilitate the exploitation of the fast local
access modes offered by industry standard dynamic RAMs.
1.2 ARM7TDMI Architecture
The ARM7TDMI processor employs a unique architectural strategy known as
THUMB
,
which makes it ideally suited to high-volume applications with memory restrictions, or
applications where code density is an issue.
1.2.1 The THUMB Concept
The key idea behind THUMB is that of a super-reduced instruction set. Essentially , the
ARM7TDMI processor has two instruction sets:
• the standard 32-bit ARM set
• a 16-bit THUMB set
The THUMB set’s 16-bit instruction length allows it to approach twice the density of
standard ARM code while retaining most of the ARM’s performance advantage over a
traditional 16-bit processor using 16-bit registers. This is possible because THUMB
code operates on the same 32-bit register set as ARM code.
THUMB code is able to provide up to 65% of the code size of ARM, and 160% of the
performance of an equivalent ARM processor connected to a 16-bit memory system.
Introduction
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1.2.2 THUMB’s Advantages
THUMB instructions operate with the standard ARM register configuration, allowing
excellent interoperability between ARM and THUMB states. Each 16-bit THUMB
instruction has a corresponding 32-bit ARM instruction with the same effect on the
processor model.
The major advantage of a 32-bit (ARM) architecture over a 16-bit architecture is its
ability to manipulate 32-bit integers with single instructions, and to address a large
address space efficiently. When processing 32-bit data, a 16-bit architecture will take
at least two instructions to perform the same task as a single ARM instruction.
However, not all the code in a program will process 32-bit data (for example, code that
performs character string handling), and some instructions, like Branches, do not
process any data at all.
If a 16-bit architecture only has 16-bit instructions, and a 32-bit architecture only has
32-bit instructions, then overall the 16-bit architecture will have better code density,
and better than one half the performance of the 32-bit architecture. Clearly 32-bit
performance comes at the cost of code density.
THUMB breaks this constraint by implementing a 16-bit instruction length on a 32-bit
architecture, making the processing of 32-bit data efficient with a compact instruction
coding. This provides far better performance than a 16-bit architecture, with better
code density than a 32-bit architecture.
THUMB also has a major advantage over other 32-bit architectures with 16-bit
instructions. This is the ability to switch back to full ARM code and execute at full
speed. Thus critical loops for applications such as
• fast interrupts
• DSP algorithms
can be coded using the full ARM instruction set, and linked with THUMB code. The
overhead of switching from THUMB code to ARM code is folded into sub-routine entry
time. Various portions of a system can be optimised for speed or for code density by
switching between THUMB and ARM execution as appropriate.
Introduction
ARM7TDMI Data Sheet
ARM DDI 0029E
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1.3 ARM7TDMI Block Diagram
Figure 1-1: ARM7TDMI block diagram
•
•
Scan Chain 0
A[31:0]
Core
Scan Chain 1
D[31:0]
nOPC
nRW
All
Other
Signals
TCK TMS TDInTRST TDO
EXTERN1
EXTERN0
nTRANS
nMREQ
Scan Chain 2
ICEBreaker
TAP controller
MAS[1:0]
Bus Splitter
DIN[31:0]
DOUT[31:0]
RANGEOUT1
RANGEOUT0
TAPSM[3:0] IR[3:0] SCREG[3:0]
Introduction
ARM7TDMI Data Sheet
ARM DDI 0029E
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1.4 ARM7TDMI Core Diagram
Figure 1-2: ARM7TDMI core
nRESET
nMREQ
SEQ
ABORT
nIRQ
nFIQ
nRW
LOCK
nCPI
CPA
CPB
nWAIT
MCLK
nOPC
nTRANS
Instruction
Decoder
&
Control
Logic
Instruction Pipeline
& Read Data Register
DBE D[31:0]
32-bit ALU
Barrel
Shifter
Address
Incrementer
Address Register
Register Bank
(31 x 32-bit registers)
(6 status registers)
A[31:0]
ALE
Multiplier
ABE
Write Data Register
nM[4:0]
32 x 8
nENOUT nENIN
TBE
Scan
Control
BREAKPTI
DBGRQI
nEXEC
DBGACK
ECLK
ISYNC
B
b
u
s
A
L
U
b
u
sA
b
u
s
P
C
b
u
s
I
n
c
r
e
m
e
n
t
e
r
b
u
s
APE
BL[3:0]
MAS[1:0]
TBIT
HIGHZ
& Thumb Instruction Decoder
Introduction
ARM7TDMI Data Sheet
ARM DDI 0029E
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1.5 ARM7TDMI Functional Diagram
Figure 1-3: ARM7TDMI functional diagram
LOCK
A[31:0]
ABORT Memory
Management
nOPC
nCPI
CPA
CPB Coprocessor
Interface
nTRANS
Memory
Interface
Interface
D[31:0]
TCK
TMS
TDI
nTRST
Boundary
Scan
TDO
Processor
Mode
nRW
nMREQ
SEQ
BL[3:0]
MAS[1:0]
APE
TBIT Processor
State
nM[4:0]
ARM7TDMI
DIN[31:0]
DOUT[31:0]
TAPSM[3:0]
IR[3:0]
Boundary Scan
TCK1
TCK2
11 Control Signals
nTDOEN
SCREG[3:0]
ABE
ALE
nIRQ
nFIQ
Bus
Interrupts ISYNC
nRESET
MCLK
nWAIT
Clocks
VDD
VSS
Power
DBGRQ
BREAKPT
DBGACK
nEXEC
Debug
Controls
EXTERN 1
DBE
TBE
EXTERN 0
nENOUT
nENIN
ECLK
DBGEN
APE
HIGHZ
BIGEND
BUSEN
RANGEOUT0
RANGEOUT1
DBGRQI
COMMRX
COMMTX
nENOUTI
ECAPCLK
BUSDIS
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Signal Description
This chapter lists and describes the signals for the ARM7TDMI.
2.1 Signal Description 2-2
2
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-2
Open Access
2.1 Signal Description
The following table lists and describes all the signals for the ARM7TDMI.
Transistor sizes
For a 0.6 µm ARM7TDMI:
INV4 driver has transistor sizes of p = 22.32 µm/0.6 µm
N = 12.6 µm/0.6 µm
INV8 driver has transistor sizes of p = 44.64 µm/0.6 µm
N = 25.2 µm/0.6 µm
Key to signal types
IC Input CMOS thresholds
P Power
O4 Output with INV4 driver
O8 Output with INV8 driver
Name Type Description
A[31:0]
Addresses 08 This is the processor address bus. If ALE (address latch enable)
is HIGH and APE (Address Pipeline Enable) is LOW, the
addresses become valid during phase 2 of the cycle before the
one to which they refer and remain so during phase 1 of the
referenced cycle. Their stable period may be controlled by ALE
or APE as described below.
ABE
Address bus enable IC This is an input signal which, when LOW, puts the address bus
drivers into a high impedance state. This signal has a similar
effect on the following control signals: MAS[1:0],nRW,LOCK,
nOPC and nTRANS. ABE must be tied HIGH when there is no
system requirement to turn off the address drivers.
ABORT
Memory Abort IC This is an input which allows the memory system to tell the
processor that a requested access is not allowed.
ALE
Address latch enable. IC This input is used to control transparent latches on the address
outputs. Normally the addresses change during phase 2 to the
value required during the next cycle, but for direct interfacing to
ROMs they are required to be stable to the end of phase 2.
Taking ALE LOW until the end of phase 2 will ensure that this
happens. This signal has a similar effect on the following control
signals: MAS[1:0],nRW,LOCK,nOPC and nTRANS. If the
system does not require address lines to be held in this way,
ALE must be tied HIGH. The address latch is static, so ALE may
be held LOW for long periods to freeze addresses.
Table 2-1: Signal Description
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-3
Open Access
APE
Address pipeline enable. IC When HIGH, this signal enables the address timing pipeline. In
this state, the address bus plus MAS[1:0],nRW,nTRANS,
LOCK and nOPC change in the phase 2 prior to the memory
cycle to which they refer. When APE is LOW, these signals
change in the phase 1 of the actual cycle. Please refer to ➲
Chapter 6, Memory Interface
for details of this timing.
BIGEND
Big Endian configuration. IC When this signal is HIGH the processor treats bytes in memory
as being in Big Endian format. When it is LOW, memory is
treated as Little Endian.
BL[3:0]
Byte Latch Control. IC These signals control when data and instructions are latched
from the external data bus. When BL[3] is HIGH, the data on
D[31:24] is latched on the falling edge of MCLK. When BL[2] is
HIGH, the data on D[23:16] is latched and so on. Please refer
to ➲
Chapter 6, Memory Interface
for details on the use of
these signals.
BREAKPT
Breakpoint. IC This signal allows external hardware to halt the execution of the
processor for debug purposes. When HIGH causes the current
memory access to be breakpointed. If the memory access is an
instruction fetch, ARM7TDMI will enter debug state if the
instruction reaches the execute stage of the ARM7TDMI pipeline.
If the memory access is for data, ARM7TDMI will enter debug
state after the current instruction completes execution.This
allows extension of the internal breakpoints provided by the
ICEBreaker module. See ➲
Chapter 9, ICEBreaker Module
.
BUSDIS
Bus Disable OThis signal is HIGH when INTEST is selected on scan chain 0 or
4 and may be used to disable external logic driving onto the
bidirectional data bus during scan testing. This signal changes on
the falling edge of TCK.
BUSEN
Data bus configuration IC This is a static configuration signal which determines whether the
bidirectional data bus,D[31:0], or the unidirectional data busses,
DIN[31:0] and DOUT[31:0], are to be used for transfer of data
between the processor and memory. Refer also to ➲
Chapter 6,
Memory Interface
.
When BUSEN is LOW, the bidirectional data bus, D[31:0] is
used. In this case, DOUT[31:0] is driven to value 0x00000000,
and any data presented on DIN[31:0] is ignored.
When BUSEN is HIGH, the bidirectional data bus, D[31:0] is
ignored and must be left unconnected. Input data and
instructions are presented on the input data bus, DIN[31:0],
output data appears on DOUT[31:0].
COMMRX
Communications Channel
Receive
OWhen HIGH, this signal denotes that the comms channel receive
buffer is empty. This signal changes on the rising edge of MCLK.
See ➲
9.11 Debug Communications Channel
on page 9-14
for more information on the debug comms channel.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
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COMMTX
Communications Channel
Transmit
O When HIGH, this signal denotes that the comms channel
transmit buffer is empty. This signal changes on the rising edge
of MCLK. See ➲
9.11 Debug Communications Channel
on
page 9-14 for more information on the debug comms channel.
CPA
Coprocessor absent. IC A coprocessor which is capable of performing the operation that
ARM7TDMI is requesting (by asserting nCPI) should take CPA
LOW immediately. If CPA is HIGH at the end of phase 1 of the
cycle in which nCPI went LOW, ARM7TDMI will abort the
coprocessor handshake and take the undefined instruction trap.
If CPA is LOW and remains LOW, ARM7TDMI will busy-wait until
CPB is LOW and then complete the coprocessor instruction.
CPB
Coprocessor busy. IC A coprocessor which is capable of performing the operation
which ARM7TDMI is requesting (by asserting nCPI), but cannot
commit to starting it immediately, should indicate this by driving
CPB HIGH. When the coprocessor is ready to start it should take
CPB LOW. ARM7TDMI samples CPB at the end of phase 1 of
each cycle in which nCPI is LOW.
D[31:0]
Data Bus. IC
08 These are bidirectional signal paths which are used for data
transfers between the processor and external memory. During
read cycles (when nRW is LOW), the input data must be valid
before the end of phase 2 of the transfer cycle. During write
cycles (when nRW is HIGH), the output data will become valid
during phase 1 and remain valid throughout phase 2 of the
transfer cycle.
Note that this bus is driven at all times, irrespective of whether
BUSEN is HIGH or LOW. When D[31:0] is not being used to
connect to the memory system it must be left unconnected. See
➲
Chapter 6, Memory Interface
.
DBE
Data Bus Enable. IC This is an input signal which, when driven LOW, puts the data
bus D[31:0] into the high impedance state. This is included for
test purposes, and should be tied HIGH at all times.
DBGACK
Debug acknowledge. 04 When HIGH indicates ARM is in debug state.
DBGEN
Debug Enable. IC This input signal allows the debug features of ARM7TDMI to be
disabled. This signal should be driven LOW when debugging is
not required.
DBGRQ
Debug request. IC This is a level-sensitive input, which when HIGH causes
ARM7TDMI to enter debug state after executing the current
instruction. This allows external hardware to force ARM7TDMI
into the debug state, in addition to the debugging features
provided by the ICEBreaker block. See ➲
Chapter 9,
ICEBreaker Module
for details.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-5
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DBGRQI
Internal debug request04 This signal represents the debug request signal which is
presented to the processor. This is the combination of external
DBGRQ, as presented to the ARM7TDMI macrocell, and bit 1 of
the debug control register. Thus there are two conditions where
this signal can change. Firstly, when DBGRQ changes, DBGRQI
will change after a propagation delay. When bit 1 of the debug
control register has been written, this signal will change on the
falling edge of TCK when the TAP controller state machine is in
the RUN-TEST/IDLE state. See ➲
Chapter 9, ICEBreaker
Module
for details.
DIN[31:0]
Data input bus IC This is the input data bus which may be used to transfer
instructions and data between the processor and memory.This
data input bus is only used when BUSEN is HIGH. The data on
this bus is sampled by the processor at the end of phase 2 during
read cycles (i.e. when nRW is LOW).
DOUT[31:0]
Data output bus 08 This is the data out bus, used to transfer data from the processor
to the memory system. Output data only appears on this bus
when BUSEN is HIGH. At all other times, this bus is driven to
value 0x00000000. When in use, data on this bus changes
during phase 1 of store cycles (i.e. when nRW is HIGH) and
remains valid throughout phase 2.
DRIVEBS
Boundary scan
cell enable
04 This signal is used to control the multiplexers in the scan cells of
an external boundary scan chain. This signal changes in the
UPDATE-IR state when scan chain 3 is selected and either the
INTEST, EXTEST, CLAMP or CLAMPZ instruction is loaded.
When an external boundary scan chain is not connected, this
output should be left unconnected.
ECAPCLK
Extest capture clock O This signal removes the need for the external logic in the test
chip which was required to enable the internal tristate bus during
scan testing. This need not be brought out as an external pin on
the test chip.
ECAPCLKBS
Extest capture clock for
Boundary Scan
04 This is a TCK2 wide pulse generated when the TAP controller
state machine is in the CAPTURE-DR state, the current
instruction is EXTEST and scan chain 3 is selected. This is used
to capture the macrocell outputs during EXTEST. When an
external boundary scan chain is not connected, this output
should be left unconnected.
ECLK
External clock output. 04 In normal operation, this is simply MCLK (optionally stretched
with nWAIT) exported from the core. When the core is being
debugged, this is DCLK. This allows external hardware to track
when the ARM7DM core is clocked.
EXTERN0
External input 0. IC This is an input to the ICEBreaker logic in the ARM7TDMI which
allows breakpoints and/or watchpoints to be dependent on an
external condition.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
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EXTERN1
External input 1. IC This is an input to the ICEBreaker logic in the ARM7TDMI which
allows breakpoints and/or watchpoints to be dependent on an
external condition.
HIGHZ 04 This signal denotes that the HIGHZ instruction has been loaded
into the TAP controller. See ➲
Chapter 8, Debug Interface
for
details.
ICAPCLKBS
Intest capture clock 04 This is a TCK2 wide pulse generated when the TAP controller
state machine is in the CAPTURE-DR state, the current
instruction is INTEST and scan chain 3 is selected. This is used
to capture the macrocell outputs during INTEST. When an
external boundary scan chain is not connected, this output
should be left unconnected.
IR[3:0]
TAP controller Instruction
register
04 These 4 bits reflect the current instruction loaded into the TAP
controller instruction register. The instruction encoding is as
described in➲
8.8 Public Instructions
on page 8-9. These bits
change on the falling edge of TCK when the state machine is in
the UPDATE-IR state.
ISYNC
Synchronous interrupts. IC When LOW indicates that the nIRQ and nFIQ inputs are to be
synchronised by the ARM core. When HIGH disables this
synchronisation for inputs that are already synchronous.
LOCK
Locked operation. 08 When LOCK is HIGH, the processor is performing a “locked”
memory access, and the memory controller must wait until LOCK
goes LOW before allowing another device to access the memory.
LOCK changes while MCLK is HIGH, and remains HIGH for the
duration of the locked memory accesses. It is active only during
the data swap (SWP) instruction. The timing of this signal may be
modified by the use of ALE and APE in a similar way to the
address, please refer to the ALE and APE descriptions. This
signal may also be driven to a high impedance state by driving
ABE LOW.
MAS[1:0]
Memory Access Size. 08 These are output signals used by the processor to indicate to the
external memory system when a word transfer or a half-word or
byte length is required. The signals take the value 10 (binary) for
words, 01 for half-words and 00 for bytes. 11 is reserved. These
values are valid for both read and write cycles. The signals will
normally become valid during phase 2 of the cycle before the one
in which the transfer will take place. They will remain stable
throughout phase 1 of the transfer cycle. The timing of the
signals may be modified by the use of ALE and APE in a similar
way to the address, please refer to the ALE and APE
descriptions. The signals may also be driven to high impedance
state by driving ABE LOW.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-7
Open Access
MCLK
Memory clock input. IC This clock times all ARM7TDMI memory accesses and internal
operations. The clock has two distinct phases -
phase 1
in which
MCLK is LOW and
phase 2
in which MCLK (and nWAIT) is
HIGH. The clock may be stretched indefinitely in either phase to
allow access to slow peripherals or memory. Alternatively, the
nWAIT input may be used with a free running MCLK to achieve
the same effect.
nCPI
Not Coprocessor
instruction.
04 When ARM7TDMI executes a coprocessor instruction, it will take
this output LOW and wait for a response from the coprocessor.
The action taken will depend on this response, which the
coprocessor signals on the CPA and CPB inputs.
nENIN
NOT enable input. IC This signal may be used in conjunction with nENOUT to control
the data bus during write cycles. See ➲
Chapter 6, Memory
Interface
.
nENOUT
Not enable output. 04 During a data write cycle, this signal is driven LOW during phase
1, and remains LOW for the entire cycle. This may be used to aid
arbitration in shared bus applications. See ➲
Chapter 6,
Memory Interface
.
nENOUTI
Not enable output. O During a coprocessor register transfer C-cycle from the
ICEbreaker comms channel coprocessor to the ARM core, this
signal goes LOW during phase 1 and stays LOW for the entire
cycle. This may be used to aid arbitration in shared bus systems.
nEXEC
Not executed. 04 When HIGH indicates that the instruction in the execution unit is
not being executed, because for example it has failed its
condition code check.
nFIQ
Not fast interrupt request. IC This is an interrupt request to the processor which causes it to be
interrupted if taken LOW when the appropriate enable in the
processor is active. The signal is level-sensitive and must be
held LOW until a suitable response is received from the
processor. nFIQ may be synchronous or asynchronous,
depending on the state of ISYNC.
nHIGHZ
Not HIGHZ 04 This signal is generated by the TAP controller when the current
instruction is HIGHZ. This is used to place the scan cells of that
scan chain in the high impedance state. When a external
boundary scan chain is not connected, this output should be left
unconnected.
nIRQ
Not interrupt request. IC As nFIQ, but with lower priority. May be taken LOW to interrupt
the processor when the appropriate enable is active. nIRQ may
be synchronous or asynchronous, depending on the state of
ISYNC.
nM[4:0]
Not processor mode. 04 These are output signals which are the inverses of the internal
status bits indicating the processor operation mode.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-8
Open Access
nMREQ
Not memory request. 04 This signal, when LOW, indicates that the processor requires
memory access during the following cycle. The signal becomes
valid during phase 1, remaining valid through phase 2 of the
cycle preceding that to which it refers.
nOPC
Not op-code fetch. 08 When LOW this signal indicates that the processor is fetching an
instruction from memory; when HIGH, data (if present) is being
transferred. The signal becomes valid during phase 2 of the
previous cycle, remaining valid through phase 1 of the
referenced cycle. The timing of this signal may be modified by
the use of ALE and APE in a similar way to the address, please
refer to the ALE and APE descriptions. This signal may also be
driven to a high impedance state by driving ABE LOW.
nRESET
Not reset. IC This is a level sensitive input signal which is used to start the
processor from a known address. A LOW level will cause the
instruction being executed to terminate abnormally. When
nRESET becomes HIGH for at least one clock cycle, the
processor will re-start from address 0. nRESET must remain
LOW (and nWAIT must remain HIGH) for at least two clock
cycles. During the LOW period the processor will perform dummy
instruction fetches with the address incrementing from the point
where reset was activated. The address will overflow to zero if
nRESET is held beyond the maximum address limit.
nRW
Not read/write. 08 When HIGH this signal indicates a processor write cycle; when
LOW, a read cycle. It becomes valid during phase 2 of the cycle
before that to which it refers, and remains valid to the end of
phase 1 of the referenced cycle. The timing of this signal may be
modified by the use of ALE and APE in a similar way to the
address, please refer to the ALE and APE descriptions. This
signal may also be driven to a high impedance state by driving
ABE LOW.
nTDOEN
Not TDO Enable. 04 When LOW, this signal denotes that serial data is being driven
out on the TDO output. nTDOEN would normally be used as an
output enable for a TDO pin in a packaged part.
nTRANS
Not memory translate. 08 When this signal is LOW it indicates that the processor is in user
mode. It may be used to tell memory management hardware
when translation of the addresses should be turned on, or as an
indicator of non-user mode activity. The timing of this signal may
be modified by the use of ALE and APE in a similar way to the
address, please refer to the ALE and APE description. This
signal may also be driven to a high impedance state by driving
ABE LOW.
nTRST
Not Test Reset. IC Active-low reset signal for the boundary scan logic. This pin must
be pulsed or driven LOW to achieve normal device operation, in
addition to the normal device reset (nRESET). For more
information, see ➲
Chapter 8, Debug Interface
.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-9
Open Access
nWAIT
Not wait. IC When accessing slow peripherals, ARM7TDMI can be made to
wait for an integer number of MCLK cycles by driving nWAIT
LOW. Internally, nWAIT is ANDed with MCLK and must only
change when MCLK is LOW. If nWAIT is not used it must be tied
HIGH.
PCLKBS
Boundary scan
update clock
04 This is a TCK2 wide pulse generated when the TAP controller
state machine is in the UPDATE-DR state and scan chain 3 is
selected. This is used by an external boundary scan chain as the
update clock. When an external boundary scan chain is not
connected, this output should be left unconnected.
RANGEOUT0
ICEbreaker Rangeout0 04 This signal indicates that ICEbreaker watchpoint register 0 has
matched the conditions currently present on the address, data
and control busses. This signal is independent of the state of the
watchpoint’s enable control bit. RANGEOUT0 changes when
ECLK is LOW.
RANGEOUT1
ICEbreaker Rangeout1 04 As RANGEOUT0 but corresponds to ICEbreaker’s watchpoint
register 1.
RSTCLKBS
Boundary Scan
Reset Clock
O This signal denotes that either the TAP controller state machine
is in the RESET state or that nTRST has been asserted. This
may be used to reset external boundary scan cells.
SCREG[3:0]
Scan Chain Register O These 4 bits reflect the ID number of the scan chain currently
selected by the TAP controller. These bits change on the falling
edge of TCK when the TAP state machine is in the UPDATE-DR
state.
SDINBS
Boundary Scan
Serial Input Data
O This signal contains the serial data to be applied to an external
scan chain and is valid around the falling edge of TCK.
SDOUTBS
Boundary scan serial
output data
IC This control signal is provided to ease the connection of an
external boundary scan chain. This is the serial data out of the
boundary scan chain. It should be set up to the rising edge of
TCK. When an external boundary scan chain is not connected,
this input should be tied LOW.
SEQ
Sequential address. O4 This output signal will become HIGH when the address of the
next memory cycle will be related to that of the last memory
access. The new address will either be the same as the previous
one or 4 greater in ARM state, or 2 greater in THUMB state.
The signal becomes valid during phase 1 and remains so
through phase 2 of the cycle before the cycle whose address it
anticipates. It may be used, in combination with the low-order
address lines, to indicate that the next cycle can use a fast
memory mode (for example DRAM page mode) and/or to bypass
the address translation system.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-10
Open Access
SHCLKBS
Boundary scan shift clock,
phase 1
04 This control signal is provided to ease the connection of an
external boundary scan chain. SHCLKBS is used to clock the
master half of the external scan cells. When in the SHIFT-DR
state of the state machine and scan chain 3 is selected,
SHCLKBS follows TCK1. When not in the SHIFT-DR state or
when scan chain 3 is not selected, this clock is LOW. When an
external boundary scan chain is not connected, this output
should be left unconnected.
SHCLK2BS
Boundary scan shift clock,
phase 2
04 This control signal is provided to ease the connection of an
external boundary scan chain. SHCLK2BS is used to clock the
master half of the external scan cells. When in the SHIFT-DR
state of the state machine and scan chain 3 is selected,
SHCLK2BS follows TCK2. When not in the SHIFT-DR state or
when scan chain 3 is not selected, this clock is LOW. When an
external boundary scan chain is not connected, this output
should be left unconnected.
TAPSM[3:0]
TAP controller
state machine
04 This bus reflects the current state of the TAP controller state
machine, as shown in ➲
8.4.2 The JTAG state machine
on
page 8-8. These bits change off the rising edge of TCK.
TBE
Test Bus Enable. IC When driven LOW, TBE forces the data bus D[31:0], the
Address bus A[31:0], plus LOCK,MAS[1:0],nRW,nTRANS
and nOPC to high impedance. This is as if both ABE and DBE
had both been driven LOW. However, TBE does not have an
associated scan cell and so allows external signals to be driven
high impedance during scan testing. Under normal operating
conditions, TBE should be held HIGH at all times.
TBIT O4 When HIGH, this signal denotes that the processor is executing
the THUMB instruction set. When LOW, the processor is
executing the ARM instruction set. This signal changes in phase
2 in the first execute cycle of a BX instruction.
TCK IC Test Clock.
TCK1
TCK, phase 1 04 This clock represents phase 1 of TCK.TCK1 is HIGH when TCK
is HIGH, although there is a slight phase lag due to the internal
clock non-overlap.
TCK2
TCK, phase 2 04 This clock represents phase 2 of TCK.TCK2 is HIGH when TCK
is LOW, although there is a slight phase lag due to the internal
clock non-overlap.TCK2 is the non-overlapping compliment of
TCK1.
TDI IC Test Data Input.
TDO
Test Data Output. O4 Output from the boundary scan logic.
TMS IC Test Mode Select.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-11
Open Access
VDD
Power supply. P These connections provide power to the device.
VSS
Ground. P These connections are the ground reference for all signals.
Name Type Description
Table 2-1: Signal Description (Continued)
Signal Description
ARM7TDMI Data Sheet
ARM DDI 0029E
2-12
Open Access
ARM7TDMI Data Sheet
ARM DDI 0029E
3-1
11
1
Open Access
Programmer’s Model
This chapter describes the two operating states of the ARM7TDMI.
3.1 Processor Operating States 3-2
3.2 Switching State 3-2
3.3 Memory Formats 3-2
3.4 Instruction Length 3-3
3.5 Data Types 3-3
3.6 Operating Modes 3-4
3.7 Registers 3-4
3.8 The Program Status Registers 3-8
3.9 Exceptions 3-10
3.11 Reset 3-15
3
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-2
Open Access
3.1 Processor Operating States
From the programmer’s point of view, the ARM7TDMI can be in one of two states:
ARM state
which executes 32-bit, word-aligned ARM instructions.
THUMB state
which operates with 16-bit, halfword-aligned THUMB
instructions. In this state, the PC uses bit 1 to select between
alternate halfwords.
Note
Transition between these two states does not affect the processor mode or the
contents of the registers.
3.2 Switching State
Entering THUMB state
Entry into THUMB state can be achieved by executing a BX instruction with the state
bit (bit 0) set in the operand register.
Transition to THUMB state will also occur automatically on return from an exception
(IRQ, FIQ, UNDEF, ABORT, SWI etc.), if the exception was entered with the processor
in THUMB state.
Entering ARM state
Entry into ARM state happens:
1 On execution of the BX instruction with the state bit clear in the operand
register.
2 On the processor taking an exception (IRQ, FIQ, RESET, UNDEF, ABORT,
SWI etc.).
In this case, the PC is placed in the exception mode’s link register, and
execution commences at the exception’s vector address.
3.3 Memory Formats
ARM7TDMI views memory as a linear collection of bytes numbered upwards from
zero. Bytes 0 to 3 hold the first stored word, bytes 4 to 7 the second and so on.
ARM7TDMI can treat words in memory as being stored either in
Big Endian
or
Little
Endian
format.
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-3
Open Access
3.3.1 Big endian format
In Big Endian format, the most significant byte of a word is stored at the lowest
numbered byte and the least significant byte at the highest numbered byte. Byte 0 of
the memory system is therefore connected to data lines 31 through 24.
3.3.2 Little endian format
In Little Endian format, the lowest numbered byte in a word is considered the word’s
least significant byte, and the highest numbered byte the most significant. Byte 0 of
the memory system is therefore connected to data lines 7 through 0.
3.4 Instruction Length
Instructions are either 32 bits long (in ARM state) or 16 bits long (in THUMB state).
3.5 Data Types
ARM7TDMI supports byte (8-bit), halfword (16-bit) and word (32-bit) data types.
Words must be aligned to four-byte boundaries and half words to two-byte boundaries.
Higher Address 31 24 23 16 15 8 7 0 Word Address
8 9 10 11 8
45674
01230
Lower Address • Most significant byte is at lowest address
• Word is addressed by byte address of most significant byte
Figure 3-1: Big endian addresses of bytes within words
Higher Address 31 24 23 16 15 8 7 0 Word Address
11 10 9 8 8
76544
32100
Lower Address • Least significant byte is at lowest address
• Word is addressed by byte address of least significant byte
Figure 3-2: Little endian addresses of bytes within words
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-4
Open Access
3.6 Operating Modes
ARM7TDMI supports seven modes of operation:
User (usr): The normal ARM program execution state
FIQ (fiq): Designed to support a data transfer or channel process
IRQ (irq): Used for general-purpose interrupt handling
Supervisor (svc): Protected mode for the operating system
Abort mode (abt): Entered after a data or instruction prefetch abort
System (sys): A privileged user mode for the operating system
Undefined (und): Entered when an undefined instruction is executed
Mode changes may be made under software control, or may be brought about by
external interrupts or exception processing. Most application programs will execute in
User mode. The non-user modes - known as
privileged modes
- are entered in order
to service interrupts or exceptions, or to access protected resources.
3.7 Registers
ARM7TDMI has a total of 37 registers - 31 general-purpose 32-bit registers and six
status registers - but these cannot all be seen at once. The processor state and
operating mode dictate which registers are available to the programmer.
3.7.1 The ARM state register set
In ARM state, 16 general registers and one or two status registers are visible at any
one time. In privileged (non-User) modes, mode-specific banked registers are
switched in. ➲
Figure 3-3: Register organization in ARM state
shows which registers
are available in each mode: the banked registers are marked with a shaded triangle.
The ARM state register set contains 16 directly accessible registers: R0 to R15. All of
these except R15 are general-purpose, and may be used to hold either data or
address values. In addition to these, there is a seventeenth register used to store
status information
Register 14 is used as the subroutine link register . This receives a copy of
R15 when a Branch and Link (BL) instruction is executed. At
all other times it may be treated as a general-purpose
register. The corresponding banked registers R14_svc,
R14_irq, R14_fiq, R14_abt and R14_und are similarly used
to hold the return values of R15 when interrupts and
exceptions arise, or when Branch and Link instructions are
executed within interrupt or exception routines.
Register 15 holds the Program Counter (PC). In ARM state, bits [1:0] of
R15 are zero and bits [31:2] contain the PC. In THUMB state,
bit [0] is zero and bits [31:1] contain the PC.
Register 16 is the CPSR (Current Program Status Register). This
contains condition code flags and the current mode bits.
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-5
Open Access
FIQ mode has seven banked registers mapped to R8-14 (R8_fiq-R14_fiq). In ARM
state, many FIQ handlers do not need to save any registers. User, IRQ, Supervisor,
Abort and Undefined each have two banked registers mapped to R13 and R14,
allowing each of these modes to have a private stack pointer and link registers.
Figure 3-3: Register organization in ARM state
ARM State General Registers and Program Counter
R0
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13
R14
R15 (PC)
R0
R1
R2
R3
R4
R5
R6
R7
R8_fiq
R9_fiq
R10_fiq
R11_fiq
R12_fiq
R13_fiq
R14_fiq
R15 (PC)
R0
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13_svc
R14_svc
R15 (PC)
R0
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13_abt
R14_abt
R15 (PC)
R0
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13_irq
R14_irq
R15 (PC)
R0
R1
R2
R3
R4
R5
R6
R7
R8
R9
R10
R11
R12
R13_und
R14_und
R15 (PC)
System & User FIQ Supervisor Abort IRQ Undefined
CPSR CPSR
SPSR_fiq
CPSR
SPSR_svc
CPSR
SPSR_abt
CPSR
SPSR_irq
CPSR
SPSR_und
ARM State Program Status Registers
= banked register
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-6
Open Access
3.7.2 The THUMB state register set
The THUMB state register set is a subset of the ARM state set. The programmer has
direct access to eight general registers, R0-R7, as well as the Program Counter (PC),
a stack pointer register (SP), a link register (LR), and the CPSR. There are banked
Stack Pointers, Link Registers and Saved Process Status Registers (SPSRs) for each
privileged mode. This is shown in ➲
Figure 3-4: Register organization in THUMB state
.
Figure 3-4: Register organization in THUMB state
3.7.3 The relationship between ARM and THUMB state registers
The THUMB state registers relate to the ARM state registers in the following way:
• THUMB state R0-R7 and ARM state R0-R7 are identical
• THUMB state CPSR and SPSRs and ARM state CPSR and SPSRs are
identical
• THUMB state SP maps onto ARM state R13
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-7
Open Access
• THUMB state LR maps onto ARM state R14
• The THUMB state Program Counter maps onto the ARM state Program
Counter (R15)
This relationship is shown in ➲
Figure 3-5: Mapping of THUMB state registers onto
ARM state registers
.
Figure 3-5: Mapping of THUMB state registers onto ARM state registers
3.7.4 Accessing Hi registers in THUMB state
In THUMB state, registers R8-R15 (the
Hi registers
) are not part of the standard
register set. However, the assembly language programmer has limited access to
them, and can use them for fast temporary storage.
A value may be transferred from a register in the range R0-R7 (a
Lo register
) to a Hi
register, and from a Hi register to a Lo register, using special variants of the MOV
instruction. Hi register values can also be compared against or added to Lo register
values with the CMP and ADD instructions. See ➲
5.5 Format 5: Hi register operations/
branch exchange
on page 5-13.
R0
R1
R2
R3
R5
R6
R7
R8
R9
R10
R11
R12
Stack Pointer (R13)
Link Register (R14)
Program Counter (R15)
R0
R1
R2
R3
R5
R6
R7
Stack Pointer (SP)
Link Register (LR)
Program Counter (PC)
CPSR CPSR
SPSR SPSR
THUMB state ARM state
R4R4
Lo registersHi registers
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-8
Open Access
3.8 The Program Status Registers
The ARM7TDMI contains a Current Program Status Register (CPSR), plus five Saved
Program Status Registers (SPSRs) for use by exception handlers. These registers
• hold information about the most recently performed ALU operation
• control the enabling and disabling of interrupts
• set the processor operating mode
The arrangement of bits is shown in ➲
Figure 3-6: Program status register format
.
Figure 3-6: Program status register format
3.8.1 The condition code flags
The N, Z, C and V bits are the condition code flags. These may be changed as a result
of arithmetic and logical operations, and may be tested to determine whether an
instruction should be executed.
In ARM state, all instructions may be executed conditionally: see ➲
4.2 The Condition
Field
on page 4-5 for details.
In THUMB state, only the Branch instruction is capable of conditional execution: see
➲
5.17 Format 17: software interrupt
on page 5-38
3.8.2 The control bits
The bottom 8 bits of a PSR (incorporating I, F, T and M[4:0]) are known collectively as
the control bits. These will change when an exception arises. If the processor is
operating in a privileged mode, they can also be manipulated by software.
The T bit
This reflects the operating state. When this bit is set, the
processor is executing in THUMB state, otherwise it is
executing in ARM state. This is reflected on the TBIT
external signal.
Note that the software must never change the state of the
TBIT in the CPSR. If this happens, the processor will
enter an unpredictable state.
01234
5
6782728293031
M0M1M2M3M4
.FIVCZN
Overflow
Carry / Borrow
Zero
Negative / Less Than
Mode bits
FIQ disable
IRQ disable
. .
condition code flags control bits
State bit
(reserved)
23
..
24
T
25
.
26
.
/ Extend
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-9
Open Access
Interrupt disable bits
The I and F bits are the interrupt disable bits. When set,
these disable the IRQ and FIQ interrupts respectively.
The mode bits
The M4, M3, M2, M1 and M0 bits (M[4:0]) are the mode
bits. These determine the processor’s operating mode,
as shown in ➲
Table 3-1: PSR mode bit values
on page
3-9. Not all combinations of the mode bits define a valid
processor mode. Only those explicitly described shall be
used. The user should be aware that if any illegal value
is programmed into the mode bits, M[4:0], then the
processor will enter an unrecoverable state. If this
occurs, reset should be applied.
Reserved bits
The remaining bits in the PSRs are reserved. When
changing a PSR’s flag or control bits, you must ensure
that these unused bits are not altered. Also, your
program should not rely on them containing specific
values, since in future processors they may read as one
or zero.
M[4:0] Mode Visible THUMB state
registers Visible ARM state
registers
10000 User R7..R0,
LR, SP
PC, CPSR
R14..R0,
PC, CPSR
10001 FIQ R7..R0,
LR_fiq, SP_fiq
PC, CPSR, SPSR_fiq
R7..R0,
R14_fiq..R8_fiq,
PC, CPSR, SPSR_fiq
10010 IRQ R7..R0,
LR_irq, SP_irq
PC, CPSR, SPSR_irq
R12..R0,
R14_irq..R13_irq,
PC, CPSR, SPSR_irq
10011 Supervisor R7..R0,
LR_svc, SP_svc,
PC, CPSR, SPSR_svc
R12..R0,
R14_svc..R13_svc,
PC, CPSR, SPSR_svc
10111 Abort R7..R0,
LR_abt, SP_abt,
PC, CPSR, SPSR_abt
R12..R0,
R14_abt..R13_abt,
PC, CPSR, SPSR_abt
11011 Undefined R7..R0
LR_und, SP_und,
PC, CPSR, SPSR_und
R12..R0,
R14_und..R13_und,
PC, CPSR
11111 System R7..R0,
LR, SP
PC, CPSR
R14..R0,
PC, CPSR
Table 3-1: PSR mode bit values
Programmer’s Model
ARM7TDMI Data Sheet
ARM DDI 0029E
3-10
Open Access
3.9 Exceptions
Exceptions arise whenever the normal flow of a program has to be halted temporarily,
for example to service an interrupt from a peripheral. Before an exception can be
handled, the current processor state must be preserved so that the original program
can resume when the handler routine has finished.
It is possible for several exceptions to arise at the same time. If this happens, they are
dealt with in a fixed order - see ➲
3.9.10 Exception priorities
on page 3-14.
3.9.1 Action on entering an exception
When handling an exception, the ARM7TDMI:
1Preserves the address of the next instruction in the appropriate Link Register .
If the exception has been entered from ARM state, then the address of the
next instruction is copied into the Link Register (that is, current PC + 4 or PC
+ 8 depending on the exception. See ➲
Table 3-2: Exception entry/exit
on
page 3-11 for details). If the exception has been entered from THUMB state,
then the value written into the Link Register is the current PC offset by a value
such that the program resumes from the correct place on return from the
exception. This means that the exception handler need not determine which
state the exception was entered from. For example, in the case of SWI, MOVS
PC, R14_svc will always return to the next instruction regardless of whether
the SWI was executed in ARM or THUMB state.
2 Copies the CPSR into the appropriate SPSR
3 Forces the CPSR mode bits to a value which depends on the exception
4Forces the PC to fetch the next instruction from the relevant exception vector
It may also set the interrupt disable flags to prevent otherwise unmanageable nestings
of exceptions.
If the processor is in THUMB state when an exception occurs, it will automatically
switch into ARM state when the PC is loaded with the exception vector address.
3.9.2 Action on leaving an exception
On completion, the exception handler:
1 Moves the Link Register, minus an offset where appropriate, to the PC. (The
offset will vary depending on the type of exception.)
2 Copies the SPSR back to the CPSR
3 Clears the interrupt disable flags, if they were set on entry
Note
An explicit switch back to THUMB state is never needed, since restoring the CPSR
from the SPSR automatically sets the T bit to the value it held immediately prior to the
exception.
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3.9.3 Exception entry/exit summary
➲
Table 3-2: Exception entry/exit
summarises the PC value preserved in the relevant
R14 on exception entry, and the recommended instruction for exiting the exception
handler.
Notes
1Where PC is the address of the BL/SWI/Undefined Instruction fetch which had
the prefetch abort.
2 Where PC is the address of the instruction which did not get executed since
the FIQ or IRQ took priority.
3Where PC is the address of the Load or Store instruction which generated the
data abort.
4 The value saved in R14_svc upon reset is unpredictable.
3.9.4 FIQ
The FIQ (Fast Interrupt Request) exception is designed to support a data transfer or
channel process, and in ARM state has sufficient private registers to remove the need
for register saving (thus minimising the overhead of context switching).
FIQ is externally generated by taking thenFIQ input LOW . This input can except either
synchronous or asynchronous transitions, depending on the state of the ISYNC input
signal. When ISYNC is LOW, nFIQ and nIRQ are considered asynchronous, and a
cycle delay for synchronization is incurred before the interrupt can affect the processor
flow.
Irrespective of whether the exception was entered from ARM or Thumb state, a FIQ
handler should leave the interrupt by executing
SUBS PC,R14_fiq,#4
Return Instruction Previous State
ARM THUMB
R14_x R14_x
Notes
BL MOV PC, R14 PC + 4 PC + 2 1
SWI MOVS PC, R14_svc PC + 4 PC + 2 1
UDEF MOVS PC, R14_und PC + 4 PC + 2 1
FIQ SUBS PC, R14_fiq, #4 PC + 4 PC + 4 2
IRQ SUBS PC, R14_irq, #4 PC + 4 PC + 4 2
PABT SUBS PC, R14_abt, #4 PC + 4 PC + 4 1
DABT SUBS PC, R14_abt, #8 PC + 8 PC + 8 3
RESET NA - - 4
Table 3-2: Exception entry/exit
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FIQ may be disabled by setting the CPSR’s F flag (but note that this is not possible
from User mode). If the F flag is clear, ARM7TDMI checks for a LOW level on the
output of the FIQ synchroniser at the end of each instruction.
3.9.5 IRQ
The IRQ (Interrupt Request) exception is a normal interrupt caused by a LOW level on
the nIRQ input. IRQ has a lower priority than FIQ and is masked out when a FIQ
sequence is entered. It may be disabled at any time by setting the I bit in the CPSR,
though this can only be done from a privileged (non-User) mode.
Irrespective of whether the exception was entered from ARM or Thumb state, an IRQ
handler should return from the interrupt by executing
SUBS PC,R14_irq,#4
3.9.6 Abort
An abort indicates that the current memory access cannot be completed. It can be
signalled by the external ABORT input. ARM7TDMI checks for the abort exception
during memory access cycles.
There are two types of abort:
Prefetch abort
occurs during an instruction prefetch.
Data abort
occurs during a data access.
If a prefetch abort occurs, the prefetched instruction is marked as invalid, but the
exception will not be taken until the instruction reaches the head of the pipeline. If the
instruction is not executed - for example because a branch occurs while it is in the
pipeline - the abort does not take place.
If a data abort occurs, the action taken depends on the instruction type:
1 Single data transfer instructions (LDR, STR) write back modified base
registers: the Abort handler must be aware of this.
2 The swap instruction (SWP) is aborted as though it had not been executed.
3Block data transfer instructions (LDM, STM) complete. If write-back is set, the
base is updated. If the instruction would have overwritten the base with data
(ie it has the base in the transfer list), the overwriting is prevented. All register
overwriting is prevented after an abort is indicated, which means in particular
that R15 (always the last register to be transferred) is preserved in an aborted
LDM instruction.
The abort mechanism allows the implementation of a demand paged virtual memory
system. In such a system the processor is allowed to generate arbitrary addresses.
When the data at an address is unavailable, the Memory Management Unit (MMU)
signals an abort. The abort handler must then work out the cause of the abort, make
the requested data available, and retry the aborted instruction. The application
program needs no knowledge of the amount of memory available to it, nor is its state
in any way affected by the abort.
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After fixing the reason for the abort, the handler should execute the following
irrespective of the state (ARM or Thumb):
SUBS PC,R14_abt,#4 for a prefetch abort, or
SUBS PC,R14_abt,#8 for a data abort
This restores both the PC and the CPSR, and retries the aborted instruction.
3.9.7 Software interrupt
The software interrupt instruction (SWI) is used for entering Supervisor mode, usually
to request a particular supervisor function. A SWI handler should return by executing
the following irrespective of the state (ARM or Thumb):
MOV PC, R14_svc
This restores the PC and CPSR, and returns to the instruction following the SWI.
3.9.8 Undefined instruction
When ARM7TDMI comes across an instruction which it cannot handle, it takes the
undefined instruction trap. This mechanism may be used to extend either the THUMB
or ARM instruction set by software emulation.
After emulating the failed instruction, the trap handler should execute the following
irrespective of the state (ARM or Thumb):
MOVS PC,R14_und
This restores the CPSR and returns to the instruction following the undefined
instruction.
3.9.9 Exception vectors
The following table shows the exception vector addresses.
Address Exception Mode on entry
0x00000000 Reset Supervisor
0x00000004 Undefined instruction Undefined
0x00000008 Software interrupt Supervisor
0x0000000C Abort (prefetch) Abort
0x00000010 Abort (data) Abort
0x00000014
Reserved Reserved
0x00000018 IRQ IRQ
0x0000001C FIQ FIQ
Table 3-3: Exception vectors
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3.9.10 Exception priorities
When multiple exceptions arise at the same time, a fixed priority system determines
the order in which they are handled:
Highest priority:
1 Reset
2 Data abort
3 FIQ
4 IRQ
5 Prefetch abort
Lowest priority:
6 Undefined Instruction, Software interrupt.
Not all exceptions can occur at once:
Undefined Instruction and Software Interrupt are mutually exclusive, since they each
correspond to particular (non-overlapping) decodings of the current instruction.
If a data abort occurs at the same time as a FIQ, and FIQs are enabled (ie the CPSR’s
F flag is clear), ARM7TDMI enters the data abort handler and then immediately
proceeds to the FIQ vector . A normal return from FIQ will cause the data abort handler
to resume execution. Placing data abort at a higher priority than FIQ is necessary to
ensure that the transfer error does not escape detection. The time for this exception
entry should be added to worst-case FIQ latency calculations.
3.10 Interrupt Latencies
The worst case latency for FIQ, assuming that it is enabled, consists of the longest
time the request can take to pass through the synchroniser (
Tsyncmax
if
asynchronous), plus the time for the longest instruction to complete (
Tldm
, the longest
instruction is an LDM which loads all the registers including the PC), plus the time for
the data abort entry (
Texc
), plus the time for FIQ entry (
Tfiq
). At the end of this time
ARM7TDMI will be executing the instruction at 0x1C.
Tsyncmax
is 3 processor cycles,
Tldm
is 20 cycles,
Texc
is 3 cycles, and
Tfiq
is 2
cycles. The total time is therefore 28 processor cycles. This is just over 1.4
microseconds in a system which uses a continuous 20 MHz processor clock. The
maximum IRQ latency calculation is similar, but must allow for the fact that FIQ has
higher priority and could delay entry into the IRQ handling routine for an arbitrary
length of time. The minimum latency for FIQ or IRQ consists of the shortest time the
request can take through the synchroniser (
Tsyncmin
) plus
Tfiq
. This is 4 processor
cycles.
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3.11 Reset
When the nRESET signal goes LOW , ARM7TDMI abandons the executing instruction
and then continues to fetch instructions from incrementing word addresses.
When nRESET goes HIGH again, ARM7TDMI:
1Overwrites R14_svc and SPSR_svc by copying the current values of the PC
and CPSR into them. The value of the saved PC and SPSR is not defined.
2Forces M[4:0] to 10011 (Supervisor mode), sets the I and F bits in the CPSR,
and clears the CPSR’s T bit.
3 Forces the PC to fetch the next instruction from address 0x00.
4 Execution resumes in ARM state.
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ARM Instruction Set
This chapter describes the ARM instruction set.
4.1 Instruction Set Summary 4-2
4.2 The Condition Field 4-5
4.3 Branch and Exchange (BX) 4-6
4.4 Branch and Branch with Link (B, BL) 4-8
4.5 Data Processing 4-10
4.6 PSR Transfer (MRS, MSR) 4-18
4.7 Multiply and Multiply-Accumulate (MUL, MLA) 4-23
4.8 Multiply Long and Multiply-Accumulate Long (MULL,MLAL) 4-25
4.9 Single Data Transfer (LDR, STR) 4-28
4.10 Halfword and Signed Data Transfer 4-34
4.11 Block Data Transfer (LDM, STM) 4-40
4.12 Single Data Swap (SWP) 4-47
4.13 Software Interrupt (SWI) 4-49
4.14 Coprocessor Data Operations (CDP) 4-51
4.15 Coprocessor Data Transfers (LDC, STC) 4-53
4.16 Coprocessor Register Transfers (MRC, MCR) 4-57
4.17 Undefined Instruction 4-60
4.18 Instruction Set Examples 4-61
4
ARM Instruction Set - Summary
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4.1 Instruction Set Summary
4.1.1 Format summary
The ARM instruction set formats are shown below.
Figure 4-1: ARM instruction set formats
Note
Some instruction codes are not defined but do not cause the Undefined instruction trap
to be taken, for instance a Multiply instruction with bit 6 changed to a 1. These
instructions should not be used, as their action may change in future ARM
implementations.
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Cond 0 0 I Opcode S Rn Rd Operand 2
Data Processing /
PSR Transfer
Cond 0 0 0 0 0 0 A S Rd Rn Rs 1 0 0 1 Rm
Multiply
Cond 0 0 0 0 1 U A S RdHi RdLo Rn 1 0 0 1 Rm
Multiply Long
Cond 0 0 0 1 0 B 0 0 Rn Rd 0 0 0 0 1 0 0 1 Rm
Single Data Swap
Cond 0 0 0 1 0 0 1 0 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 1 Rn
Branch and Exchange
Cond 0 0 0 P U 0 W L Rn Rd 0 0 0 0 1 S H 1 Rm
Halfword Data Transfer:
register offset
Cond 0 0 0 P U 1 W L Rn Rd Offset 1 S H 1 Offset
Halfword Data Transfer:
immediate offset
Cond 0 1 I P U B W L Rn Rd Offset
Single Data Transfer
Cond 0 1 1 1
Undefined
Cond 1 0 0 P U S W L Rn Register List
Block Data Transfer
Cond 1 0 1 L Offset
Branch
Cond 1 1 0 P U N W L Rn CRd CP# Offset
Coprocessor Data
Transfer
Cond 1 1 1 0 CP Opc CRn CRd CP# CP 0 CRm
Coprocessor Data
Operation
Cond 1 1 1 0 CP Opc L CRn Rd CP# CP 1 CRm
Coprocessor Register
Transfer
Cond 1 1 1 1 Ignored by processor
Software Interrupt
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
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4.1.2 Instruction summary
Mnemonic Instruction Action See Section:
ADC Add with carry Rd := Rn + Op2 + Carry 4.5
ADD Add Rd := Rn + Op2 4.5
AND AND Rd := Rn AND Op2 4.5
B Branch R15 := address 4.4
BIC Bit Clear Rd := Rn AND NOT Op2 4.5
BL Branch with Link R14 := R15, R15 := address 4.4
BX Branch and Exchange R15 := Rn,
T bit := Rn[0] 4.3
CDP Coprocesor Data Processing (Coprocessor-specific) 4.14
CMN Compare Negative CPSR flags := Rn + Op2 4.5
CMP Compare CPSR flags := Rn - Op2 4.5
EOR Exclusive OR Rd := (Rn AND NOT Op2)
OR (op2 AND NOT Rn) 4.5
LDC Load coprocessor from
memory Coprocessor load 4.15
LDM Load multiple registers Stack manipulation (Pop) 4.11
LDR Load register from memory Rd := (address) 4.9, 4.10
MCR Move CPU register to
coprocessor register cRn := rRn {<op>cRm} 4.16
MLA Multiply Accumulate Rd := (Rm * Rs) + Rn 4.7, 4.8
MOV Move register or constant Rd : = Op2 4.5
MRC Move from coprocessor
register to CPU register Rn := cRn {<op>cRm} 4.16
MRS Move PSR status/flags to
register Rn := PSR 4.6
MSR Move register to PSR
status/flags PSR := Rm 4.6
MUL Multiply Rd := Rm * Rs 4.7, 4.8
MVN Move negative register Rd := 0xFFFFFFFF EOR Op2 4.5
ORR OR Rd := Rn OR Op2 4.5
Table 4-1: The ARM Instruction set
ARM Instruction Set - Summary
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RSB Reverse Subtract Rd := Op2 - Rn 4.5
RSC Reverse Subtract with Carry Rd := Op2 - Rn - 1 + Carry 4.5
SBC Subtract with Carry Rd := Rn - Op2 - 1 + Carry 4.5
STC Store coprocessor register to
memory address := CRn 4.15
STM Store Multiple Stack manipulation (Push) 4.11
STR Store register to memory <address> := Rd 4.9, 4.10
SUB Subtract Rd := Rn - Op2 4.5
SWI Software Interrupt OS call 4.13
SWP Swap register with memory Rd := [Rn], [Rn] := Rm 4.12
TEQ Test bitwise equality CPSR flags := Rn EOR Op2 4.5
TST Test bits CPSR flags := Rn AND Op2 4.5
Mnemonic Instruction Action See Section:
Table 4-1: The ARM Instruction set (Continued)
ARM Instruction Set - Condition Field
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4.2 The Condition Field
In ARM state, all instructions are conditionally executed according to the state of the
CPSR condition codes and the instruction’s condition field. This field (bits 31:28)
determines the circumstances under which an instruction is to be executed. If the state
of the C, N, Z and V flags fulfils the conditions encoded by the field, the instruction is
executed, otherwise it is ignored.
There are sixteen possible conditions, each represented by a two-character suffix that
can be appended to the instruction’s mnemonic. For example, a Branch (B in assembly
language) becomes BEQ for "Branch if Equal", which means the Branch will only be
taken if the Z flag is set.
In practice, fifteen different conditions may be used: these are listed in ➲
Table 4-2:
Condition code summary
. The sixteenth (1111) is reserved, and must not be used.
In the absence of a suffix, the condition field of most instructions is set to "Always"
(sufix AL). This means the instruction will always be executed regardless of the CPSR
condition codes.
Code Suffix Flags Meaning
0000 EQ Z set equal
0001 NE Z clear not equal
0010 CS C set unsigned higher or same
0011 CC C clear unsigned lower
0100 MI N set negative
0101 PL N clear positive or zero
0110 VS V set overflow
0111 VC V clear no overflow
1000 HI C set and Z clear unsigned higher
1001 LS C clear or Z set unsigned lower or same
1010 GE N equals V greater or equal
1011 LT N not equal to V less than
1100 GT Z clear AND (N equals V) greater than
1101 LE Z set OR (N not equal to V) less than or equal
1110 AL (ignored) always
Table 4-2: Condition code summary
ARM Instruction Set - Condition Field
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4.3 Branch and Exchange (BX)
This instruction is only executed if the condition is true. The various conditions are
defined in ➲
Table 4-2: Condition code summary
on page 4-5.
This instruction performs a branch by copying the contents of a general register, Rn,
into the program counter, PC. The branch causes a pipeline flush and refill from the
address specified by Rn. This instruction also permits the instruction set to be
exchanged. When the instruction is executed, the value of Rn[0] determines whether
the instruction stream will be decoded as ARM or THUMB instructions.
Figure 4-2: Branch and Exchange instructions
4.3.1 Instruction cycle times
The BX instruction takes 2S + 1N cycles to execute, where S and N are as defined in
➲
6.2 Cycle Types
on page 6-2.
4.3.2 Assembler syntax
BX - branch and exchange.
BX{cond} Rn
{cond} Two character condition mnemonic. See ➲
Table 4-2: Condition code
summary
on page 4-5.
Rn is an expression evaluating to a valid register number.
4.3.3 Using R15 as an operand
If R15 is used as an operand, the behaviour is undefined.
Cond 0 0 0 1 0 0 1 0 0 0 0 1 Rn
034781112151619202324272831
Operand register
If bit 0 of Rn = 1, subsequent instructions decoded as THUMB instructions
If bit 0 of Rn = 0, subsequent instructions decoded as ARM instructions
Condition Field
111111111111
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4.3.4 Examples
ADR R0, Into_THUMB + 1 ; Generate branch target address
; and set bit 0 high - hence
; arrive in THUMB state.
BX R0 ; Branch and change to THUMB
; state.
CODE16 ; Assemble subsequent code as
Into_THUMB ; THUMB instructions
.
.ADR R5, Back_to_ARM : Generate branch target to word
: aligned ; address - hence bit 0
; is low and so change back to ARM
; state.
BX R5 ; Branch and change back to ARM
; state.
.
.
ALIGN ; Word align
CODE32 ; Assemble subsequent code as ARM
Back_to_ARM ; instructions
.
.
ARM Instruction Set - B, BL
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4.4 Branch and Branch with Link (B, BL)
The instruction is only executed if the condition is true. The various conditions are
defined ➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-3: Branch instructions
, below.
Figure 4-3: Branch instructions
Branch instructions contain a signed 2's complement 24 bit offset. This is shifted left
two bits, sign extended to 32 bits, and added to the PC. The instruction can therefore
specify a branch of +/- 32Mbytes. The branch offset must take account of the prefetch
operation, which causes the PC to be 2 words (8 bytes) ahead of the current
instruction.
Branches beyond +/- 32Mbytes must use an offset or absolute destination which has
been previously loaded into a register. In this case the PC should be manually saved
in R14 if a Branch with Link type operation is required.
4.4.1 The link bit
Branch with Link (BL) writes the old PC into the link register (R14) of the current bank.
The PC value written into R14 is adjusted to allow for the prefetch, and contains the
address of the instruction following the branch and link instruction. Note that the CPSR
is not saved with the PC and R14[1:0] are always cleared.
To return from a routine called by Branch with Link use MOV PC,R14 if the link register
is still valid or LDM Rn!,{..PC} if the link register has been saved onto a stack pointed
to by Rn.
4.4.2 Instruction cycle times
Branch and Branch with Link instructions take 2S + 1N incremental cycles, where S
and N are as defined in ➲
6.2 Cycle Types
on page 6-2.
Cond 101 L offset
31 28 27 25 24 23 0
Link bit
0 = Branch
1 = Branch with Link
Condition field
ARM Instruction Set - B, BL
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4.4.3 Assembler syntax
Items in {} are optional. Items in <> must be present.
B{L}{cond} <expression>
{L} is used to request the Branch with Link form of the instruction.
If absent, R14 will not be affected by the instruction.
{cond} is a two-character mnemonic as shown in ➲
Table 4-2:
Condition code summary
on page 4-5. If absent then AL
(ALways) will be used.
<expression> is the destination. The assembler calculates the offset.
4.4.4 Examples
here BAL here ; assembles to 0xEAFFFFFE (note effect of
; PC offset).
B there ; Always condition used as default.
CMP R1,#0 ; Compare R1 with zero and branch to fred
; if R1 was zero, otherwise continue
BEQ fred ; continue to next instruction.
BL sub+ROM ; Call subroutine at computed address.
ADDS R1,#1 ; Add 1 to register 1, setting CPSR flags
; on the result then call subroutine if
BLCC sub ; the C flag is clear, which will be the
; case unless R1 held 0xFFFFFFFF.
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4.5 DataProcessing
The data processing instruction is only executed if the condition is true. The conditions
are defined in ➲
Table 4-2: Condition code summary
on page 4-5.
The instruction encoding is shown in ➲
Figure 4-4: Data processing instructions
below.
Figure 4-4: Data processing instructions
The instruction produces a result by performing a specified arithmetic or logical
operation on one or two operands. The first operand is always a register (Rn).
Cond 00 I OpCode Rn Rd Operand 2
011121516192021242526272831
Destination register
1st operand register
Set condition codes
Operation Code
0 = do not alter condition codes
1 = set condition codes
0000 = AND - Rd:= Op1 AND Op2
0010 = SUB - Rd:= Op1 - Op2
0011 = RSB - Rd:= Op2 - Op1
0100 = ADD - Rd:= Op1 + Op2
0101 = ADC - Rd:= Op1 + Op2 + C
0110 = SBC - Rd:= Op1 - Op2 + C
0111 = RSC - Rd:= Op2 - Op1 + C
1000 = TST - set condition codes on Op1 AND Op2
1001 = TEQ - set condition codes on Op1 EOR Op2
1010 = CMP - set condition codes on Op1 - Op2
1011 = CMN - set condition codes on Op1 + Op2
1100 = ORR - Rd:= Op1 OR Op2
1101 = MOV - Rd:= Op2
1110 = BIC - Rd:= Op1 AND NOT Op2
1111 = MVN - Rd:= NOT Op2
Immediate Operand
0 = operand 2 is a register
1 = operand 2 is an immediate value
Shift Rm
Rotate
S
Unsigned 8 bit immediate value
2nd operand register
shift applied to Rm
shift applied to Imm
Imm
Condition field
11 8 7 0
03411
0001 = EOR - Rd:= Op1 EOR Op2
- 1
- 1
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The second operand may be a shifted register (Rm) or a rotated 8 bit immediate value
(Imm) according to the value of the I bit in the instruction. The condition codes in the
CPSR may be preserved or updated as a result of this instruction, according to the
value of the S bit in the instruction.
Certain operations (TST, TEQ, CMP, CMN) do not write the result to Rd. They are used
only to perform tests and to set the condition codes on the result and always have the
S bit set. The instructions and their effects are listed in ➲
Table 4-3: ARM Data
processing instructions
on page 4-11.
4.5.1 CPSR flags
The data processing operations may be classified as logical or arithmetic. The logical
operations (AND, EOR, TST, TEQ, ORR, MOV, BIC, MVN) perform the logical action
on all corresponding bits of the operand or operands to produce the result. If the S bit
is set (and Rd is not R15, see below) the V flag in the CPSR will be unaf fected, the C
flag will be set to the carry out from the barrel shifter (or preserved when the shift
operation is LSL #0), the Z flag will be set if and only if the result is all zeros, and the
N flag will be set to the logical value of bit 31 of the result.
Assembler
Mnemonic OpCode Action
AND 0000 operand1 AND operand2
EOR 0001 operand1 EOR operand2
SUB 0010 operand1 - operand2
RSB 0011 operand2 - operand1
ADD 0100 operand1 + operand2
ADC 0101 operand1 + operand2 + carry
SBC 0110 operand1 - operand2 + carry - 1
RSC 0111 operand2 - operand1 + carry - 1
TST 1000 as AND, but result is not written
TEQ 1001 as EOR, but result is not written
CMP 1010 as SUB, but result is not written
CMN 1011 as ADD, but result is not written
ORR 1100 operand1 OR operand2
MOV 1101 operand2 (operand1 is ignored)
BIC 1110 operand1 AND NOT operand2 (Bit clear)
MVN 1111 NOT operand2 (operand1 is ignored)
Table 4-3: ARM Data processing instructions
ARM Instruction Set - Shifts
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The arithmetic operations (SUB, RSB, ADD, ADC, SBC, RSC, CMP, CMN) treat each
operand as a 32 bit integer (either unsigned or 2's complement signed, the two are
equivalent). If the S bit is set (and Rd is not R15) the V flag in the CPSR will be set if
an overflow occurs into bit 31 of the result; this may be ignored if the operands were
considered unsigned, but warns of a possible error if the operands were 2's
complement signed. The C flag will be set to the carry out of bit 31 of the ALU, the Z
flag will be set if and only if the result was zero, and the N flag will be set to the value
of bit 31 of the result (indicating a negative result if the operands are considered to be
2's complement signed).
4.5.2 Shifts
When the second operand is specified to be a shifted register, the operation of the
barrel shifter is controlled by the Shift field in the instruction. This field indicates the
type of shift to be performed (logical left or right, arithmetic right or rotate right). The
amount by which the register should be shifted may be contained in an immediate field
in the instruction, or in the bottom byte of another register (other than R15). The
encoding for the different shift types is shown in ➲
Figure 4-5: ARM shift operations
.
Figure 4-5: ARM shift operations
Instruction specified shift amount
When the shift amount is specified in the instruction, it is contained in a 5 bit field which
may take any value from 0 to 31. A logical shift left (LSL) takes the contents of Rm and
moves each bit by the specified amount to a more significant position. The least
significant bits of the result are filled with zeros, and the high bits of Rm which do not
map into the result are discarded, except that the least significant discarded bit
becomes the shifter carry output which may be latched into the C bit of the CPSR when
the ALU operation is in the logical class (see above). For example, the effect of LSL #5
is shown in ➲
Figure 4-6: Logical shift left
.
0 0 1Rs
11 8 7 6 5 411 7 6 5 4
Shift type
Shift amount
5 bit unsigned integer
00 = logical left
01 = logical right
10 = arithmetic right
11 = rotate right
Shift type
Shift register
00 = logical left
01 = logical right
10 = arithmetic right
11 = rotate right
Shift amount specified in
bottom byte of Rs
ARM Instruction Set - Shifts
ARM7TDMI Data Sheet
ARM DDI 0029E
4-13
Open Access
Figure 4-6: Logical shift left
Note
LSL #0 is a special case, where the shifter carry out is the old value of the CPSR C
flag. The contents of Rm are used directly as the second operand.
A logical shift right (LSR) is similar, but the contents of Rm are moved to less
significant positions in the result. LSR #5 has the effect shown in ➲
Figure 4-7: Logical
shift right
.
Figure 4-7: Logical shift right
The form of the shift field which might be expected to correspond to LSR #0 is used to
encode LSR #32, which has a zero result with bit 31 of Rm as the carry output. Logical
shift right zero is redundant as it is the same as logical shift left zero, so the assembler
will convert LSR #0 (and ASR #0 and ROR #0) into LSL #0, and allow LSR #32 to be
specified.
An arithmetic shift right (ASR) is similar to logical shift right, except that the high bits
are filled with bit 31 of Rm instead of zeros. This preserves the sign in 2's complement
notation. For example, ASR #5 is shown in ➲
Figure 4-8: Arithmetic shift right
.
0 0 0 0 0
contents of Rm
value of operand 2
31 27 26 0
carry out
contents of Rm
value of operand 2
31 0
carry out
0 0 0 0 0
5 4
ARM Instruction Set - Shifts
ARM7TDMI Data Sheet
ARM DDI 0029E
4-14
Open Access
Figure 4-8: Arithmetic shift right
The form of the shift field which might be expected to give ASR #0 is used to encode
ASR #32. Bit 31 of Rm is again used as the carry output, and each bit of operand 2 is
also equal to bit 31 of Rm. The result is therefore all ones or all zeros, according to the
value of bit 31 of Rm.
Rotate right (ROR) operations reuse the bits which “overshoot” in a logical shift right
operation by reintroducing them at the high end of the result, in place of the zeros used
to fill the high end in logical right operations. For example, ROR #5 is shown in➲
Figure
4-9: Rotate right
on page 4-14.
Figure 4-9: Rotate right
The form of the shift field which might be expected to give ROR #0 is used to encode
a special function of the barrel shifter , rotate right extended (RRX). This is a rotate right
by one bit position of the 33 bit quantity formed by appending the CPSR C flag to the
most significant end of the contents of Rm as shown in ➲
Figure 4-10: Rotate right
extended
.
contents of Rm
value of operand 2
31 0
carry out
5 430
contents of Rm
value of operand 2
31 0
carry out
5 4
ARM Instruction Set - Shifts
ARM7TDMI Data Sheet
ARM DDI 0029E
4-15
Open Access
Figure 4-10: Rotate right extended
Register specified shift amount
Only the least significant byte of the contents of Rs is used to determine the shift
amount. Rs can be any general register other than R15.
If this byte is zero, the unchanged contents of Rm will be used as the second operand,
and the old value of the CPSR C flag will be passed on as the shifter carry output.
If the byte has a value between 1 and 31, the shifted result will exactly match that of
an instruction specified shift with the same value and shift operation.
If the value in the byte is 32 or more, the result will be a logical extension of the shift
described above:
1 LSL by 32 has result zero, carry out equal to bit 0 of Rm.
2 LSL by more than 32 has result zero, carry out zero.
3 LSR by 32 has result zero, carry out equal to bit 31 of Rm.
4 LSR by more than 32 has result zero, carry out zero.
5 ASR by 32 or more has result filled with and carry out equal to bit 31 of Rm.
6 ROR by 32 has result equal to Rm, carry out equal to bit 31 of Rm.
7 ROR by n where n is greater than 32 will give the same result and carry out
as ROR by n-32; therefore repeatedly subtract 32 from n until the amount is
in the range 1 to 32 and see above.
Note The zero in bit 7 of an instruction with a register controlled shift is compulsory; a one
in this bit will cause the instruction to be a multiply or undefined instruction.
4.5.3 Immediate operand rotates
The immediate operand rotate field is a 4 bit unsigned integer which specifies a shift
operation on the 8 bit immediate value. This value is zero extended to 32 bits, and then
subject to a rotate right by twice the value in the rotate field. This enables many
common constants to be generated, for example all powers of 2.
contents of Rm
value of operand 2
31 0
carry
out
1
C
in
ARM Instruction Set - TEQ, TST, CMP & CMN
ARM7TDMI Data Sheet
ARM DDI 0029E
4-16
Open Access
4.5.4 Writing to R15
When Rd is a register other than R15, the condition code flags in the CPSR may be
updated from the ALU flags as described above.
When Rd is R15 and the S flag in the instruction is not set the result of the operation
is placed in R15 and the CPSR is unaffected.
When Rd is R15 and the S flag is set the result of the operation is placed in R15 and
the SPSR corresponding to the current mode is moved to the CPSR. This allows state
changes which atomically restore both PC and CPSR. This form of instruction should
not be used in User mode.
4.5.5 Using R15 as an operand
If R15 (the PC) is used as an operand in a data processing instruction the register is
used directly.
The PC value will be the address of the instruction, plus 8 or 12 bytes due to instruction
prefetching. If the shift amount is specified in the instruction, the PC will be 8 bytes
ahead. If a register is used to specify the shift amount the PC will be 12 bytes ahead.
4.5.6 TEQ, TST, CMP and CMN opcodes
Note
TEQ, TST, CMP and CMN do not write the result of their operation but do set flags in
the CPSR. An assembler should always set the S flag for these instructions even if this
is not specified in the mnemonic.
The TEQP form of the TEQ instruction used in earlier ARM processors must not be
used: the PSR transfer operations should be used instead.
The action of TEQP in the ARM7TDMI is to move SPSR_<mode> to the CPSR if the
processor is in a privileged mode and to do nothing if in User mode.
4.5.7 Instruction cycle times
Data Processing instructions vary in the number of incremental cycles taken as
follows:
S, N and I are as defined in ➲
6.2 Cycle Types
on page 6-2.
Processing Type Cycles
Normal Data Processing 1S
Data Processing with register specified shift 1S + 1I
Data Processing with PC written 2S + 1N
Data Processing with register specified shift and PC written 2S + 1N + 1I
Table 4-4: Incremental cycle times
ARM Instruction Set - TEQ, TST, CMP & CMN
ARM7TDMI Data Sheet
ARM DDI 0029E
4-17
Open Access
4.5.8 Assembler syntax
1 MOV,MVN (single operand instructions.)
<opcode>{cond}{S} Rd,<Op2>
2 CMP,CMN,TEQ,TST (instructions which do not produce a result.)
<opcode>{cond} Rn,<Op2>
3 AND,EOR,SUB,RSB,ADD,ADC,SBC,RSC,ORR,BIC
<opcode>{cond}{S} Rd,Rn,<Op2>
where:
<Op2> is Rm{,<shift>} or,<#expression>
{cond} is a two-character condition mnemonic. See ➲
Table 4-2:
Condition code summary
on page 4-5.
{S} set condition codes if S present (implied for CMP, CMN, TEQ,
TST).
Rd, Rn and Rm are expressions evaluating to a register number.
<#expression> if this is used, the assembler will attempt to generate a shifted
immediate 8-bit field to match the expression. If this is
impossible, it will give an error.
<shift> is <shiftname> <register> or <shiftname> #expression, or
RRX (rotate right one bit with extend).
<shiftname>s are: ASL, LSL, LSR, ASR, ROR. (ASL is a synonym for LSL,
they assemble to the same code.)
4.5.9 Examples
ADDEQ R2,R4,R5 ; If the Z flag is set make R2:=R4+R5
TEQS R4,#3 ; test R4 for equality with 3.
; (The S is in fact redundant as the
; assembler inserts it automatically.)
SUB R4,R5,R7,LSR R2; Logical right shift R7 by the number in
; the bottom byte of R2, subtract result
; from R5, and put the answer into R4.
MOV PC,R14 ; Return from subroutine.
MOVS PC,R14 ; Return from exception and restore CPSR
; from SPSR_mode.
ARM Instruction Set - MRS, MSR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-18
Open Access
4.6 PSR Transfer (MRS, MSR)
The instruction is only executed if the condition is true. The various conditions are
defined in ➲
Table 4-2: Condition code summary
on page 4-5.
The MRS and MSR instructions are formed from a subset of the Data Processing
operations and are implemented using the TEQ, TST, CMN and CMP instructions
without the S flag set. The encoding is shown in ➲
Figure 4-11: PSR transfer
on page
4-19.
These instructions allow access to the CPSR and SPSR registers. The MRS
instruction allows the contents of the CPSR or SPSR_<mode> to be moved to a
general register. The MSR instruction allows the contents of a general register to be
moved to the CPSR or SPSR_<mode> register.
The MSR instruction also allows an immediate value or register contents to be
transferred to the condition code flags (N,Z,C and V) of CPSR or SPSR_<mode>
without affecting the control bits. In this case, the top four bits of the specified register
contents or 32 bit immediate value are written to the top four bits of the relevant PSR.
4.6.1 Operand restrictions
•In User mode, the control bits of the CPSR are protected from change, so only
the condition code flags of the CPSR can be changed. In other (privileged)
modes the entire CPSR can be changed.
Note that the software must never change the state of the T bit in the CPSR.
If this happens, the processor will enter an unpredictable state.
• The SPSR register which is accessed depends on the mode at the time of
execution. For example, only SPSR_fiq is accessible when the processor is in
FIQ mode.
• You must not specify R15 as the source or destination register.
•Also, do not attempt to access an SPSR in User mode, since no such register
exists.
ARM Instruction Set - MRS, MSR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-19
Open Access
Figure 4-11: PSR transfer
Cond
0
000000000000
Rd
P
00010 s001111
16 15 12 11
22
28 21
31 23
27
MRS (transfer PSR contents to a register)
Destination register
Source PSR
Condition field
0=CPSR
1=SPSR_<current mode>
Cond
0
00000000 Rm
P
00010 d1010011111
43
12 11
22
28 21
31 23
27
MSR (transfer register contents to PSR)
Source register
Destination PSR
Condition field
0=CPSR
1=SPSR_<current mode>
Cond
0
Source operand
P
00 d1010001111
12 11
22
28 21
31 23
27
MSR (transfer register contents or immdiate value to PSR flag bits only)
Destination PSR
Immediate Operand
0=CPSR
1=SPSR_<current mode>
I10
11 430
0=source operand is a register
1=source operand is an immediate value
11 8 7 0
Condition field
00000000
Rotate Imm
Rm
Source register
Unsigned 8 bit immediate value
shift applied to Imm
ARM Instruction Set - MRS, MSR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-20
Open Access
4.6.2 Reserved bits
Only twelve bits of the PSR are defined in ARM7TDMI (N,Z,C,V,I,F, T & M[4:0]); the
remaining bits are reserved for use in future versions of the processor. Refer to
➲
Figure 3-6: Program status register format
on page 3-8 for a full description of the
PSR bits.
To ensure the maximum compatibility between ARM7TDMI programs and future
processors, the following rules should be observed:
• The reserved bits should be preserved when changing the value in a PSR.
• Programs should not rely on specific values from the reserved bits when
checking the PSR status, since they may read as one or zero in future
processors.
A read-modify-write strategy should therefore be used when altering the control bits of
any PSR register; this involves transferring the appropriate PSR register to a general
register using the MRS instruction, changing only the relevant bits and then
transferring the modified value back to the PSR register using the MSR instruction.
Example
The following sequence performs a mode change:
MRS R0,CPSR ; Take a copy of the CPSR.
BIC R0,R0,#0x1F ; Clear the mode bits.
ORR R0,R0,#new_mode ; Select new mode
MSR CPSR,R0 ; Write back the modified
; CPSR.
When the aim is simply to change the condition code flags in a PSR, a value can be
written directly to the flag bits without disturbing the control bits. The following
instruction sets the N,Z,C and V flags:
MSR CPSR_flg,#0xF0000000 ; Set all the flags
; regardless of their
; previous state (does not
; affect any control bits).
No attempt should be made to write an 8 bit immediate value into the whole PSR since
such an operation cannot preserve the reserved bits.
4.6.3 Instruction cycle times
PSR T ransfers take 1S incremental cycles, where S is as defined in ➲
6.2 Cycle Types
on page 6-2.
ARM Instruction Set - MRS, MSR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-21
Open Access
4.6.4 Assembler syntax
1 MRS - transfer PSR contents to a register
MRS{cond} Rd,<psr>
2 MSR - transfer register contents to PSR
MSR{cond} <psr>,Rm
3 MSR - transfer register contents to PSR flag bits only
MSR{cond} <psrf>,Rm
The most significant four bits of the register contents are written to the N,Z,C
& V flags respectively.
4 MSR - transfer immediate value to PSR flag bits only
MSR{cond} <psrf>,<#expression>
The expression should symbolise a 32 bit value of which the most significant
four bits are written to the N,Z,C and V flags respectively.
Key:
{cond} two-character condition mnemonic. See ➲
Table 4-2:
Condition code summary
on page 4-5.
Rd and Rm are expressions evaluating to a register number other than
R15
<psr> is CPSR, CPSR_all, SPSR or SPSR_all. (CPSR and
CPSR_all are synonyms as are SPSR and SPSR_all)
<psrf> is CPSR_flg or SPSR_flg
<#expression> where this is used, the assembler will attempt to generate a
shifted immediate 8-bit field to match the expression. If this is
impossible, it will give an error.
ARM Instruction Set - MRS, MSR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-22
Open Access
4.6.5 Examples
In User mode the instructions behave as follows:
MSR CPSR_all,Rm ; CPSR[31:28] <- Rm[31:28]
MSR CPSR_flg,Rm ; CPSR[31:28] <- Rm[31:28]
MSR CPSR_flg,#0xA0000000 ; CPSR[31:28] <- 0xA
;(set N,C; clear Z,V)
MRS Rd,CPSR ; Rd[31:0] <- CPSR[31:0]
In privileged modes the instructions behave as follows:
MSR CPSR_all,Rm ; CPSR[31:0] <- Rm[31:0]
MSR CPSR_flg,Rm ; CPSR[31:28] <- Rm[31:28]
MSR CPSR_flg,#0x50000000 ; CPSR[31:28] <- 0x5
;(set Z,V; clear N,C)
MRS Rd,CPSR ; Rd[31:0] <- CPSR[31:0]
MSR SPSR_all,Rm ;SPSR_<mode>[31:0]<- Rm[31:0]
MSR SPSR_flg,Rm ; SPSR_<mode>[31:28] <- Rm[31:28]
MSR SPSR_flg,#0xC0000000 ; SPSR_<mode>[31:28] <- 0xC
;(set N,Z; clear C,V)
MRS Rd,SPSR ; Rd[31:0] <- SPSR_<mode>[31:0]
ARM Instruction Set - MUL, MLA
ARM7TDMI Data Sheet
ARM DDI 0029E
4-23
Open Access
4.7 Multiply and Multiply-Accumulate (MUL, MLA)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-12: Multiply instructions
.
The multiply and multiply-accumulate instructions use an 8 bit Booth's algorithm to
perform integer multiplication.
Figure 4-12: Multiply instructions
The multiply form of the instruction gives Rd:=Rm*Rs. Rn is ignored, and should be
set to zero for compatibility with possible future upgrades to the instruction set.
The multiply-accumulate form gives Rd:=Rm*Rs+Rn, which can save an explicit ADD
instruction in some circumstances.
Both forms of the instruction work on operands which may be considered as signed
(2’s complement) or unsigned integers.
The results of a signed multiply and of an unsigned multiply of 32 bit operands differ
only in the upper 32 bits - the low 32 bits of the signed and unsigned results are
identical. As these instructions only produce the low 32 bits of a multiply, they can be
used for both signed and unsigned multiplies.
For example consider the multiplication of the operands:
Operand A Operand B Result
0xFFFFFFF6 0x0000001 0xFFFFFF38
If the operands are interpreted as signed
Operand A has the value -10, operand B has the value 20, and the result is -200 which
is correctly represented as 0xFFFFFF38
If the operands are interpreted as unsigned
Operand A has the value 4294967286, operand B has the value 20 and the result is
85899345720, which is represented as 0x13FFFFFF38, so the least significant 32 bits
are 0xFFFFFF38.
Cond 0 0 0 0 0 0 A S Rd Rn Rs 1 0 0 1 Rm
034781112151619202122272831
Operand registers
Destination register
Set condition code
Accumulate
0 = do not alter condition codes
1 = set condition codes
0 = multiply only
1 = multiply and accumulate
Condition Field
ARM Instruction Set - MUL, MLA
ARM7TDMI Data Sheet
ARM DDI 0029E
4-24
Open Access
4.7.1 Operand restrictions
The destination register Rd must not be the same as the operand register Rm. R15
must not be used as an operand or as the destination register.
All other register combinations will give correct results, and Rd, Rn and Rs may use
the same register when required.
4.7.2 CPSR flags
Setting the CPSR flags is optional, and is controlled by the S bit in the instruction. The
N (Negative) and Z (Zero) flags are set correctly on the result (N is made equal to bit
31 of the result, and Z is set if and only if the result is zero). The C (Carry) flag is set
to a meaningless value and the V (oVerflow) flag is unaffected.
4.7.3 Instruction cycle times
MUL takes 1S + mI and MLA 1S + (m+1)I cycles to execute, where S and I are as
defined in ➲
6.2 Cycle Types
on page 6-2.
mis the number of 8 bit multiplier array cycles required to complete the
multiply, which is controlled by the value of the multiplier operand
specified by Rs. Its possible values are as follows
1 if bits [32:8] of the multiplier operand are all zero or all one.
2 if bits [32:16] of the multiplier operand are all zero or all one.
3 if bits [32:24] of the multiplier operand are all zero or all one.
4 in all other cases.
4.7.4 Assembler syntax
MUL{cond}{S} Rd,Rm,Rs
MLA{cond}{S} Rd,Rm,Rs,Rn
{cond} two-character condition mnemonic. See ➲
Table 4-2:
Condition code summary
on page 4-5.
{S} set condition codes if S present
Rd, Rm, Rs and Rn are expressions evaluating to a register number other
than R15.
4.7.5 Examples
MUL R1,R2,R3 ; R1:=R2*R3
MLAEQS R1,R2,R3,R4 ; Conditionally R1:=R2*R3+R4,
; setting condition codes.
ARM Instruction Set - MULL,MLAL
ARM7TDMI Data Sheet
ARM DDI 0029E
4-25
Open Access
4.8 Multiply Long and Multiply-Accumulate Long (MULL,MLAL)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-13: Multiply long instructions
.
The multiply long instructions perform integer multiplication on two 32 bit operands
and produce 64 bit results. Signed and unsigned multiplication each with optional
accumulate give rise to four variations.
Figure 4-13: Multiply long instructions
The multiply forms (UMULL and SMULL) take two 32 bit numbers and multiply them
to produce a 64 bit result of the form RdHi,RdLo := Rm * Rs. The lower 32 bits of the
64 bit result are written to RdLo, the upper 32 bits of the result are written to RdHi.
The multiply-accumulate forms (UMLAL and SMLAL) take two 32 bit numbers, multiply
them and add a 64 bit number to produce a 64 bit result of the form RdHi,RdLo := Rm
* Rs + RdHi,RdLo. The lower 32 bits of the 64 bit number to add is read from RdLo.
The upper 32 bits of the 64 bit number to add is read from RdHi. The lower 32 bits of
the 64 bit result are written to RdLo. The upper 32 bits of the 64 bit result are written
to RdHi.
The UMULL and UMLAL instructions treat all of their operands as unsigned binary
numbers and write an unsigned 64 bit result. The SMULL and SMLAL instructions
treat all of their operands as two's-complement signed numbers and write a two's-
complement signed 64 bit result.
4.8.1 Operand restrictions
• R15 must not be used as an operand or as a destination register.
• RdHi, RdLo, and Rm must all specify different registers.
Cond 0 0 0 0 1 U A S RdHi RdLo Rs 1 0 0 1 Rm
03478111215161920212223272831
Operand registers
Source destination registers
Set condition code
Accumulate
Unsigned
0 = do not alter condition codes
1 = set condition codes
0 = multiply only
1 = multiply and accumulate
0 = unsigned
1 = signed
Condition Field
ARM Instruction Set - MULL,MLAL
ARM7TDMI Data Sheet
ARM DDI 0029E
4-26
Open Access
4.8.2 CPSR flags
Setting the CPSR flags is optional, and is controlled by the S bit in the instruction. The
N and Z flags are set correctly on the result (N is equal to bit 63 of the result, Z is set
if and only if all 64 bits of the result are zero). Both the C and V flags are set to
meaningless values.
4.8.3 Instruction cycle times
MULL takes 1S + (m+1)I and MLAL 1S + (m+2)I cycles to execute, where
m
is the
number of 8 bit multiplier array cycles required to complete the multiply, which is
controlled by the value of the multiplier operand specified by Rs.
Its possible values are as follows:
For signed instructions SMULL, SMLAL:
1 if bits [31:8] of the multiplier operand are all zero or all one.
2 if bits [31:16] of the multiplier operand are all zero or all one.
3 if bits [31:24] of the multiplier operand are all zero or all one.
4 in all other cases.
For unsigned instructions UMULL, UMLAL:
1 if bits [31:8] of the multiplier operand are all zero.
2 if bits [31:16] of the multiplier operand are all zero.
3 if bits [31:24] of the multiplier operand are all zero.
4 in all other cases.
S and I are as defined in ➲
6.2 Cycle Types
on page 6-2.
4.8.4 Assembler syntax
Mnemonic Description Purpose
UMULL{cond}{S} RdLo,RdHi,Rm,Rs Unsigned Multiply Long 32 x 32 = 64
UMLAL{cond}{S} RdLo,RdHi,Rm,Rs Unsigned Multiply & Accumulate Long 32 x 32 + 64 = 64
SMULL{cond}{S} RdLo,RdHi,Rm,Rs Signed Multiply Long 32 x 32 = 64
SMLAL{cond}{S} RdLo,RdHi,Rm,Rs Signed Multiply & Accumulate Long 32 x 32 + 64 = 64
Table 4-5: Assembler syntax descriptions
ARM Instruction Set - MULL,MLAL
ARM7TDMI Data Sheet
ARM DDI 0029E
4-27
Open Access
where:
{cond} two-character condition mnemonic. See ➲
Table 4-2:
Condition code summary
on page 4-5.
{S} set condition codes if S present
RdLo, RdHi, Rm, Rs are expressions evaluating to a register number other
than R15.
4.8.5 Examples
UMULL R1,R4,R2,R3 ; R4,R1:=R2*R3
UMLALS R1,R5,R2,R3 ; R5,R1:=R2*R3+R5,R1 also setting
; condition codes
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-28
Open Access
4.9 Single Data Transfer (LDR, STR)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-14: Single data transfer instructions
on page 4-28.
The single data transfer instructions are used to load or store single bytes or words of
data. The memory address used in the transfer is calculated by adding an offset to or
subtracting an offset from a base register.
The result of this calculation may be written back into the base register if auto-indexing
is required.
Figure 4-14: Single data transfer instructions
Cond I Rn Rd
011121516192021242526272831
01 P U B W L Offset
2223
011
Source/Destination register
Base register
Load/Store bit
0 = Store to memory
1 = Load from memory
Write-back bit
Byte/Word bit
0 = no write-back
1 = write address into base
0 = transfer word quantity
1 = transfer byte quantity
Up/Down bit
Pre/Post indexing bit
0 = offset is an immediate value
Immediate offset
Immediate offset
Unsigned 12 bit immediate offset
1 = offset is a register
11 0
shift applied to Rm
34
Condition field
0 = down; subtract offset from base
1 = up; add offset to base
0 = post; add offset after transfer
1 = pre; add offset before transfer
Offset register
Shift Rm
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-29
Open Access
4.9.1 Offsets and auto-indexing
The offset from the base may be either a 12 bit unsigned binary immediate value in
the instruction, or a second register (possibly shifted in some way). The offset may be
added to (U=1) or subtracted from (U=0) the base register Rn. The offset modification
may be performed either before (pre-indexed, P=1) or after (post-indexed, P=0) the
base is used as the transfer address.
The W bit gives optional auto increment and decrement addressing modes. The
modified base value may be written back into the base (W=1), or the old base value
may be kept (W=0). In the case of post-indexed addressing, the write back bit is
redundant and is always set to zero, since the old base value can be retained by
setting the offset to zero. Therefore post-indexed data transfers always write back the
modified base. The only use of the W bit in a post-indexed data transfer is in privileged
mode code, where setting the W bit forces non-privileged mode for the transfer,
allowing the operating system to generate a user address in a system where the
memory management hardware makes suitable use of this hardware.
4.9.2 Shifted register offset
The 8 shift control bits are described in the data processing instructions section.
However, the register specified shift amounts are not available in this instruction class.
See ➲
4.5.2 Shifts
on page 4-12.
4.9.3 Bytes and words
This instruction class may be used to transfer a byte (B=1) or a word (B=0) between
an ARM7TDMI register and memory.
The action of LDR(B) and STR(B) instructions is influenced by the BIGEND control
signal. The two possible configurations are described below.
Little endian configuration
A byte load (LDRB) expects the data on data bus inputs 7 through 0 if the supplied
address is on a word boundary, on data bus inputs 15 through 8 if it is a word address
plus one byte, and so on. The selected byte is placed in the bottom 8 bits of the
destination register, and the remaining bits of the register are filled with zeros. Please
see ➲
Figure 3-2: Little endian addresses of bytes within words
on page 3-3.
A byte store (STRB) repeats the bottom 8 bits of the source register four times across
data bus outputs 31 through 0. The external memory system should activate the
appropriate byte subsystem to store the data.
A word load (LDR) will normally use a word aligned address. However, an address
offset from a word boundary will cause the data to be rotated into the register so that
the addressed byte occupies bits 0 to 7. This means that half-words accessed at
offsets 0 and 2 from the word boundary will be correctly loaded into bits 0 through 15
of the register. Two shift operations are then required to clear or to sign extend the
upper 16 bits. This is illustrated in ➲
Figure 4-15: Little endian offset addressing
on
page 4-30.
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-30
Open Access
Figure 4-15: Little endian offset addressing
A word store (STR) should generate a word aligned address. The word presented to
the data bus is not affected if the address is not word aligned. That is, bit 31 of the
register being stored always appears on data bus output 31.
Big endian configuration
A byte load (LDRB) expects the data on data bus inputs 31 through 24 if the supplied
address is on a word boundary , on data bus inputs 23 through 16 if it is a word address
plus one byte, and so on. The selected byte is placed in the bottom 8 bits of the
destination register and the remaining bits of the register are filled with zeros. Please
see ➲
Figure 3-1: Big endian addresses of bytes within words
on page 3-3.
A byte store (STRB) repeats the bottom 8 bits of the source register four times across
data bus outputs 31 through 0. The external memory system should activate the
appropriate byte subsystem to store the data.
A word load (LDR) should generate a word aligned address. An address offset of 0 or
2 from a word boundary will cause the data to be rotated into the register so that the
addressed byte occupies bits 31 through 24. This means that half-words accessed at
these offsets will be correctly loaded into bits 16 through 31 of the register. A shift
operation is then required to move (and optionally sign extend) the data into the
bottom 16 bits. An address offset of 1 or 3 from a word boundary will cause the data
to be rotated into the register so that the addressed byte occupies bits 15 through 8.
A
B
C
D
memory
A+3
A+2
A+1
A
24
16
8
0
A
B
C
D
register
24
16
8
0
LDR from word aligned address
A
B
C
D
A+3
A+2
A+1
A
24
16
8
0
A
B
C
D
24
16
8
0
LDR from address offset by 2
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-31
Open Access
A word store (STR) should generate a word aligned address. The word presented to
the data bus is not affected if the address is not word aligned. That is, bit 31 of the
register being stored always appears on data bus output 31.
4.9.4 Use of R15
Write-back must not be specified if R15 is specified as the base register (Rn). When
using R15 as the base register you must remember it contains an address 8 bytes on
from the address of the current instruction.
R15 must not be specified as the register offset (Rm).
When R15 is the source register (Rd) of a register store (STR) instruction, the stored
value will be address of the instruction plus 12.
4.9.5 Restriction on the use of base register
When configured for late aborts, the following example code is difficult to unwind as
the base register , Rn, gets updated before the abort handler starts. Sometimes it may
be impossible to calculate the initial value.
After an abort, the following example code is difficult to unwind as the base register,
Rn, gets updated before the abort handler starts. Sometimes it may be impossible to
calculate the initial value.
Example:
LDR R0,[R1],R1
Therefore a post-indexed LDR or STR where Rm is the same register as Rn should
not be used.
4.9.6 Data aborts
A transfer to or from a legal address may cause problems for a memory management
system. For instance, in a system which uses virtual memory the required data may
be absent from main memory. The memory manager can signal a problem by taking
the processor ABORT input HIGH whereupon the Data Abort trap will be taken. It is
up to the system software to resolve the cause of the problem, then the instruction can
be restarted and the original program continued.
4.9.7 Instruction cycle times
Normal LDR instructions take 1S + 1N + 1I and LDR PC take 2S + 2N +1I incremental
cycles, where S,N and I are as defined in ➲
6.2 Cycle Types
on page 6-2.
STR instructions take 2N incremental cycles to execute.
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-32
Open Access
4.9.8 Assembler syntax
<LDR|STR>{cond}{B}{T} Rd,<Address>
where:
LDR load from memory into a register
STR store from a register into memory
{cond} two-character condition mnemonic. See ➲
Table 4-2: Condition code
summary
on page 4-5.
{B} if B is present then byte transfer, otherwise word transfer
{T} if T is present the W bit will be set in a post-indexed instruction, forcing
non-privileged mode for the transfer cycle. T is not allowed when a
pre-indexed addressing mode is specified or implied.
Rd is an expression evaluating to a valid register number.
Rn and Rm are expressions evaluating to a register number . If Rn is R15 then the
assembler will subtract 8 from the offset value to allow for ARM7TDMI
pipelining. In this case base write-back should not be specified.
<Address> can be:
1 An expression which generates an address:
<expression>
The assembler will attempt to generate an instruction using
the PC as a base and a corrected immediate offset to address
the location given by evaluating the expression. This will be a
PC relative, pre-indexed address. If the address is out of
range, an error will be generated.
2 A pre-indexed addressing specification:
[Rn] offset of zero
[Rn,<#expression>]{!} offset of <expression>
bytes
[Rn,{+/-}Rm{,<shift>}]{!} offset of +/- contents of
index register, shifted
by <shift>
3 A post-indexed addressing specification:
[Rn],<#expression> offset of <expression>
bytes
[Rn],{+/-}Rm{,<shift>} offset of +/- contents of
index register, shifted
as by <shift>.
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-33
Open Access
<shift> general shift operation (see data processing instructions) but
you cannot specify the shift amount by a register.
{!} writes back the base register (set the W bit) if! is present.
4.9.9 Examples
STR R1,[R2,R4]! ; Store R1 at R2+R4 (both of which are
; registers) and write back address to
; R2.
STR R1,[R2],R4 ; Store R1 at R2 and write back
; R2+R4 to R2.
LDR R1,[R2,#16] ; Load R1 from contents of R2+16, but
; don't write back.
LDR R1,[R2,R3,LSL#2] ; Load R1 from contents of R2+R3*4.
LDREQBR1,[R6,#5] ; Conditionally load byte at R6+5 into
; R1 bits 0 to 7, filling bits 8 to 31
; with zeros.
STR R1,PLACE ; Generate PC relative offset to
; address PLACE.
•
PLACE
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-34
Open Access
4.10 Halfword and Signed Data Transfer
(LDRH/STRH/LDRSB/LDRSH)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-16: Halfword and signed data transfer with register offset
,
below, and ➲
Figure 4-17: Halfword and signed data transfer with immediate offset
on
page 4-35.
These instructions are used to load or store half-words of data and also load
sign-extended bytes or half-words of data. The memory address used in the transfer
is calculated by adding an offset to or subtracting an offset from a base register. The
result of this calculation may be written back into the base register if auto-indexing is
required.
Figure 4-16: Halfword and signed data transfer with register offset
Cond 0 0 0 P U 0 W L Rn Rd 0 0 0 0 Rm
034781112151619202122272831
Offset register
Base register
S H
Source/Destination
00 = SWP instruction
01 = Unsigned halfwords
0 = store to memory
1 = load from memory
Load/Store
1 S H 1
10 = Signed byte
11 = Signed halfwords
register
0 = no write-back
1 = write address into base
Write-back
0 = down: subtract offset from
base
Up/Down
1 = up: add offset to base
0 = post: add/subtract offset
Pre/Post indexing
after transfer
1 = pre: add/subtract offset
before transfer
Condition field
232425 56
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-35
Open Access
Figure 4-17: Halfword and signed data transfer with immediate offset
4.10.1 Offsets and auto-indexing
The offset from the base may be either a 8-bit unsigned binary immediate value in the
instruction, or a second register. The 8-bit offset is formed by concatenating bits 11 to
8 and bits 3 to 0 of the instruction word, such that bit 11 becomes the MSB and bit 0
becomes the LSB. The offset may be added to (U=1) or subtracted from (U=0) the
base register Rn. The offset modification may be performed either before (pre-
indexed, P=1) or after (post-indexed, P=0) the base register is used as the transfer
address.
The W bit gives optional auto-increment and decrement addressing modes. The
modified base value may be written back into the base (W=1), or the old base may be
kept (W=0). In the case of post-indexed addressing, the write back bit is redundant and
is always set to zero, since the old base value can be retained if necessary by setting
the offset to zero. Therefore post-indexed data transfers always write back the
modified base.
The Write-back bit should not be set high (W=1) when post-indexed addressing is
selected.
Cond 0 0 0 P U 1 W L Rn Rd Offset
034781112151619202122272831
Immediate Offset
Base register
S H
Source/Destination
00 = SWP instruction
01 = Unsigned halfwords
0 = store to memory
1 = load from memory
Load/Store
1 S H 1
10 = Signed byte
11 = Signed halfwords
register
0 = no write-back
1 = write address into base
Write-back
0 = down: subtract offset from
base
Up/Down
1 = up: add offset to base
0 = post: add/subtract offset
Pre/Post indexing
after transfer
1 = pre: add/subtract offset
before transfer
Condition field
232425 56
Offset
Immediate Offset
(High nibble)
(Low nibble)
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-36
Open Access
4.10.2 Halfword load and stores
Setting S=0 and H=1 may be used to transfer unsigned Half-words between an
ARM7TDMI register and memory.
The action of LDRH and STRH instructions is influenced by the BIGEND control
signal. The two possible configurations are described in the section below.
4.10.3 Signed byte and halfword loads
The S bit controls the loading of sign-extended data. When S=1 the H bit selects
between Bytes (H=0) and Half-words (H=1). The L bit should not be set low (Store)
when Signed (S=1) operations have been selected.
The LDRSB instruction loads the selected Byte into bits 7 to 0 of the destination
register and bits 31 to 8 of the destination register are set to the value of bit 7, the sign
bit.
The LDRSH instruction loads the selected Half-word into bits 15 to 0 of the destination
register and bits 31 to 16 of the destination register are set to the value of bit 15, the
sign bit.
The action of the LDRSB and LDRSH instructions is influenced by the BIGEND control
signal. The two possible configurations are described in the following section.
4.10.4 Endianness and byte/halfword selection
Little endian configuration
A signed byte load (LDRSB) expects data on data bus inputs 7 through to 0 if the
supplied address is on a word boundary, on data bus inputs 15 through to 8 if it is a
word address plus one byte, and so on. The selected byte is placed in the bottom 8 bit
of the destination register , and the remaining bits of the register are filled with the sign
bit, bit 7 of the byte. Please see ➲
Figure 3-2: Little endian addresses of bytes within
words
on page 3-3
A halfword load (LDRSH or LDRH) expects data on data bus inputs 15 through to 0 if
the supplied address is on a word boundary and on data bus inputs 31 through to 16
if it is a halfword boundary, (A[1]=1).The supplied address should always be on a
halfword boundary. If bit 0 of the supplied address is HIGH then the ARM7TDMI will
load an unpredictable value. The selected halfword is placed in the bottom 16 bits of
the destination register . For unsigned half-words (LDRH), the top 16 bits of the register
are filled with zeros and for signed half-words (LDRSH) the top 16 bits are filled with
the sign bit, bit 15 of the halfword.
A halfword store (STRH) repeats the bottom 16 bits of the source register twice across
the data bus outputs 31 through to 0. The external memory system should activate the
appropriate halfword subsystem to store the data. Note that the address must be
halfword aligned, if bit 0 of the address is HIGH this will cause unpredictable
behaviour.
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-37
Open Access
Big endian configuration
A signed byte load (LDRSB) expects data on data bus inputs 31 through to 24 if the
supplied address is on a word boundary, on data bus inputs 23 through to 16 if it is a
word address plus one byte, and so on. The selected byte is placed in the bottom 8 bit
of the destination register , and the remaining bits of the register are filled with the sign
bit, bit 7 of the byte. Please see ➲
Figure 3-1: Big endian addresses of bytes within
words
on page 3-3
A halfword load (LDRSH or LDRH) expects data on data bus inputs 31 through to 16
if the supplied address is on a word boundary and on data bus inputs 15 through to 0
if it is a halfword boundary, (A[1]=1). The supplied address should always be on a
halfword boundary. If bit 0 of the supplied address is HIGH then the ARM7TDMI will
load an unpredictable value. The selected halfword is placed in the bottom 16 bits of
the destination register . For unsigned half-words (LDRH), the top 16 bits of the register
are filled with zeros and for signed half-words (LDRSH) the top 16 bits are filled with
the sign bit, bit 15 of the halfword.
A halfword store (STRH) repeats the bottom 16 bits of the source register twice across
the data bus outputs 31 through to 0. The external memory system should activate the
appropriate halfword subsystem to store the data. Note that the address must be
halfword aligned, if bit 0 of the address is HIGH this will cause unpredictable
behaviour.
4.10.5 Use of R15
Write-back should not be specified if R15 is specified as the base register (Rn). When
using R15 as the base register you must remember it contains an address 8 bytes on
from the address of the current instruction.
R15 should not be specified as the register offset (Rm).
When R15 is the source register (Rd) of a Half-word store (STRH) instruction, the
stored address will be address of the instruction plus 12.
4.10.6 Data aborts
A transfer to or from a legal address may cause problems for a memory management
system. For instance, in a system which uses virtual memory the required data may
be absent from the main memory. The memory manager can signal a problem by
taking the processor ABORT input HIGH whereupon the Data Abort trap will be taken.
It is up to the system software to resolve the cause of the problem, then the instruction
can be restarted and the original program continued.
4.10.7 Instruction cycle times
Normal LDR(H,SH,SB) instructions take 1S + 1N + 1I
LDR(H,SH,SB) PC take 2S + 2N + 1I incremental cycles.
S,N and I are defined in➲
6.2 Cycle Types
on page 6-2.
STRH instructions take 2N incremental cycles to execute.
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-38
Open Access
4.10.8 Assembler syntax
<LDR|STR>{cond}<H|SH|SB> Rd,<address>
LDR load from memory into a register
STR Store from a register into memory
{cond} two-character condition mnemonic. See ➲
Table 4-2: Condition code
summary
on page 4-5.
H Transfer halfword quantity
SB Load sign extended byte (Only valid for LDR)
SH Load sign extended halfword (Only valid for LDR)
Rd is an expression evaluating to a valid register number.
<address> can be:
1 An expression which generates an address:
<expression>
The assembler will attempt to generate an instruction using
the PC as a base and a corrected immediate offset to address
the location given by evaluating the expression. This will be a
PC relative, pre-indexed address. If the address is out of
range, an error will be generated.
2 A pre-indexed addressing specification:
[Rn] offset of zero
[Rn,<#expression>]{!} offset of <expression> bytes
[Rn,{+/-}Rm]{!} offset of +/- contents of
index register
3 A post-indexed addressing specification:
[Rn],<#expression> offset of <expression> bytes
[Rn],{+/-}Rm offset of +/- contents of
index register.
Rn and Rm are expressions evaluating to a register number.
If Rn is R15 then the assembler will subtract 8 from the offset
value to allow for ARM7TDMI pipelining. In this case base
write-back should not be specified.
{!} writes back the base register (set the W bit) if ! is present.
ARM Instruction Set - LDR, STR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-39
Open Access
4.10.9 Examples
LDRH R1,[R2,-R3]! ; Load R1 from the contents of the
; halfword address contained in
; R2-R3 (both of which are registers)
; and write back address to R2
STRH R3,[R4,#14] ; Store the halfword in R3 at R14+14
; but don't write back.
LDRSB R8,[R2],#-223 ; Load R8 with the sign extended
; contents of the byte address
; contained in R2 and write back
; R2-223 to R2.
LDRNESH R11,[R0] ; conditionally load R11 with the sign
; extended contents of the halfword
; address contained in R0.
HERE ; Generate PC relative offset to
; address FRED.
; Store the halfword in R5 at address
; FRED.
STRH R5, [PC, #(FRED-HERE-8)]
.
FRED
ARM Instruction Set - LDM, STM
ARM7TDMI Data Sheet
ARM DDI 0029E
4-40
Open Access
4.11 Block Data Transfer (LDM, STM)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-18: Block data transfer instructions
.
Block data transfer instructions are used to load (LDM) or store (STM) any subset of
the currently visible registers. They support all possible stacking modes, maintaining
full or empty stacks which can grow up or down memory, and are very efficient
instructions for saving or restoring context, or for moving large blocks of data around
main memory.
4.11.1 The register list
The instruction can cause the transfer of any registers in the current bank (and
non-user mode programs can also transfer to and from the user bank, see below). The
register list is a 16 bit field in the instruction, with each bit corresponding to a register.
A 1 in bit 0 of the register field will cause R0 to be transferred, a 0 will cause it not to
be transferred; similarly bit 1 controls the transfer of R1, and so on.
Any subset of the registers, or all the registers, may be specified. The only restriction
is that the register list should not be empty.
Whenever R15 is stored to memory the stored value is the address of the STM
instruction plus 12.
Figure 4-18: Block data transfer instructions
Cond Rn
015161920212425272831
P U W L
2223
100 S Register list
Base register
Load/Store bit
0 = Store to memory
1 = Load from memory
Write-back bit
0 = no write-back
1 = write address into base
Up/Down bit
Pre/Post indexing bit
0 = down; subtract offset from base
1 = up; add offset to base
0 = post; add offset after transfer
1 = pre; add offset before transfer
PSR & force user bit
0 = do not load PSR or force user mode
1 = load PSR or force user mode
Condition field
ARM Instruction Set - LDM, STM
ARM7TDMI Data Sheet
ARM DDI 0029E
4-41
Open Access
4.11.2 Addressing modes
The transfer addresses are determined by the contents of the base register (Rn), the
pre/post bit (P) and the up/down bit (U). The registers are transferred in the order
lowest to highest, so R15 (if in the list) will always be transferred last. The lowest
register also gets transferred to/from the lowest memory address. By way of
illustration, consider the transfer of R1, R5 and R7 in the case where Rn=0x1000 and
write back of the modified base is required (W=1). ➲
Figure 4-19: Post-increment
addressing
,➲
Figure 4-20: Pre-increment addressing
,➲
Figure 4-21: Post-decrement
addressing
and ➲
Figure 4-22: Pre-decrement addressing
show the sequence of
register transfers, the addresses used, and the value of Rn after the instruction has
completed.
In all cases, had write back of the modified base not been required (W=0), Rn would
have retained its initial value of 0x1000 unless it was also in the transfer list of a load
multiple register instruction, when it would have been overwritten with the loaded
value.
4.11.3 Address alignment
The address should normally be a word aligned quantity and non-word aligned
addresses do not affect the instruction. However, the bottom 2 bits of the address will
appear on A[1:0] and might be interpreted by the memory system.
Figure 4-19: Post-increment addressing
0x100C
0x1000
0x0FF4
Rn
1
0x100C
0x1000
0x0FF4
2
R1
0x100C
0x1000
0x0FF4
3
0x100C
0x1000
0x0FF4
4
R1
R7
R5
R1
R5
Rn
ARM Instruction Set - LDM, STM
ARM7TDMI Data Sheet
ARM DDI 0029E
4-42
Open Access
Figure 4-20: Pre-increment addressing
Figure 4-21: Post-decrement addressing
0x100C
0x1000
0x0FF4
Rn
1
0x100C
0x1000
0x0FF4
2
R1
0x100C
0x1000
0x0FF4
3
0x100C
0x1000
0x0FF4
4
R1
R7
R5
R1
R5 Rn
0x100C
0x1000
0x0FF4
Rn
1
0x100C
0x1000
0x0FF4
2
R1
0x100C
0x1000
0x0FF4
3
0x100C
0x1000
0x0FF4
4
R1
R7
R5
R1
R5
Rn
ARM Instruction Set - LDM, STM
ARM7TDMI Data Sheet
ARM DDI 0029E
4-43
Open Access
Figure 4-22: Pre-decrement addressing
4.11.4 Use of the S bit
When the S bit is set in a LDM/STM instruction its meaning depends on whether or not
R15 is in the transfer list and on the type of instruction. The S bit should only be set if
the instruction is to execute in a privileged mode.
LDM with R15 in transfer list and S bit set (Mode changes)
If the instruction is a LDM then SPSR_<mode> is transferred to CPSR at the same
time as R15 is loaded.
STM with R15 in transfer list and S bit set (User bank transfer)
The registers transferred are taken from the User bank rather than the bank
corresponding to the current mode. This is useful for saving the user state on process
switches. Base write-back should not be used when this mechanism is employed.
R15 not in list and S bit set (User bank transfer)
For both LDM and STM instructions, the User bank registers are transferred rather
than the register bank corresponding to the current mode. This is useful for saving the
user state on process switches. Base write-back should not be used when this
mechanism is employed.
When the instruction is LDM, care must be taken not to read from a banked register
during the following cycle (inserting a dummy instruction such as MOV R0, R0 after
the LDM will ensure safety).
0x100C
0x1000
0x0FF4
Rn
1
0x100C
0x1000
0x0FF4
2
R1
0x100C
0x1000
0x0FF4
3
0x100C
0x1000
0x0FF4
4
R1
R7
R5
R1
R5 Rn
ARM Instruction Set - LDM, STM
ARM7TDMI Data Sheet
ARM DDI 0029E
4-44
Open Access
4.11.5 Use of R15 as the base
R15 should not be used as the base register in any LDM or STM instruction.
4.11.6 Inclusion of the base in the register list
When write-back is specified, the base is written back at the end of the second cycle
of the instruction. During a STM, the first register is written out at the start of the
second cycle. A STM which includes storing the base, with the base as the first register
to be stored, will therefore store the unchanged value, whereas with the base second
or later in the transfer order , will store the modified value. A LDM will always overwrite
the updated base if the base is in the list.
4.11.7 Data aborts
Some legal addresses may be unacceptable to a memory management system, and
the memory manager can indicate a problem with an address by taking the ABORT
signal HIGH. This can happen on any transfer during a multiple register load or store,
and must be recoverable if ARM7TDMI is to be used in a virtual memory system.
Aborts during STM instructions
If the abort occurs during a store multiple instruction, ARM7TDMI takes little action
until the instruction completes, whereupon it enters the data abort trap. The memory
manager is responsible for preventing erroneous writes to the memory. The only
change to the internal state of the processor will be the modification of the base
register if write-back was specified, and this must be reversed by software (and the
cause of the abort resolved) before the instruction may be retried.
Aborts during LDM instructions
When ARM7TDMI detects a data abort during a load multiple instruction, it modifies
the operation of the instruction to ensure that recovery is possible.
1Overwriting of registers stops when the abort happens. The aborting load will
not take place but earlier ones may have overwritten registers. The PC is
always the last register to be written and so will always be preserved.
2 The base register is restored, to its modified value if write-back was
requested. This ensures recoverability in the case where the base register is
also in the transfer list, and may have been overwritten before the abort
occurred.
The data abort trap is taken when the load multiple has completed, and the system
software must undo any base modification (and resolve the cause of the abort) before
restarting the instruction.
4.11.8 Instruction cycle times
Normal LDM instructions take nS + 1N + 1I and LDM PC takes (n+1)S + 2N + 1I
incremental cycles, where S,N and I are as defined in ➲
6.2 Cycle Types
on page 6-2.
STM instructions take (n-1)S + 2N incremental cycles to execute, where
n
is the
number of words transferred.
ARM Instruction Set - LDM, STM
ARM7TDMI Data Sheet
ARM DDI 0029E
4-45
Open Access
4.11.9 Assembler syntax
<LDM|STM>{cond}<FD|ED|FA|EA|IA|IB|DA|DB> Rn{!},<Rlist>{^}
where:
{cond} two character condition mnemonic. See ➲
Table 4-2: Condition code
summary
on page 4-5.
Rn is an expression evaluating to a valid register number
<Rlist> is a list of registers and register ranges enclosed in {} (e.g. {R0,R2-
R7,R10}).
{!} if present requests write-back (W=1), otherwise W=0
{^} if present set S bit to load the CPSR along with the PC, or force
transfer of user bank when in privileged mode
Addressing mode names
There are different assembler mnemonics for each of the addressing modes,
depending on whether the instruction is being used to support stacks or for other
purposes. The equivalence between the names and the values of the bits in the
instruction are shown in the following table:
FD, ED, F A, EA define pre/post indexing and the up/down bit by reference to the form
of stack required. The F and E refer to a “full” or “empty” stack, i.e. whether a pre-index
has to be done (full) before storing to the stack. The A and D refer to whether the stack
is ascending or descending. If ascending, a STM will go up and LDM down, if
descending, vice-versa.
IA, IB, DA, DB allow control when LDM/STM are not being used for stacks and simply
mean Increment After, Increment Before, Decrement After, Decrement Before.
Name Stack Other L bit P bit U bit
pre-increment load LDMED LDMIB 1 1 1
post-increment load LDMFD LDMIA 1 0 1
pre-decrement load LDMEA LDMDB 1 1 0
post-decrement load LDMFA LDMDA 1 0 0
pre-increment store STMFA STMIB 0 1 1
post-increment store STMEA STMIA 0 0 1
pre-decrement store STMFD STMDB 0 1 0
post-decrement store STMED STMDA 0 0 0
Table 4-6: Addressing mode names
ARM Instruction Set - LDM, STM
ARM7TDMI Data Sheet
ARM DDI 0029E
4-46
Open Access
4.11.10Examples
LDMFD SP!,{R0,R1,R2} ; Unstack 3 registers.
STMIA R0,{R0-R15} ; Save all registers.
LDMFD SP!,{R15} ; R15 <- (SP),CPSR unchanged.
LDMFD SP!,{R15}^ ; R15 <- (SP), CPSR <- SPSR_mode
; (allowed only in privileged modes).
STMFD R13,{R0-R14}^ ; Save user mode regs on stack
; (allowed only in privileged modes).
These instructions may be used to save state on subroutine entry, and restore it
efficiently on return to the calling routine:
STMED SP!,{R0-R3,R14} ; Save R0 to R3 to use as workspace
; and R14 for returning.
BL somewhere ; This nested call will overwrite R14
LDMED SP!,{R0-R3,R15} ; restore workspace and return.
ARM Instruction Set - SWP
ARM7TDMI Data Sheet
ARM DDI 0029E
4-47
Open Access
4.12 Single Data Swap (SWP)
Figure 4-23: Swap instruction
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-23: Swap instruction
.
The data swap instruction is used to swap a byte or word quantity between a register
and external memory. This instruction is implemented as a memory read followed by
a memory write which are “locked” together (the processor cannot be interrupted until
both operations have completed, and the memory manager is warned to treat them as
inseparable). This class of instruction is particularly useful for implementing software
semaphores.
The swap address is determined by the contents of the base register (Rn). The
processor first reads the contents of the swap address. Then it writes the contents of
the source register (Rm) to the swap address, and stores the old memory contents in
the destination register (Rd). The same register may be specified as both the source
and destination.
TheLOCK output goes HIGH for the duration of the read and write operations to signal
to the external memory manager that they are locked together , and should be allowed
to complete without interruption. This is important in multi-processor systems where
the swap instruction is the only indivisible instruction which may be used to implement
semaphores; control of the memory must not be removed from a processor while it is
performing a locked operation.
4.12.1 Bytes and words
This instruction class may be used to swap a byte (B=1) or a word (B=0) between an
ARM7TDMI register and memory. The SWP instruction is implemented as a LDR
followed by a STR and the action of these is as described in the section on single data
transfers. In particular , the description of Big and Little Endian configuration applies to
the SWP instruction.
0111215161920272831 23 78 4 3
Condition field
Cond Rn Rd 10010000 Rm00B00010
22 21
Destination register
Source register
Base register
Byte/Word bit
0 = swap word quantity
1 = swap byte quantity
ARM Instruction Set - SWP
ARM7TDMI Data Sheet
ARM DDI 0029E
4-48
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4.12.2 Use of R15
Do not use R15 as an operand (Rd, Rn or Rs) in a SWP instruction.
4.12.3 Data aborts
If the address used for the swap is unacceptable to a memory management system,
the memory manager can flag the problem by driving ABORT HIGH. This can happen
on either the read or the write cycle (or both), and in either case, the Data Abort trap
will be taken. It is up to the system software to resolve the cause of the problem, then
the instruction can be restarted and the original program continued.
4.12.4 Instruction cycle times
Swap instructions take 1S + 2N +1I incremental cycles to execute, where S,N and I
are as defined in ➲
6.2 Cycle Types
on page 6-2.
4.12.5 Assembler syntax
<SWP>{cond}{B} Rd,Rm,[Rn]
{cond} two-character condition mnemonic. See ➲
Table 4-2:
Condition code summary
on page 4-5.
{B} if B is present then byte transfer, otherwise word transfer
Rd,Rm,Rn are expressions evaluating to valid register numbers
4.12.6 Examples
SWP R0,R1,[R2] ; Load R0 with the word addressed by R2, and
; store R1 at R2.
SWPB R2,R3,[R4] ; Load R2 with the byte addressed by R4, and
; store bits 0 to 7 of R3 at R4.
SWPEQ R0,R0,[R1] ; Conditionally swap the contents of the
; word addressed by R1 with R0.
ARM Instruction Set - SWI
ARM7TDMI Data Sheet
ARM DDI 0029E
4-49
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4.13 Software Interrupt (SWI)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-24: Software interrupt instruction
, below.
Figure 4-24: Software interrupt instruction
The software interrupt instruction is used to enter Supervisor mode in a controlled
manner. The instruction causes the software interrupt trap to be taken, which effects
the mode change. The PC is then forced to a fixed value (0x08) and the CPSR is
saved in SPSR_svc. If the SWI vector address is suitably protected (by external
memory management hardware) from modification by the user, a fully protected
operating system may be constructed.
4.13.1 Return from the supervisor
The PC is saved in R14_svc upon entering the software interrupt trap, with the PC
adjusted to point to the word after the SWI instruction. MOVS PC,R14_svc will return
to the calling program and restore the CPSR.
Note that the link mechanism is not re-entrant, so if the supervisor code wishes to use
software interrupts within itself it must first save a copy of the return address and
SPSR.
4.13.2 Comment field
The bottom 24 bits of the instruction are ignored by the processor, and may be used
to communicate information to the supervisor code. For instance, the supervisor may
look at this field and use it to index into an array of entry points for routines which
perform the various supervisor functions.
4.13.3 Instruction cycle times
Software interrupt instructions take 2S + 1N incremental cycles to execute, where S
and N are as defined in ➲
6.2 Cycle Types
on page 6-2.
31 28 27 24 23 0
Condition field
1111Cond Comment field (ignored by Processor)
ARM Instruction Set - SWI
ARM7TDMI Data Sheet
ARM DDI 0029E
4-50
Open Access
4.13.4 Assembler syntax
SWI{cond} <expression>
{cond} two character condition mnemonic, ➲
Table 4-2: Condition
code summary
on page 4-5.
<expression> is evaluated and placed in the comment field (which is
ignored by ARM7TDMI).
4.13.5 Examples
SWI ReadC ; Get next character from read stream.
SWI WriteI+”k” ; Output a “k” to the write stream.
SWINE 0 ; Conditionally call supervisor
; with 0 in comment field.
Supervisor code
The previous examples assume that suitable supervisor code exists, for instance:
0x08 B Supervisor ; SWI entry point
EntryTable ; addresses of supervisor routines
DCD ZeroRtn
DCD ReadCRtn
DCD WriteIRtn
. . .
Zero EQU 0
ReadC EQU 256
WriteI EQU 512
Supervisor
; SWI has routine required in bits 8-23 and data (if any) in
; bits 0-7.
; Assumes R13_svc points to a suitable stack
STMFD R13,{R0-R2,R14} ; Save work registers and return
; address.
LDR R0,[R14,#-4] ; Get SWI instruction.
BIC R0,R0,#0xFF000000 ; Clear top 8 bits.
MOV R1,R0,LSR#8 ; Get routine offset.
ADR R2,EntryTable ; Get start address of entry table.
LDR R15,[R2,R1,LSL#2] ; Branch to appropriate routine.
WriteIRtn ; Enter with character in R0 bits 0-7.
. . . . . .
LDMFD R13,{R0-R2,R15}^ ; Restore workspace and return,
; restoring processor mode and flags.
ARM Instruction Set - CDP
ARM7TDMI Data Sheet
ARM DDI 0029E
4-51
Open Access
4.14 Coprocessor Data Operations (CDP)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-25: Coprocessor data operation instruction
.
This class of instruction is used to tell a coprocessor to perform some internal
operation. No result is communicated back to ARM7TDMI, and it will not wait for the
operation to complete. The coprocessor could contain a queue of such instructions
awaiting execution, and their execution can overlap other activity, allowing the
coprocessor and ARM7TDMI to perform independent tasks in parallel.
Figure 4-25: Coprocessor data operation instruction
4.14.1 The coprocessor fields
Only bit 4 and bits 24 to 31 are significant to ARM7TDMI. The remaining bits are used
by coprocessors. The above field names are used by convention, and particular
coprocessors may redefine the use of all fields except CP# as appropriate. The CP#
field is used to contain an identifying number (in the range 0 to 15) for each
coprocessor, and a coprocessor will ignore any instruction which does not contain its
number in the CP# field.
The conventional interpretation of the instruction is that the coprocessor should
perform an operation specified in the CP Opc field (and possibly in the CP field) on the
contents of CRn and CRm, and place the result in CRd.
4.14.2 Instruction cycle times
Coprocessor data operations take 1S + bI incremental cycles to execute, where
b
is
the number of cycles spent in the coprocessor busy-wait loop.
S and I are as defined in ➲
6.2 Cycle Types
on page 6-2.
Cond
011121516192024272831 23
CRd CP#
78
1110 CP Opc CRn CP 0 CRm
543
Coprocessor number
Condition field
Coprocessor information
Coprocessor operand register
Coprocessor destination register
Coprocessor operand register
Coprocessor operation code
ARM Instruction Set - CDP
ARM7TDMI Data Sheet
ARM DDI 0029E
4-52
Open Access
4.14.3 Assembler syntax
CDP{cond} p#,<expression1>,cd,cn,cm{,<expression2>}
{cond} two character condition mnemonic. See ➲
Table 4-2:
Condition code summary
on page 4-5.
p# the unique number of the required coprocessor
<expression1> evaluated to a constant and placed in the CP Opc field
cd, cn and cm evaluate to the valid coprocessor register numbers CRd, CRn
and CRm respectively
<expression2> where present is evaluated to a constant and placed in the
CP field
4.14.4 Examples
CDP p1,10,c1,c2,c3 ; Request coproc 1 to do operation 10
; on CR2 and CR3, and put the result
; in CR1.
CDPEQ p2,5,c1,c2,c3,2 ; If Z flag is set request coproc 2
; to do operation 5 (type 2) on CR2
; and CR3,and put the result in CR1.
ARM Instruction Set - LDC, STC
ARM7TDMI Data Sheet
ARM DDI 0029E
4-53
Open Access
4.15 Coprocessor DataTransfers (LDC, STC)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-26: Coprocessor data transfer instructions
.
This class of instruction is used to load (LDC) or store (STC) a subset of a
coprocessors’s registers directly to memory. ARM7TDMI is responsible for supplying
the memory address, and the coprocessor supplies or accepts the data and controls
the number of words transferred.
Figure 4-26: Coprocessor data transfer instructions
4.15.1 The coprocessor fields
The CP# field is used to identify the coprocessor which is required to supply or accept
the data, and a coprocessor will only respond if its number matches the contents of
this field.
The CRd field and the N bit contain information for the coprocessor which may be
interpreted in different ways by different coprocessors, but by convention CRd is the
register to be transferred (or the first register where more than one is to be
transferred), and the N bit is used to choose one of two transfer length options. For
instance N=0 could select the transfer of a single register, and N=1 could select the
transfer of all the registers for context switching.
Cond Rn
0111215161920212425272831
P U W L
2223
110 N CRd CP# Offset
78
Coprocessor number
Unsigned 8 bit immediate offset
Base register
Load/Store bit
0 = Store to memory
1 = Load from memory
Write-back bit
0 = no write-back
1 = write address into base
Coprocessor source/destination register
Pre/Post indexing bit
Up/Down bit
0 = down; subtract offset from base
1 = up; add offset to base
0 = post; add offset after transfer
Transfer length
Condition field
1 = pre; add offset before transfer
ARM Instruction Set - LDC, STC
ARM7TDMI Data Sheet
ARM DDI 0029E
4-54
Open Access
4.15.2 Addressing modes
ARM7TDMI is responsible for providing the address used by the memory system for
the transfer, and the addressing modes available are a subset of those used in single
data transfer instructions. Note, however, that the immediate offsets are 8 bits wide
and specify word offsets for coprocessor data transfers, whereas they are 12 bits wide
and specify byte offsets for single data transfers.
The 8 bit unsigned immediate offset is shifted left 2 bits and either added to (U=1) or
subtracted from (U=0) the base register (Rn); this calculation may be performed either
before (P=1) or after (P=0) the base is used as the transfer address. The modified
base value may be overwritten back into the base register (if W=1), or the old value of
the base may be preserved (W=0). Note that post-indexed addressing modes require
explicit setting of the W bit, unlike LDR and STR which always write-back when post-
indexed.
The value of the base register, modified by the offset in a pre-indexed instruction, is
used as the address for the transfer of the first word. The second word (if more than
one is transferred) will go to or come from an address one word (4 bytes) higher than
the first transfer, and the address will be incremented by one word for each
subsequent transfer.
4.15.3 Address alignment
The base address should normally be a word aligned quantity . The bottom 2 bits of the
address will appear on A[1:0] and might be interpreted by the memory system.
4.15.4 Use of R15
If Rn is R15, the value used will be the address of the instruction plus 8 bytes. Base
write-back to R15 must not be specified.
4.15.5 Data aborts
If the address is legal but the memory manager generates an abort, the data trap will
be taken. The write-back of the modified base will take place, but all other processor
state will be preserved. The coprocessor is partly responsible for ensuring that the
data transfer can be restarted after the cause of theabort has been resolved, and must
ensure that any subsequent actions it undertakes can be repeated when the
instruction is retried.
4.15.6 Instruction cycle times
Coprocessor data transfer instructions take (n-1)S + 2N + bI incremental cycles to
execute, where:
n is the number of words transferred.
b is the number of cycles spent in the coprocessor busy-wait loop.
S, N and I are as defined in ➲
6.2 Cycle Types
on page 6-2.
ARM Instruction Set - LDC, STC
ARM7TDMI Data Sheet
ARM DDI 0029E
4-55
Open Access
4.15.7 Assembler syntax
<LDC|STC>{cond}{L} p#,cd,<Address>
LDC load from memory to coprocessor
STC store from coprocessor to memory
{L} when present perform long transfer (N=1), otherwise perform short
transfer (N=0)
{cond} two character condition mnemonic. See ➲
Table 4-2: Condition code
summary
on page 4-5.
p# the unique number of the required coprocessor
cd is an expression evaluating to a valid coprocessor register number
that is placed in the CRd field
<Address> can be:
1 An expression which generates an address:
<expression>
The assembler will attempt to generate an instruction using
the PC as a base and a corrected immediate offset to address
the location given by evaluating the expression. This will be a
PC relative, pre-indexed address. If the address is out of
range, an error will be generated.
2 A pre-indexed addressing specification:
[Rn] offset of zero
[Rn,<#expression>]{!} offset of <expression> bytes
3 A post-indexed addressing specification:
[Rn],<#expression> offset of <expression> bytes
{!} write back the base register
(set the W bit) if! is present
Rn is an expression evaluating
to a valid ARM7TDMI
register number.
Note
If Rn is R15, the assembler will subtract 8 from the offset value to allow for ARM7TDMI
pipelining.
ARM Instruction Set - LDC, STC
ARM7TDMI Data Sheet
ARM DDI 0029E
4-56
Open Access
4.15.8 Examples
LDC p1,c2,table ; Load c2 of coproc 1 from address
; table, using a PC relative address.
STCEQL p2,c3,[R5,#24]!; Conditionally store c3 of coproc 2
; into an address 24 bytes up from R5,
; write this address back to R5, and use
; long transfer option (probably to
; store multiple words).
Note
Although the address offset is expressed in bytes, the instruction offset field is in
words. The assembler will adjust the offset appropriately.
ARM Instruction Set - MRC, MCR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-57
Open Access
4.16 Coprocessor Register Transfers (MRC, MCR)
The instruction is only executed if the condition is true. The various conditions are
defined in➲
Table 4-2: Condition code summary
on page 4-5. The instruction encoding
is shown in ➲
Figure 4-27: Coprocessor register transfer instructions
.
This class of instruction is used to communicate information directly between
ARM7TDMI and a coprocessor. An example of a coprocessor to ARM7TDMI register
transfer (MRC) instruction would be a FIX of a floating point value held in a
coprocessor, where the floating point number is converted into a 32 bit integer within
the coprocessor , and the result is then transferred to ARM7TDMI register . A FLOA T of
a 32 bit value in ARM7TDMI register into a floating point value within the coprocessor
illustrates the use of ARM7TDMI register to coprocessor transfer (MCR).
An important use of this instruction is to communicate control information directly from
the coprocessor into the ARM7TDMI CPSR flags. As an example, the result of a
comparison of two floating point values within a coprocessor can be moved to the
CPSR to control the subsequent flow of execution.
Figure 4-27: Coprocessor register transfer instructions
4.16.1 The coprocessor fields
The CP# field is used, as for all coprocessor instructions, to specify which coprocessor
is being called upon.
The CP Opc, CRn, CP and CRm fields are used only by the coprocessor, and the
interpretation presented here is derived from convention only. Other interpretations
are allowed where the coprocessor functionality is incompatible with this one. The
conventional interpretation is that the CP Opc and CP fields specify the operation the
coprocessor is required to perform, CRn is the coprocessor register which is the
21
Cond
011121516192024272831 23
CP#
78
1110 CRn CP CRm
543
1LCP Opc Rd
Coprocessor number
Coprocessor information
Coprocessor operand register
Coprocessor operation mode
Condition field
Load/Store bit
0 = Store to Co-Processor
1 = Load from Co-Processor
ARM source/destination register
Coprocessor source/destination register
ARM Instruction Set - MRC, MCR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-58
Open Access
source or destination of the transferred information, and CRm is a second coprocessor
register which may be involved in some way which depends on the particular operation
specified.
4.16.2 Transfers to R15
When a coprocessor register transfer to ARM7TDMI has R15 as the destination, bits
31, 30, 29 and 28 of the transferred word are copied into the N, Z, C and V flags
respectively. The other bits of the transferred word are ignored, and the PC and other
CPSR bits are unaffected by the transfer.
4.16.3 Transfers from R15
A coprocessor register transfer from ARM7TDMI with R15 as the source register will
store the PC+12.
4.16.4 Instruction cycle times
MRC instructions take 1S + (b+1)I +1C incremental cycles to execute, where S, I and
C are as defined in ➲
6.2 Cycle Types
on page 6-2.
MCR instructions take 1S + bI +1C incremental cycles to execute, where
b
is the
number of cycles spent in the coprocessor busy-wait loop.
4.16.5 Assembler syntax
<MCR|MRC>{cond} p#,<expression1>,Rd,cn,cm{,<expression2>}
MRC move from coprocessor to ARM7TDMI register (L=1)
MCR move from ARM7TDMI register to coprocessor (L=0)
{cond} two character condition mnemonic. See ➲
Table 4-2:
Condition code summary
on page 4-5.
p# the unique number of the required coprocessor
<expression1> evaluated to a constant and placed in the CP Opc field
Rd is an expression evaluating to a valid ARM7TDMI register
number
cn and cm are expressions evaluating to the valid coprocessor register
numbers CRn and CRm respectively
<expression2> where present is evaluated to a constant and placed in the
CP field
ARM Instruction Set - MRC, MCR
ARM7TDMI Data Sheet
ARM DDI 0029E
4-59
Open Access
4.16.6 Examples
MRC p2,5,R3,c5,c6 ; Request coproc 2 to perform operation 5
; on c5 and c6, and transfer the (single
; 32 bit word) result back to R3.
MCR p6,0,R4,c5,c6 ; Request coproc 6 to perform operation 0
; on R4 and place the result in c6.
MRCEQ p3,9,R3,c5,c6,2 ; Conditionally request coproc 3 to
; perform operation 9 (type 2) on c5 and
; c6, and transfer the result back to R3.
ARM Instruction Set - Undefined
ARM7TDMI Data Sheet
ARM DDI 0029E
4-60
Open Access
4.17 Undefined Instruction
The instruction is only executed if the condition is true. The various conditions are
defined in ➲
Table 4-2: Condition code summary
on page 4-5. The instruction format
is shown in ➲
Figure 4-28: Undefined instruction
.
Figure 4-28: Undefined instruction
If the condition is true, the undefined instruction trap will be taken.
Note that the undefined instruction mechanism involves offering this instruction to any
coprocessors which may be present, and all coprocessors must refuse to accept it by
driving CPA and CPB HIGH.
4.17.1 Instruction cycle times
This instruction takes 2S + 1I + 1N cycles, where S, N and I are as defined in ➲
6.2
Cycle Types
on page 6-2.
4.17.2 Assembler syntax
The assembler has no mnemonics for generating this instruction. If it is adopted in the
future for some specified use, suitable mnemonics will be added to the assembler.
Until such time, this instruction must not be used.
Cond
024272831 5 4 3
1011 xxxx
25
xxxxxxxxxxxxxxxxxxxx
ARM Instruction Set - Examples
ARM7TDMI Data Sheet
ARM DDI 0029E
4-61
Open Access
4.18 Instruction Set Examples
The following examples show ways in which the basic ARM7TDMI instructions can
combine to give efficient code. None of these methods saves a great deal of execution
time (although they may save some), mostly they just save code.
4.18.1 Using the conditional instructions
Using conditionals for logical OR
CMP Rn,#p ; If Rn=p OR Rm=q THEN GOTO Label.
BEQ Label
CMP Rm,#q
BEQ Label
This can be replaced by
CMP Rn,#p
CMPNE Rm,#q ; If condition not satisfied try
; other test.
BEQ Label
Absolute value
TEQ Rn,#0 ; Test sign
RSBMI Rn,Rn,#0 ; and 2's complement if necessary.
Multiplication by 4, 5 or 6 (run time)
MOV Rc,Ra,LSL#2 ; Multiply by 4,
CMP Rb,#5 ; test value,
ADDCS Rc,Rc,Ra ; complete multiply by 5,
ADDHI Rc,Rc,Ra ; complete multiply by 6.
Combining discrete and range tests
TEQ Rc,#127 ; Discrete test,
CMPNE Rc,#” ”-1 ; range test
MOVLS Rc,#”.” ; IF Rc<=” ” OR Rc=ASCII(127)
; THEN Rc:=”.”
Division and remainder
A number of divide routines for specific applications are provided in source form as
part of the ANSI C library provided with the ARM Cross Development Toolkit, available
from your supplier. A short general purpose divide routine follows.
; Enter with numbers in Ra and Rb.
;
MOV Rcnt,#1 ; Bit to control the division.
Div1 CMP Rb,#0x80000000 ; Move Rb until greater than Ra.
CMPCC Rb,Ra
MOVCC Rb,Rb,ASL#1
MOVCC Rcnt,Rcnt,ASL#1
BCC Div1
MOV Rc,#0
ARM Instruction Set - Examples
ARM7TDMI Data Sheet
ARM DDI 0029E
4-62
Open Access
Div2 CMP Ra,Rb ; Test for possible subtraction.
SUBCS Ra,Ra,Rb ; Subtract if ok,
ADDCS Rc,Rc,Rcnt ; put relevant bit into result
MOVS Rcnt,Rcnt,LSR#1 ; shift control bit
MOVNE Rb,Rb,LSR#1 ; halve unless finished.
BNE Div2 ;
; Divide result in Rc,
; remainder in Ra.
Overflow detection in the ARM7TDMI
1 Overflow in unsigned multiply with a 32 bit result
UMULL Rd,Rt,Rm,Rn ;3 to 6 cycles
TEQ Rt,#0 ;+1 cycle and a register
BNE overflow
2 Overflow in signed multiply with a 32 bit result
SMULL Rd,Rt,Rm,Rn ;3 to 6 cycles
TEQ Rt,Rd ASR#31 ;+1 cycle and a register
BNE overflow
3 Overflow in unsigned multiply accumulate with a 32 bit result
UMLAL Rd,Rt,Rm,Rn ;4 to 7 cycles
TEQ Rt,#0 ;+1 cycle and a register
BNE overflow
4 Overflow in signed multiply accumulate with a 32 bit result
SMLAL Rd,Rt,Rm,Rn ;4 to 7 cycles
TEQ Rt,Rd, ASR#31 ;+1 cycle and a register
BNE overflow
5 Overflow in unsigned multiply accumulate with a 64 bit result
UMULL Rl,Rh,Rm,Rn ;3 to 6 cycles
ADDS Rl,Rl,Ra1 ;lower accumulate
ADC Rh,Rh,Ra2 ;upper accumulate
BCS overflow ;1 cycle and 2 registers
6 Overflow in signed multiply accumulate with a 64 bit result
SMULL Rl,Rh,Rm,Rn ;3 to 6 cycles
ADDS Rl,Rl,Ra1 ;lower accumulate
ADC Rh,Rh,Ra2 ;upper accumulate
BVS overflow ;1 cycle and 2 registers
Note Overflow checking is not applicable to unsigned and signed multiplies with a 64-bit
result, since overflow does not occur in such calculations.
ARM Instruction Set - Examples
ARM7TDMI Data Sheet
ARM DDI 0029E
4-63
Open Access
4.18.2 Pseudo-random binary sequence generator
It is often necessary to generate (pseudo-) random numbers and the most efficient
algorithms are based on shift generators with exclusive-OR feedback rather like a
cyclic redundancy check generator. Unfortunately the sequence of a 32 bit generator
needs more than one feedback tap to be maximal length (i.e. 2^32-1 cycles before
repetition), so this example uses a 33 bit register with taps at bits 33 and 20. The basic
algorithm is newbit:=bit 33 eor bit 20, shift left the 33 bit number and put in newbit at
the bottom; this operation is performed for all the newbits needed (i.e. 32 bits). The
entire operation can be done in 5 S cycles:
; Enter with seed in Ra (32 bits),
Rb (1 bit in Rb lsb), uses Rc.
;
TST Rb,Rb,LSR#1 ; Top bit into carry
MOVS Rc,Ra,RRX ; 33 bit rotate right
ADC Rb,Rb,Rb ; carry into lsb of Rb
EOR Rc,Rc,Ra,LSL#12 ; (involved!)
EOR Ra,Rc,Rc,LSR#20 ; (similarly involved!)
; new seed in Ra, Rb as before
4.18.3 Multiplication by constant using the barrel shifter
Multiplication by 2^n (1,2,4,8,16,32..)
MOV Ra, Rb, LSL #n
Multiplication by 2^n+1 (3,5,9,17..)
ADDRa,Ra,Ra,LSL #n
Multiplication by 2^n-1 (3,7,15..)
RSB Ra,Ra,Ra,LSL #n
Multiplication by 6
ADD Ra,Ra,Ra,LSL #1; multiply by 3
MOV Ra,Ra,LSL#1; and then by 2
Multiply by 10 and add in extra number
ADD Ra,Ra,Ra,LSL#2; multiply by 5
ADD Ra,Rc,Ra,LSL#1; multiply by 2 and add in next digit
General recursive method for Rb := Ra*C, C a constant:
1 If C even, say C = 2^n*D, D odd:
D=1: MOV Rb,Ra,LSL #n
D<>1: {Rb := Ra*D}
MOV Rb,Rb,LSL #n
2 If C MOD 4 = 1, say C = 2^n*D+1, D odd, n>1:
D=1: ADD Rb,Ra,Ra,LSL #n
ARM Instruction Set - Examples
ARM7TDMI Data Sheet
ARM DDI 0029E
4-64
Open Access
D<>1: {Rb := Ra*D}
ADD Rb,Ra,Rb,LSL #n
3 If C MOD 4 = 3, say C = 2^n*D-1, D odd, n>1:
D=1: RSB Rb,Ra,Ra,LSL #n
D<>1: {Rb := Ra*D}
RSB Rb,Ra,Rb,LSL #n
This is not quite optimal, but close. An example of its non-optimality is multiply
by 45 which is done by:
RSB Rb,Ra,Ra,LSL#2 ; multiply by 3
RSB Rb,Ra,Rb,LSL#2 ; multiply by 4*3-1 = 11
ADD Rb,Ra,Rb,LSL# 2; multiply by 4*11+1 = 45
rather than by:
ADD Rb,Ra,Ra,LSL#3 ; multiply by 9
ADD Rb,Rb,Rb,LSL#2 ; multiply by 5*9 = 45
4.18.4 Loading a word from an unknown alignment
; enter with address in Ra (32 bits)
; uses Rb, Rc; result in Rd.
; Note d must be less than c e.g. 0,1
;
BIC Rb,Ra,#3 ; get word aligned address
LDMIA Rb,{Rd,Rc} ; get 64 bits containing answer
AND Rb,Ra,#3 ; correction factor in bytes
MOVS Rb,Rb,LSL#3 ; ...now in bits and test if aligned
MOVNE Rd,Rd,LSR Rb ; produce bottom of result word
; (if not aligned)
RSBNE Rb,Rb,#32 ; get other shift amount
ORRNE Rd,Rd,Rc,LSL Rb; combine two halves to get result
ARM7TDMI Data Sheet
ARM DDI 0029E
5-1
11
1
Open Access
THUMB Instruction Set
This chapter describes the THUMB instruction set.
Format Summary 5-2
Opcode Summary 5-3
5.1 Format 1: move shifted register 5-5
5.2 Format 2: add/subtract 5-7
5.3 Format 3: move/compare/add/subtract immediate 5-9
5.4 Format 4: ALU operations 5-11
5.5 Format 5: Hi register operations/branch exchange 5-13
5.6 Format 6: PC-relative load 5-16
5.7 Format 7: load/store with register offset 5-18
5.8 Format 8: load/store sign-extended byte/halfword 5-20
5.9 Format 9: load/store with immediate offset 5-22
5.10 Format 10: load/store halfword 5-24
5.11 Format 11: SP-relative load/store 5-26
5.12 Format 12: load address 5-28
5.13 Format 13: add offset to Stack Pointer 5-30
5.14 Format 14: push/pop registers 5-32
5.15 Format 15: multiple load/store 5-34
5.16 Format 16: conditional branch 5-36
5.17 Format 17: software interrupt 5-38
5.18 Format 18: unconditional branch 5-39
5.19 Format 19: long branch with link 5-40
5.20 Instruction Set Examples 5-42
5
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-2
Open Access
Format Summary
The THUMB instruction set formats are shown in the following figure.
Figure 5-1: THUMB instruction set formats
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
1
0 0 0 Op Offset5 Rs Rd
Move shifted register
2
0 0 0 1 1 I Op Rn/offset3 Rs Rd
Add/subtract
3
0 0 1 Op Rd Offset8
Move/compare/add
/subtract immediate
4
0 1 0 0 0 0 Op Rs Rd
ALU operations
5
0 1 0 0 0 1 Op H1 H2 Rs/Hs Rd/Hd
Hi register operations
/branch exchange
6
0 1 0 0 1 Rd Word8
PC-relative load
7
0 1 0 1 L B 0 Ro Rb Rd
Load/store with register
offset
8
0 1 0 1 H S 1 Ro Rb Rd
Load/store sign-extended
byte/halfword
9
0 1 1 B L Offset5 Rb Rd
Load/store with immediate
offset
10
1 0 0 0 L Offset5 Rb Rd
Load/store halfword
11
1 0 0 1 L Rd Word8
SP-relative load/store
12
1 0 1 0 SP Rd Word8
Load address
13
1 0 1 1 0 0 0 0 S SWord7
Add offset to stack pointer
14
1 0 1 1 L 1 0 R Rlist
Push/pop registers
15
1 1 0 0 L Rb Rlist
Multiple load/store
16
1 1 0 1 Cond Soffset8
Conditional branch
17
1 1 0 1 1 1 1 1 Value8
Software Interrupt
18
1 1 1 0 0 Offset11
Unconditional branch
19
1 1 1 1 H Offset
Long branch with link
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-3
Open Access
Opcode Summary
The following table summarizes the THUMB instruction set. For further
information about a particular instruction please refer to the sections listed in the
right-most column.
Mnemonic Instruction Lo register
operand Hi register
operand Condition
codes set See Section:
ADC Add with Carry ✔ ✔ 5.4
ADD Add ✔ ✔ ✔➀ 5.1.3, 5.5, 5.12, 5.13
AND AND ✔ ✔ 5.4
ASR Arithmetic Shift Right ✔ ✔ 5.1, 5.4
B Unconditional branch ✔5.16
B
xx
Conditional branch ✔5.17
BIC Bit Clear ✔ ✔ 5.4
BL Branch and Link 5.19
BX Branch and Exchange ✔ ✔ 5.5
CMN Compare Negative ✔ ✔ 5.4
CMP Compare ✔ ✔ ✔ 5.3, 5.4, 5.5
EOR EOR ✔ ✔ 5.4
LDMIA Load multiple ✔5.15
LDR Load word ✔5.7, 5.6, 5.9, 5.11
LDRB Load byte ✔5.7, 5.9
LDRH Load halfword ✔5.8, 5.10
LSL Logical Shift Left ✔ ✔ 5.1, 5.4
LDSB Load sign-extended
byte ✔5.8
LDSH Load sign-extended
halfword ✔5.8
LSR Logical Shift Right ✔ ✔ 5.1, 5.4
MOV Move register ✔ ✔ ✔➁ 5.3, 5.5
MUL Multiply ✔ ✔ 5.4
MVN Move Negative register ✔ ✔ 5.4
Table 5-1: THUMB instruction set opcodes
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-4
Open Access
➀The condition codes are unaffected by the format 5, 12 and 13
versions of this instruction.
➁The condition codes are unaffected by the format 5 version of this
instruction.
NEG Negate ✔ ✔ 5.4
ORR OR ✔ ✔ 5.4
POP Pop registers ✔5.14
PUSH Push registers ✔5.14
ROR Rotate Right ✔ ✔ 5.4
SBC Subtract with Carry ✔ ✔ 5.4
STMIA Store Multiple ✔5.15
STR Store word ✔5.7, 5.9, 5.11
STRB Store byte ✔5.7
STRH Store halfword ✔5.8, 5.10
SWI Software Interrupt 5.17
SUB Subtract ✔ ✔ 5.1.3, 5.3
TST Test bits ✔ ✔ 5.4
Mnemonic Instruction Lo register
operand Hi register
operand Condition
codes set See Section:
Table 5-1: THUMB instruction set opcodes (Continued)
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-5
Open Access
5.1 Format 1: move shifted register
Figure 5-2: Format 1
5.1.1 Operation
These instructions move a shifted value between Lo registers. The THUMB assembler
syntax is shown in ➲
Table 5-2: Summary of format 1 instructions
.
Note All instructions in this group set the CPSR condition codes.
OP THUMB assembler ARM equivalent Action
00 LSL Rd, Rs, #Offset5 MOVS Rd, Rs, LSL #Offset5 Shift Rs left by a 5-bit immediate value
and store the result in Rd.
01 LSR Rd, Rs, #Offset5 MOVS Rd, Rs, LSR #Offset5 Perform logical shift right on Rs by a 5-
bit immediate value and store the result
in Rd.
10 ASR Rd, Rs, #Offset5 MOVS Rd, Rs, ASR #Offset5 Perform arithmetic shift right on Rs by a
5-bit immediate value and store the
result in Rd.
Table 5-2: Summary of format 1 instructions
0123456789101112131415
Offset5 Rs000
Destination register
Source register
Immediate value
Opcode
Op Rd
0 - LSL
1 - LSR
2 - ASR
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-6
Open Access
5.1.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-2: Summary of format 1 instructions
on page 5-5. The instruction cycle times for the
THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.1.3 Examples
LSR R2, R5, #27 ; Logical shift right the contents
; of R5 by 27 and store the result in R2.
; Set condition codes on the result.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-7
Open Access
5.2 Format 2: add/subtract
Figure 5-3: Format 2
5.2.1 Operation
These instructions allow the contents of a Lo register or a 3-bit immediate value to be
added to or subtracted from a Lo register. The THUMB assembler syntax is shown in
➲
Table 5-3: Summary of format 2 instructions
.
Note All instructions in this group set the CPSR condition codes.
Op I THUMB assembler ARM equivalent Action
0 0 ADD Rd, Rs, Rn ADDS Rd, Rs, Rn Add contents of Rn to contents of Rs. Place
result in Rd.
0 1 ADD Rd, Rs, #Offset3 ADDS Rd, Rs, #Offset3 Add 3-bit immediate value to contents of
Rs. Place result in Rd.
1 0 SUB Rd, Rs, Rn SUBS Rd, Rs, Rn Subtract contents of Rn from contents of
Rs. Place result in Rd.
1 1 SUB Rd, Rs, #Offset3 SUBS Rd, Rs, #Offset3 Subtract 3-bit immediate value from
contents of Rs. Place result in Rd.
Table 5-3: Summary of format 2 instructions
0123456789101112131415
Rn/Offset3 Rs1000
Destination register
Opcode
Source register
0 - ADD
Register/
1 - SUB
Immediate value
Immediate flag
0 - Register operand
1 - Immediate operand
1 I Op Rd
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-8
Open Access
5.2.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-3: Summary of format 2 instructions
on page 5-7. The instruction cycle times for the
THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.2.3 Examples
ADD R0, R3, R4 ; R0 := R3 + R4 and set condition codes on
; the result.
SUB R6, R2, #6 ; R6 := R2 - 6 and set condition codes.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-9
Open Access
5.3 Format 3: move/compare/add/subtract immediate
Figure 5-4: Format 3
5.3.1 Operations
The instructions in this group perform operations between a Lo register and an 8-bit
immediate value.
The THUMB assembler syntax is shown in ➲
Table 5-4: Summary of format 3
instructions
.
Note All instructions in this group set the CPSR condition codes.
Op THUMB assembler ARM equivalent Action
00 MOV Rd, #Offset8 MOVS Rd, #Offset8 Move 8-bit immediate value into Rd.
01 CMP Rd, #Offset8 CMP Rd, #Offset8 Compare contents of Rd with 8-bit
immediate value.
10 ADD Rd, #Offset8 ADDS Rd, Rd, #Offset8 Add 8-bit immediate value to contents of Rd
and place the result in Rd.
11 SUB Rd, #Offset8 SUBS Rd, Rd, #Offset8 Subtract 8-bit immediate value from
contents of Rd and place the result in Rd.
Table 5-4: Summary of format 3 instructions
0123456789101112131415
RdOp100 Offset8
Source/destination register
Immediate value
Opcode
0 - MOV
1 - CMP
2 - ADD
3 SUB
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-10
Open Access
5.3.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-4: Summary of format 3 instructions
on page 5-9. The instruction cycle times for the
THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.3.3 Examples
MOV R0, #128 ; R0 := 128 and set condition codes
CMP R2, #62 ; Set condition codes on R2 - 62
ADD R1, #255 ; R1 := R1 + 255 and set condition
; codes
SUB R6, #145 ; R6 := R6 - 145 and set condition
; codes
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-11
Open Access
5.4 Format 4: ALU operations
Figure 5-5: Format 4
5.4.1 Operation
The following instructions perform ALU operations on a Lo register pair.
Note All instructions in this group set the CPSR condition codes.
OP THUMB assembler ARM equivalent Action
0000 AND Rd, Rs ANDS Rd, Rd, Rs Rd:= Rd AND Rs
0001 EOR Rd, Rs EORS Rd, Rd, Rs Rd:= Rd EOR Rs
0010 LSL Rd, Rs MOVS Rd, Rd, LSL Rs Rd := Rd << Rs
0011 LSR Rd, Rs MOVS Rd, Rd, LSR Rs Rd := Rd >> Rs
0100 ASR Rd, Rs MOVS Rd, Rd, ASR Rs Rd := Rd ASR Rs
0101 ADC Rd, Rs ADCS Rd, Rd, Rs Rd := Rd + Rs + C-bit
0110 SBC Rd, Rs SBCS Rd, Rd, Rs Rd := Rd - Rs - NOT C-bit
0111 ROR Rd, Rs MOVS Rd, Rd, ROR Rs Rd := Rd ROR Rs
1000 TST Rd, Rs TST Rd, Rs Set condition codes on Rd AND Rs
1001 NEG Rd, Rs RSBS Rd, Rs, #0 Rd = -Rs
Table 5-5: Summary of Format 4 instructions
0123456789101112131415
Op Rs010
Source/destination
Source register 2
Opcode
Rd
register
000
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-12
Open Access
5.4.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-5: Summary of Format 4 instructions
on page 5-11. The instruction cycle times for
the THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.4.3 Examples
EOR R3, R4 ; R3 := R3 EOR R4 and set condition codes
ROR R1, R0 ; Rotate Right R1 by the value in R0, store
; the result in R1 and set condition codes
NEG R5, R3 ; Subtract the contents of R3 from zero,
; store the result in R5. Set condition codes
; ie R5 = -R3
CMP R2, R6 ; Set the condition codes on the result of
; R2 - R6
MUL R0, R7 ; R0 := R7 * R0 and set condition codes
1010 CMP Rd, Rs CMP Rd, Rs Set condition codes on Rd - Rs
1011 CMN Rd, Rs CMN Rd, Rs Set condition codes on Rd + Rs
1100 ORR Rd, Rs ORRS Rd, Rd, Rs Rd := Rd OR Rs
1101 MUL Rd, Rs MULS Rd, Rs, Rd Rd := Rs * Rd
1110 BIC Rd, Rs BICS Rd, Rd, Rs Rd := Rd AND NOT Rs
1111 MVN Rd, Rs MVNS Rd, Rs Rd := NOT Rs
OP THUMB assembler ARM equivalent Action
Table 5-5: Summary of Format 4 instructions (Continued)
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-13
Open Access
5.5 Format 5: Hi register operations/branch exchange
Figure 5-6: Format 5
5.5.1 Operation
There are four sets of instructions in this group. The first three allow ADD, CMP and
MOV operations to be performed between Lo and Hi registers, or a pair of Hi registers.
The fourth, BX, allows a Branch to be performed which may also be used to switch
processor state.
The THUMB assembler syntax is shown in ➲
Table 5-6: Summary of format 5
instructions
Note In this group only CMP (Op = 01) sets the CPSR condition codes.
The action of H1= 0, H2 = 0 for Op = 00 (ADD), Op =01 (CMP) and Op = 10 (MOV) is
undefined, and should not be used.
Op H1 H2 THUMB assembler ARM equivalent Action
00 0 1 ADD Rd, Hs ADD Rd, Rd, Hs Add a register in the range 8-15 to a
register in the range 0-7.
00 1 0 ADD Hd, Rs ADD Hd, Hd, Rs Add a register in the range 0-7 to a
register in the range 8-15.
00 1 1 ADD Hd, Hs ADD Hd, Hd, Hs Add two registers in the range 8-15
Table 5-6: Summary of format 5 instructions
0123456789101112131415
Op010 Rs/Hs
Destination register
Source register
0 0 H1
Opcode
1 H2
Hi operand flag 2
Hi operand flag 1
Rd/Hd
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-14
Open Access
5.5.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-6: Summary of format 5 instructions
on page 5-13. The instruction cycle times for the
THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.5.3 The BX instruction
BX performs a Branch to a routine whose start address is specified in a Lo or Hi
register.
Bit 0 of the address determines the processor state on entry to the routine:
Bit 0 = 0 causes the processor to enter ARM state.
Bit 0 = 1 causes the processor to enter THUMB state.
Note The action of H1 = 1 for this instruction is undefined, and should not be used.
01 0 1 CMP Rd, Hs CMP Rd, Hs Compare a register in the range 0-7
with a register in the range 8-15. Set
the condition code flags on the result.
01 1 0 CMP Hd, Rs CMP Hd, Rs Compare a register in the range 8-15
with a register in the range 0-7. Set the
condition code flags on the result.
01 1 1 CMP Hd, Hs CMP Hd, Hs Compare two registers in the range 8-
15. Set the condition code flags on the
result.
10 0 1 MOV Rd, Hs MOV Rd, Hs Move a value from a register in the
range 8-15 to a register in the range 0-
7.
10 1 0 MOV Hd, Rs MOV Hd, Rs Move a value from a register in the
range 0-7 to a register in the range 8-
15.
10 1 1 MOV Hd, Hs MOV Hd, Hs Move a value between two registers in
the range 8-15.
11 0 0 BX Rs BX Rs Perform branch (plus optional state
change) to address in a register in the
range 0-7.
11 0 1 BX Hs BX Hs Perform branch (plus optional state
change) to address in a register in the
range 8-15.
Op H1 H2 THUMB assembler ARM equivalent Action
Table 5-6: Summary of format 5 instructions (Continued)
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-15
Open Access
5.5.4 Examples
Hi register operations
ADD PC, R5 ; PC := PC + R5 but don't set the
; condition codes.
CMP R4, R12 ; Set the condition codes on the
; result of R4 - R12.
MOV R15, R14 ; Move R14 (LR) into R15 (PC)
; but don't set the condition codes,
; eg. return from subroutine.
Branch and exchange
; Switch from THUMB to ARM state.
ADR R1,outofTHUMB
; Load address of outofTHUMB
; into R1.
MOV R11,R1
BX R11 ; Transfer the contents of R11 into
; the PC.
; Bit 0 of R11 determines whether
; ARM or THUMB state is entered, ie.
; ARM state here.
...
ALIGN
CODE32
outofTHUMB ; Now processing ARM instructions...
5.5.5 Using R15 as an operand
If R15 is used as an operand, the value will be the address of the instruction + 4 with
bit 0 cleared. Executing a BX PC in THUMB state from a non-word aligned address
will result in unpredictable execution.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-16
Open Access
5.6 Format 6: PC-relative load
Figure 5-7: Format 6
5.6.1 Operation
This instruction loads a word from an address specified as a 10-bit immediate offset
from the PC.
The THUMB assembler syntax is shown below.
Note The value specified by #Imm is a full 10-bit address, but must always be word-aligned
(ie with bits 1:0 set to 0), since the assembler places #Imm >> 2 in field Word8.
Note The value of the PC will be 4 bytes greater than the address of this instruction, but bit
1 of the PC is forced to 0 to ensure it is word aligned.
THUMB assembler ARM equivalent Action
LDR Rd, [PC, #Imm] LDR Rd, [R15, #Imm] Add unsigned offset (255 words,
1020 bytes) in Imm to the current
value of the PC. Load the word
from the resulting address into Rd.
Table 5-7: Summary of PC-relative load instruction
0123456789101112131415
Rd010 Word8
Destination register
Immediate value
0 1
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-17
Open Access
5.6.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-7: Summary of PC-relative load instruction
on page 5-16. The instruction cycle times
for the THUMB instruction are identical to that of the equivalent ARM instruction. For
more information on instruction cycle times, please refer to ➲
Chapter 10, Instruction
Cycle Operations
.
5.6.3 Examples
LDR R3,[PC,#844] ; Load into R3 the word found at the
; address formed by adding 844 to PC.
; bit[1] of PC is forced to zero.
; Note that the THUMB opcode will contain
; 211 as the Word8 value.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-18
Open Access
5.7 Format 7: load/store with register offset
Figure 5-8: Format 7
5.7.1 Operation
These instructions transfer byte or word values between registers and memory.
Memory addresses are pre-indexed using an offset register in the range 0-7.
The THUMB assembler syntax is shown in ➲
Table 5-8: Summary of format 7
instructions
.
L B THUMB assembler ARM equivalent Action
0 0 STR Rd, [Rb, Ro] STR Rd, [Rb, Ro] Pre-indexed word store:
Calculate the target address by
adding together the value in Rb
and the value in Ro. Store the
contents of Rd at the address.
Table 5-8: Summary of format 7 instructions
0123456789101112131415
Ro RbL010
Source/destination
Base register
Offset register
1 B 0 Rd
Byte/Word flag
Load/Store flag
0 - Transfer word quantity
1 - Transfer byte quantity
0 - Store to memory
1 - Load from memory
register
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-19
Open Access
5.7.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-8: Summary of format 7 instructions
on page 5-18. The instruction cycle times for the
THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.7.3 Examples
STR R3, [R2,R6] ; Store word in R3 at the address
; formed by adding R6 to R2.
LDRB R2, [R0,R7] ; Load into R2 the byte found at
; the address formed by adding
; R7 to R0.
0 1 STRB Rd, [Rb, Ro] STRB Rd, [Rb, Ro] Pre-indexed byte store:
Calculate the target address by
adding together the value in Rb
and the value in Ro. Store the byte
value in Rd at the resulting
address.
1 0 LDR Rd, [Rb, Ro] LDR Rd, [Rb, Ro] Pre-indexed word load:
Calculate the source address by
adding together the value in Rb
and the value in Ro. Load the
contents of the address into Rd.
1 1 LDRB Rd, [Rb, Ro] LDRB Rd, [Rb, Ro] Pre-indexed byte load:
Calculate the source address by
adding together the value in Rb
and the value in Ro. Load the byte
value at the resulting address.
L B THUMB assembler ARM equivalent Action
Table 5-8: Summary of format 7 instructions (Continued)
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-20
Open Access
5.8 Format 8: load/store sign-extended byte/halfword
Figure 5-9: Format 8
5.8.1 Operation
These instructions load optionally sign-extended bytes or halfwords, and store
halfwords. The THUMB assembler syntax is shown below.
S H THUMB assembler ARM equivalent Action
0 0 STRH Rd, [Rb, Ro] STRH Rd, [Rb, Ro] Store halfword:
Add Ro to base address in Rb. Store bits 0-
15 of Rd at the resulting address.
0 1 LDRH Rd, [Rb, Ro] LDRH Rd, [Rb, Ro] Load halfword:
Add Ro to base address in Rb. Load bits 0-
15 of Rd from the resulting address, and set
bits 16-31 of Rd to 0.
1 0 LDSB Rd, [Rb, Ro] LDRSB Rd, [Rb, Ro] Load sign-extended byte:
Add Ro to base address in Rb. Load bits 0-
7 of Rd from the resulting address, and set
bits 8-31 of Rd to bit 7.
Table 5-9: Summary of format 8 instructions
0123456789101112131415
Ro RbH010
Destination register
Base register
Offset register
H flag
1 S 1 Rd
Sign-extended flag
0 - Operand not sign-extended
1 - Operand sign-extended
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-21
Open Access
5.8.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-9: Summary of format 8 instructions
on page 5-20. The instruction cycle times for the
THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.8.3 Examples
STRH R4, [R3, R0] ; Store the lower 16 bits of R4 at the
; address formed by adding R0 to R3.
LDSB R2, [R7, R1] ; Load into R2 the sign extended byte
; found at the address formed by adding
; R1 to R7.
LDSH R3, [R4, R2] ; Load into R3 the sign extended halfword
; found at the address formed by adding
; R2 to R4.
1 1 LDSH Rd, [Rb, Ro] LDRSH Rd, [Rb, Ro] Load sign-extended halfword:
Add Ro to base address in Rb. Load bits 0-
15 of Rd from the resulting address, and set
bits 16-31 of Rd to bit 15.
S H THUMB assembler ARM equivalent Action
Table 5-9: Summary of format 8 instructions (Continued)
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-22
Open Access
5.9 Format 9: load/store with immediate offset
Figure 5-10: Format 9
5.9.1 Operation
These instructions transfer byte or word values between registers and memory using
an immediate 5 or 7-bit offset.
The THUMB assembler syntax is shown in ➲
Table 5-10: Summary of format 9
instructions
.
L B THUMB assembler ARM equivalent Action
0 0 STR Rd, [Rb, #Imm] STR Rd, [Rb, #Imm] Calculate the target address by
adding together the value in Rb
and Imm. Store the contents of Rd
at the address.
1 0 LDR Rd, [Rb, #Imm] LDR Rd, [Rb, #Imm] Calculate the source address by
adding together the value in Rb
and Imm. Load Rd from the
address.
Table 5-10: Summary of format 9 instructions
0123456789101112131415
Offset5 RbL110
Source/destination
Base register
Offset value
B Rd
Byte/Word flag
Load/Store flag
0 - Transfer word quantity
1 - Transfer byte quantity
0 - Store to memory
1 - Load from memory
register
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-23
Open Access
Note For word accesses (B = 0), the value specified by #Imm is a full 7-bit address, but must
be word-aligned (ie with bits 1:0 set to 0), since the assembler places #Imm >> 2 in
the Offset5 field.
5.9.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-10: Summary of format 9 instructions
on page 5-22. The instruction cycle times for
the THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.9.3 Examples
LDR R2, [R5,#116] ; Load into R2 the word found at the
; address formed by adding 116 to R5.
; Note that the THUMB opcode will
; contain 29 as the Offset5 value.
STRB R1, [R0,#13] ; Store the lower 8 bits of R1 at the
; address formed by adding 13 to R0.
; Note that the THUMB opcode will
; contain 13 as the Offset5 value.
0 1 STRB Rd, [Rb, #Imm] STRB Rd, [Rb, #Imm] Calculate the target address by
adding together the value in Rb
and Imm. Store the byte value in
Rd at the address.
1 1 LDRB Rd, [Rb, #Imm] LDRB Rd, [Rb, #Imm] Calculate source address by
adding together the value in Rb
and Imm. Load the byte value at
the address into Rd.
L B THUMB assembler ARM equivalent Action
Table 5-10: Summary of format 9 instructions (Continued)
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-24
Open Access
5.10 Format 10: load/store halfword
Figure 5-11: Format 10
5.10.1 Operation
These instructions transfer halfword values between a Lo register and memory.
Addresses are pre-indexed, using a 6-bit immediate value.
The THUMB assembler syntax is shown in ➲
Table 5-11: Halfword data transfer
instructions
.
Note #Imm is a full 6-bit address but must be halfword-aligned (ie with bit 0 set to 0) since
the assembler places #Imm >> 1 in the Offset5 field.
L THUMB assembler ARM equivalent Action
0 STRH Rd, [Rb, #Imm] STRH Rd, [Rb, #Imm] Add #Imm to base address in Rb and store
bits 0-15 of Rd at the resulting address.
1 LDRH Rd, [Rb, #Imm] LDRH Rd, [Rb, #Imm] Add #Imm to base address in Rb. Load bits
0-15 from the resulting address into Rd and
set bits 16-31 to zero.
Table 5-11: Halfword data transfer instructions
0123456789101112131415
Offset5 RbL001
Source/destination
Base register
Immediate value
0 Rd
register
Load/Store bit
0 - Store to memory
1 - Load from memory
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-25
Open Access
5.10.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-11: Halfword data transfer instructions
on page 5-24. The instruction cycle times for
the THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.10.3 Examples
STRH R6, [R1, #56] ; Store the lower 16 bits of R4 at
; the address formed by adding 56
; R1.
; Note that the THUMB opcode will
; contain 28 as the Offset5 value.
LDRH R4, [R7, #4] ; Load into R4 the halfword found at
; the address formed by adding 4 to R7.
; Note that the THUMB opcode will contain
; 2 as the Offset5 value.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-26
Open Access
5.11 Format 11: SP-relative load/store
Figure 5-12: Format 11
5.11.1 Operation
The instructions in this group perform an SP-relative load or store.The THUMB
assembler syntax is shown in the following table.
Note The offset supplied in #Imm is a full 10-bit address, but must always be word-aligned
(ie bits 1:0 set to 0), since the assembler places #Imm >> 2 in the Word8 field.
L THUMB assembler ARM equivalent Action
0 STR Rd, [SP, #Imm] STR Rd, [R13 #Imm] Add unsigned offset (255 words, 1020
bytes) in Imm to the current value of the SP
(R7). Store the contents of Rd at the
resulting address.
1 LDR Rd, [SP, #Imm] LDR Rd, [R13 #Imm] Add unsigned offset (255 words, 1020
bytes) in Imm to the current value of the SP
(R7). Load the word from the resulting
address into Rd.
Table 5-12: SP-relative load/store instructions
0123456789101112131415
Rd001 Word8
Destination register
Immediate value
1 L
Load/Store bit
0 - Store to memory
1 - Load from memory
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-27
Open Access
5.11.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-12: SP-relative load/store instructions
on page 5-26. The instruction cycle times for
the THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.11.3 Examples
STR R4, [SP,#492] ; Store the contents of R4 at the address
; formed by adding 492 to SP (R13).
; Note that the THUMB opcode will contain
; 123 as the Word8 value.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-28
Open Access
5.12 Format 12: load address
Figure 5-13: Format 12
5.12.1 Operation
These instructions calculate an address by adding an 10-bit constant to either the PC
or the SP, and load the resulting address into a register.
The THUMB assembler syntax is shown in the following table.
Note The value specified by #Imm is a full 10-bit value, but this must be word-aligned (ie
with bits 1:0 set to 0) since the assembler places #Imm >> 2 in field Word8.
Where the PC is used as the source register (SP = 0), bit 1 of the PC is always read
as 0. The value of the PC will be 4 bytes greater than the address of the instruction
before bit 1 is forced to 0.
The CPSR condition codes are unaffected by these instructions.
SP THUMB assembler ARM equivalent Action
0 ADD Rd, PC, #Imm ADD Rd, R15, #Imm Add #Imm to the current value of
the program counter (PC) and load
the result into Rd.
1 ADD Rd, SP, #Imm ADD Rd, R13, #Imm Add #Imm to the current value of
the stack pointer (SP) and load the
result into Rd.
Table 5-13: Load address
0123456789101112131415
Rd101 0 SP Word8
8-bit unsigned constant
Destination register
Source
0 - PC
1 - SP
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-29
Open Access
5.12.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-13: Load address
on page 5-28. The instruction cycle times for the THUMB
instruction are identical to that of the equivalent ARM instruction. For more information
on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle Operations
.
5.12.3 Examples
ADD R2, PC, #572 ; R2 := PC + 572, but don't set the
; condition codes. bit[1] of PC is
; forced to zero.
; Note that the THUMB opcode will
; contain 143 as the Word8 value.
ADD R6, SP, #212 ; R6 := SP (R13) + 212, but don't
; set the condition codes.
; Note that the THUMB opcode will
; contain 53 as the Word8 value.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-30
Open Access
5.13 Format 13: add offset to Stack Pointer
Figure 5-14: Format 13
5.13.1 Operation
This instruction adds a 9-bit signed constant to the stack pointer. The following table
shows the THUMB assembler syntax.
Note The offset specified by #Imm can be up to -/+ 508, but must be word-aligned (ie with
bits 1:0 set to 0) since the assembler converts #Imm to an 8-bit sign + magnitude
number before placing it in field SWord7.
Note The condition codes are not set by this instruction.
5.13.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-14: The ADD SP instruction
on page 5-30. The instruction cycle times for the
THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
S THUMB assembler ARM equivalent Action
0 ADD SP, #Imm ADD R13, R13, #Imm Add #Imm to the stack pointer (SP).
1 ADD SP, #-Imm SUB R13, R13, #Imm Add #-Imm to the stack pointer (SP).
Table 5-14: The ADD SP instruction
0123456789101112131415
101 1
7-bit immediate value
SWord7000 0 S
Sign flag
0 -Offset is positive
1 -Offset is negative
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-31
Open Access
5.13.3 Examples
ADD SP, #268 ; SP (R13) := SP + 268, but don't set
; the condition codes.
; Note that the THUMB opcode will
; contain 67 as the Word7 value and S=0.
ADD SP, #-104 ; SP (R13) := SP - 104, but don't set
; the condition codes.
; Note that the THUMB opcode will contain
; 26 as the Word7 value and S=1.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-32
Open Access
5.14 Format 14: push/pop registers
Figure 5-15: Format 14
5.14.1 Operation
The instructions in this group allow registers 0-7 and optionally LR to be pushed onto
the stack, and registers 0-7 and optionally PC to be popped off the stack.
The THUMB assembler syntax is shown in ➲
Table 5-15: PUSH and POP instructions
.
Note The stack is always assumed to be Full Descending.
L R THUMB assembler ARM equivalent Action
0 0 PUSH { Rlist } STMDB R13!, { Rlist } Push the registers specified by
Rlist onto the stack. Update the
stack pointer.
0 1 PUSH { Rlist, LR } STMDB R13!, { Rlist, R14 } Push the Link Register and the
registers specified by Rlist (if any)
onto the stack. Update the stack
pointer.
1 0 POP { Rlist } LDMIA R13!, { Rlist } Pop values off the stack into the
registers specified by Rlist. Update
the stack pointer.
1 1 POP { Rlist, PC } LDMIA R13!, { Rlist, R15 } Pop values off the stack and load
into the registers specified by Rlist.
Pop the PC off the stack. Update
the stack pointer.
Table 5-15: PUSH and POP instructions
0123456789101112131415
10 0 Rlist
Register list
PC/LR bit
Load/Store bit
0 - Store to memory
1 - Load from memory
1 1 L 1
0 - Do not store LR/load PC
1 - Store LR/Load PC
R
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-33
Open Access
5.14.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-15: PUSH and POP instructions
on page 5-32. The instruction cycle times for the
THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
.
5.14.3 Examples
PUSH {R0-R4,LR} ; Store R0,R1,R2,R3,R4 and R14 (LR) at
; the stack pointed to by R13 (SP) and
; update R13.
; Useful at start of a sub-routine to
; save workspace and return address.
POP {R2,R6,PC} ; Load R2,R6 and R15 (PC) from the stack
; pointed to by R13 (SP) and update R13.
; Useful to restore workspace and return
; from sub-routine.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-34
Open Access
5.15 Format 15: multiple load/store
Figure 5-16: Format 15
5.15.1 Operation
These instructions allow multiple loading and storing of Lo registers. The THUMB
assembler syntax is shown in the following table.
5.15.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-16: The multiple load/store instructions
on page 5-34. The instruction cycle times for
the THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
L THUMB assembler ARM equivalent Action
0 STMIA Rb!, { Rlist } STMIA Rb!, { Rlist } Store the registers specified by
Rlist, starting at the base address
in Rb. Write back the new base
address.
1 LDMIA Rb!, { Rlist } LDMIA Rb!, { Rlist } Load the registers specified by
Rlist, starting at the base address
in Rb. Write back the new base
address.
Table 5-16: The multiple load/store instructions
0123456789101112131415
Rb011 0 L Rlist
Register list
Base register
Load/Store bit
0 - Store to memory
1 - Load from memory
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-35
Open Access
5.15.3 Examples
STMIA R0!, {R3-R7} ; Store the contents of registers R3-R7
; starting at the address specified in
; R0, incrementing the addresses for each
; word.
; Write back the updated value of R0.
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-36
Open Access
5.16 Format 16: conditional branch
Figure 5-17: Format 16
5.16.1 Operation
The instructions in this group all perform a conditional Branch depending on the state
of the CPSR condition codes. The branch offset must take account of the prefetch
operation, which causes the PC to be 1 word (4 bytes) ahead of the current instruction.
The THUMB assembler syntax is shown in the following table.
Cond THUMB assembler ARM equivalent Action
0000 BEQ label BEQ label Branch if Z set (equal)
0001 BNE label BNE label Branch if Z clear (not equal)
0010 BCS label BCS label Branch if C set (unsigned higher or
same)
0011 BCC label BCC label Branch if C clear (unsigned lower)
0100 BMI label BMI label Branch if N set (negative)
0101 BPL label BPL label Branch if N clear (positive or zero)
0110 BVS label BVS label Branch if V set (overflow)
0111 BVC label BVC label Branch if V clear (no overflow)
1000 BHI label BHI label Branch if C set and Z clear
(unsigned higher)
1001 BLS label BLS label Branch if C clear or Z set
(unsigned lower or same)
Table 5-17: The conditional branch instructions
0123456789101112131415
011 1
8-bit signed immediate
Condition
Cond SOffset8
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-37
Open Access
Note While label specifies a full 9-bit two’s complement address, this must always be
halfword-aligned (ie with bit 0 set to 0) since the assembler actually places label >> 1
in field SOffset8.
Note Cond = 1110 is undefined, and should not be used.
Cond = 1111 creates the SWI instruction: see ➲
5.17 Format 17: software interrupt
on
page 5-38.
5.16.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-17: The conditional branch instructions
on page 5-36. The instruction cycle times for
the THUMB instruction are identical to that of the equivalent ARM instruction. For more
information on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle
Operations
5.16.3 Examples
CMP R0, #45 ; Branch to ’over’ if R0 > 45.
BGT over ; Note that the THUMB opcode will contain
... ; the number of halfwords to offset.
...
...
over ... ; Must be halfword aligned.
...
1010 BGE label BGE label Branch if N set and V set, or N
clear and V clear (greater or
equal)
1011 BLT label BLT label Branch if N set and V clear, or N
clear and V set (less than)
1100 BGT label BGT label Branch if Z clear, and either N set
and V set or N clear and V clear
(greater than)
1101 BLE label BLE label Branch if Z set, or N set and V
clear, or N clear and V set (less
than or equal)
Cond THUMB assembler ARM equivalent Action
Table 5-17: The conditional branch instructions (Continued)
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-38
Open Access
5.17 Format 17: software interrupt
Figure 5-18: Format 17
5.17.1 Operation
The SWI instruction performs a software interrupt. On taking the SWI, the processor
switches into ARM state and enters Supervisor (SVC) mode.
The THUMB assembler syntax for this instruction is shown below.
Note Value8 is used solely by the SWI handler: it is ignored by the processor.
5.17.2 Instruction cycle times
All instructions in this format have an equivalent ARM instruction as shown in ➲
Table
5-18: The SWI instruction
on page 5-38. The instruction cycle times for the THUMB
instruction are identical to that of the equivalent ARM instruction. For more information
on instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle Operations
5.17.3 Examples
SWI 18 ; Take the software interrupt exception.
; Enter Supervisor mode with 18 as the
; requested SWI number.
THUMB assembler ARM equivalent Action
SWI Value8 SWI Value8 Perform Software Interrupt:
Move the address of the next instruction
into LR, move CPSR to SPSR, load the SWI
vector address (0x8) into the PC. Switch to
ARM state and enter SVC mode.
Table 5-18: The SWI instruction
0123456789101112131415
011 1 Value8111 1
Comment field
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-39
Open Access
5.18 Format 18: unconditional branch
Figure 5-19: Format 18
5.18.1 Operation
This instruction performs a PC-relative Branch. The THUMB assembler syntax is
shown below. The branch offset must take account of the prefetch operation, which
causes the PC to be 1 word (4 bytes) ahead of the current instruction.
Note The address specified by label is a full 12-bit two’s complement address, but must
always be halfword aligned (ie bit 0 set to 0), since the assembler places label >> 1 in
the Offset11 field.
5.18.2 Examples
here B here ; Branch onto itself.
; Assembles to 0xE7FE.
; (Note effect of PC offset).
B jimmy ; Branch to 'jimmy'.
... ; Note that the THUMB opcode will
; contain the number of halfwords
; to offset.
jimmy ... ; Must be halfword aligned.
THUMB assembler ARM equivalent Action
B label BAL label (halfword
offset) Branch PC relative +/- Offset11 << 1, where
label is PC +/- 2048 bytes.
Table 5-19: Summary of Branch instruction
0123456789101112131415
111 Offset11
Immediate value
0 0
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-40
Open Access
5.19 Format 19: long branch with link
Figure 5-20: Format 19
5.19.1 Operation
This format specifies a long branch with link.
The assembler splits the 23-bit two’s complement half-word offset specifed by the
label into two 11-bit halves, ignoring bit 0 (which must be 0), and creates two THUMB
instructions.
Instruction 1 (H = 0)
In the first instruction the Offset field contains the upper 11 bits of the target address.
This is shifted left by 12 bits and added to the current PC address. The resulting
address is placed in LR.
Instruction 2 (H =1)
In the second instruction the Offset field contains an 11-bit representation lower half of
the target address. This is shifted left by 1 bit and added to LR. LR, which now contains
the full 23-bit address, is placed in PC, the address of the instruction following the BL
is placed in LR and bit 0 of LR is set.
The branch offset must take account of the prefetch operation, which causes the PC
to be 1 word (4 bytes) ahead of the current instruction
0123456789101112131415
111 1 OffsetH
Long branch and link offset high/low
Low/high offset bit
0 - offset high
1 - offset low
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-41
Open Access
5.19.2 Instruction cycle times
This instruction format does not have an equivalent ARM instruction. For details of the
instruction cycle times, please refer to ➲
Chapter 10, Instruction Cycle Operations
.
5.19.3 Examples
BL faraway ; Unconditionally Branch to 'faraway'
next ... ; and place following instruction
; address, ie ’next’, in R14,the Link
; Register and set bit 0 of LR high.
; Note that the THUMB opcodes will
; contain the number of halfwords to
; offset.
faraway ... ; Must be Half-word aligned.
H THUMB assembler ARM equivalent Action
0 BL label none LR := PC + OffsetHigh << 12
1 temp := next instruction address
PC := LR + OffsetLow << 1
LR := temp | 1
Table 5-20: The BL instruction
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-42
Open Access
5.20 Instruction Set Examples
The following examples show ways in which the THUMB instructions may be used to
generate small and efficient code. Each example also shows the ARM equivalent so
these may be compared.
5.20.1 Multiplication by a constant using shifts and adds
The following shows code to multiply by various constants using 1, 2 or 3 Thumb
instructions alongside the ARM equivalents. For other constants it is generally better
to use the built-in MUL instruction rather than using a sequence of 4 or more
instructions.
Thumb ARM
1 Multiplication by 2^n (1,2,4,8,...)
LSL Ra, Rb, LSL #n MOV Ra, Rb, LSL #n
2 Multiplication by 2^n+1 (3,5,9,17,...)
LSL Rt, Rb, #n ADD Ra, Rb, Rb, LSL #n
ADD Ra, Rt, Rb
3 Multiplication by 2^n-1 (3,7,15,...)
LSL Rt, Rb, #n RSB Ra, Rb, Rb, LSL #n
SUB Ra, Rt, Rb
4 Multiplication by -2^n (-2, -4, -8, ...)
LSL Ra, Rb, #n MOV Ra, Rb, LSL #n
MVN Ra, Ra RSB Ra, Ra, #0
5 Multiplication by -2^n-1 (-3, -7, -15, ...)
LSL Rt, Rb, #n SUB Ra, Rb, Rb, LSL #n
SUB Ra, Rb, Rt
6 Multiplication by any C = {2^n+1, 2^n-1, -2^n or -2^n-1} * 2^n
Effectively this is any of the multiplications in 2 to 5 followed by a final shift.
This allows the following additional constants to be multiplied.
6, 10, 12, 14, 18, 20, 24, 28, 30, 34, 36, 40, 48, 56, 60, 62 .....
(2..5) (2..5)
LSL Ra, Ra, #n MOV Ra, Ra, LSL #n
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-43
Open Access
5.20.2 General purpose signed divide
This example shows a general purpose signed divide and remainder routine in both
Thumb and ARM code.
Thumb code
signed_divide
; Signed divide of R1 by R0: returns quotient in R0,
; remainder in R1
; Get abs value of R0 into R3
ASR R2, R0, #31 ; Get 0 or -1 in R2 depending on sign of R0
EOR R0, R2 ; EOR with -1 (0xFFFFFFFF) if negative
SUB R3, R0, R2 ; and ADD 1 (SUB -1) to get abs value
; SUB always sets flag so go & report division by 0 if necessary
; BEQ divide_by_zero
; Get abs value of R1 by xoring with 0xFFFFFFFF and adding 1
; if negative
ASR R0, R1, #31 ; Get 0 or -1 in R3 depending on sign of R1
EOR R1, R0 ; EOR with -1 (0xFFFFFFFF) if negative
SUB R1, R0 ; and ADD 1 (SUB -1) to get abs value
; Save signs (0 or -1 in R0 & R2) for later use in determining
; sign of quotient & remainder.
PUSH {R0, R2}
; Justification, shift 1 bit at a time until divisor (R0 value)
; is just <= than dividend (R1 value). To do this shift dividend
; right by 1 and stop as soon as shifted value becomes >.
LSR R0, R1, #1
MOV R2, R3
B %FT0
just_l LSL R2, #1
0 CMP R2, R0
BLS just_l
MOV R0, #0 ; Set accumulator to 0
B %FT0 ; Branch into division loop
div_l LSR R2, #1
0 CMP R1, R2 ; Test subtract
BCC %FT0
SUB R1, R2 ; If successful do a real
; subtract
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-44
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0 ADC R0, R0 ; Shift result and add 1 if
; subtract succeeded
CMP R2, R3 ; Terminate when R2 == R3 (ie we have just
BNE div_l ; tested subtracting the 'ones' value).
; Now fixup the signs of the quotient (R0) and remainder (R1)
POP {R2, R3} ; Get dividend/divisor signs back
EOR R3, R2 ; Result sign
EOR R0, R3 ; Negate if result sign = -1
SUB R0, R3
EOR R1, R2 ; Negate remainder if dividend sign = -1
SUB R1, R2
MOV pc, lr
ARM code
signed_divide
; effectively zero a4 as top bit will be shifted out later
ANDS a4, a1, #&80000000
RSBMI a1, a1, #0
EORS ip, a4, a2, ASR #32
; ip bit 31 = sign of result
; ip bit 30 = sign of a2
RSBCS a2, a2, #0
; central part is identical code to udiv
; (without MOV a4, #0 which comes for free as part of signed
; entry sequence)
MOVS a3, a1
BEQ divide_by_zero
just_l
; justification stage shifts 1 bit at a time
CMP a3, a2, LSR #1
MOVLS a3, a3, LSL #1
; NB: LSL #1 is always OK if LS succeeds
BLO s_loop
div_l CMP a2, a3
ADC a4, a4, a4
SUBCS a2, a2, a3
TEQ a3, a1
MOVNE a3, a3, LSR #1
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
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BNE s_loop2
MOV a1, a4
MOVS ip, ip, ASL #1
RSBCS a1, a1, #0
RSBMI a2, a2, #0
MOV pc, lr
5.20.3 Division by a constant
Division by a constant can often be performed by a short fixed sequence of shifts, adds
and subtracts. For an explanation of the algorithm see
The ARM Cookbook
(ARM
DUYI-0005B), section entitiled
Division by a constant
.
Here is an example of a divide by 10 routine based on the algorithm in the ARM
Cookbook in both Thumb and ARM code.
Thumb code
udiv10
; takes argument in a1
; returns quotient in a1, remainder in a2
MOV a2, a1
LSR a3, a1, #2
SUB a1, a3
LSR a3, a1, #4
ADD a1, a3
LSR a3, a1, #8
ADD a1, a3
LSR a3, a1, #16
ADD a1, a3
LSR a1, #3
ASL a3, a1, #2
ADD a3, a1
ASL a3, #1
SUB a2, a3
CMP a2, #10
BLT %FT0
ADD a1, #1
SUB a2, #10
0MOV pc, lr
THUMB Instruction Set
ARM7TDMI Data Sheet
ARM DDI 0029E
5-46
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ARM code
udiv10
; takes argument in a1
; returns quotient in a1, remainder in a2
SUB a2, a1, #10
SUB a1, a1, a1, lsr #2
ADD a1, a1, a1, lsr #4
ADD a1, a1, a1, lsr #8
ADD a1, a1, a1, lsr #16
MOV a1, a1, lsr #3
ADD a3, a1, a1, asl #2
SUBS a2, a2, a3, asl #1
ADDPL a1, a1, #1
ADDMI a2, a2, #10
MOV pc, lr
ARM7TDMI Data Sheet
ARM DDI 0029E
6-1
11
1
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Memory Interface
This chapter describes the ARM7TDMI memory interface.
6.1 Overview 6-2
6.2 Cycle Types 6-2
6.3 Address Timing 6-4
6.4 Data Transfer Size 6-9
6.5 Instruction Fetch 6-10
6.6 Memory Management 6-12
6.7 Locked Operations 6-12
6.8 Stretching Access Times 6-12
6.9 The ARM Data Bus 6-13
6.10 The External Data Bus 6-15
6
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-2
Open Access
6.1 Overview
ARM7TDMI’s memory interface consists of the following basic elements:
• 32-bit address bus
This specifies to memory the location to be used for the transfer.
• 32-bit data bus
Instructions and data are transferred across this bus. Data may be word,
halfword or byte wide in size.
ARM7TDMI includes a bidirectional data bus, D[31:0], plus separate
unidirectional data busses,DIN[31:0] and DOUT[31:0]. Most of the text in this
chapter describes the bus behaviour assuming that the bidirectional is in use.
However, the behaviour applies equally to the unidirectional busses.
• Control signals
These specify, for example, the size of the data to be transferred, and the
direction of the transfer together with providing privileged information.
This collection of signals allow ARM7TDMI to be simply interfaced to DRAM, SRAM
and ROM. To fully exploit page mode access to DRAM, information is provided on
whether or not the memory accesses are sequential. In general, interfacing to static
memories is much simpler than interfacing to dynamic memory.
6.2 Cycle Types
All memory transfer cycles can be placed in one of four categories:
1 Non-sequential cycle. ARM7TDMI requests a transfer to or from an address
which is unrelated to the address used in the preceding cycle.
2Sequential cycle. ARM7TDMI requests a transfer to or from an address which
is either the same as the address in the preceding cycle, or is one word or
halfword after the preceding address.
3 Internal cycle. ARM7TDMI does not require a transfer, as it is performing an
internal function and no useful prefetching can be performed at the same time.
4 Coprocessor register transfer. ARM7TDMI wishes to use the data bus to
communicate with a coprocessor, but does not require any action by the
memory system.
These four classes are distinguishable to the memory system by inspection of the
nMREQ and SEQ control lines (see ➲
Table 6-1: Memory cycle types
). These control
lines are generated during phase 1 of the cycle before the cycle whose characteristics
they forecast, and this pipelining of the control information gives the memory system
sufficient time to decide whether or not it can use a page mode access.
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ARM7TDMI Data Sheet
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➲
Figure 6-1: ARM memory cycle timing
on page 6-3 shows the pipelining of the control
signals, and suggests how the DRAM address strobes (nRAS and nCAS) might be
timed to use page mode for S-cycles. Note that the N-cycle is longer than the other
cycles. This is to allow for the DRAM precharge and row access time, and is not an
ARM7TDMI requirement.
Figure 6-1: ARM memory cycle timing
When an S-cycle follows an N-cycle, the address will always be one word or halfword
greater than the address used in the N-cycle. This address (marked “a” in the above
diagram) should be checked to ensure that it is not the last in the DRAM page before
the memory system commits to the S-cycle. If it is at the page end, the S-cycle cannot
be performed in page mode and the memory system will have to perform a full access.
The processor clock must be stretched to match the full access. When an S-cycle
follows an I-cycle, the address will be the same as that used in the I-cycle. This fact
may be used to start the DRAM access during the preceding cycle, which enables the
S-cycle to run at page mode speed whilst performing a full DRAM access. This is
shown in ➲
Figure 6-2: Memory cycle optimization.
nMREQ SEQ Cycle type
0 0 Non-sequential (N-cycle)
0 1 Sequential (S-cycle)
1 0 Internal (I-cycle)
1 1 Coprocessor register transfer (C-cycle)
Table 6-1: Memory cycle types
MCLK
A[31:0]
nMREQ
SEQ
nCAS
a a+4
I-cycleS-cycle C-cycleN-cycle
nRAS
D[31:0]
a+8
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-4
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.
Figure 6-2: Memory cycle optimization
6.3 Address Timing
ARM7TDMI’s address bus can operate in one of two configurations - pipelined or
depipelined, and this is controlled by the APE input signal. The configurability is
provided to ease the design in of ARM7TDMI to both SRAM and DRAM based
systems.
It is a requirement SRAMs and ROMs that the address be held stable throughout the
memory cycle. In a system containing SRAM and ROM only, APE may be tied
permanently LOW, producing the desired address timing. This is shown in
➲
Figure 6-3: ARM7TDMI de-pipelined addresses
.
Note APE effects the timing of the address bus A[31:0], plus nRW,MAS[1:0],LOCK,
nOPC and nTRANS.
MCLK
A[31:0]
nMREQ
SEQ
nCAS
I-cycle S-cycle
nRAS
D[31:0]
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
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Figure 6-3: ARM7TDMI de-pipelined addresses
In a DRAM based system, it is desirable to obtain the address from ARM7TDMI as
early as possible. When APE is HIGH, ARM7TDMI's address becomes valid in the
MCLK high phase before the memory cycle to which it refers. This timing allows longer
for address decoding and the generation of DRAM control signals. ➲
Figure 6-4:
ARM7TDMI pipelined addresses
on page 6-5 shows the effect on the timing when
APE is HIGH.
Figure 6-4: ARM7TDMI pipelined addresses
MCLK
APE
nMREQ
SEQ
A[31:0]
D[31:0]
MCLK
APE
nMREQ
SEQ
A[31:0]
D[31:0]
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-6
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Many systems will contain a mixture of DRAM and SRAM/ROM. To cater for the
different address timing requirements, APE may be safely changed during the low
phase ofMCLK. T ypically,APE would be held at one level during a burst of sequential
accesses to one type of memory. When a non-sequential access occurs, the timing
of most systems enforce a wait state to allow for address decoding. As a result of the
address decode, APE can be driven to the correct value for the particular bank of
memory being accessed. The value of APE can be held until the memory control
signals denote another non-sequential access.
By way of an example, ➲
Figure 6-5: Typical system timing
, shows a combination of
accesses to a mixed DRAM / SRAM system. Here, the SRAM has zero wait states,
and the DRAM has a 2:1 N-cycle / S-cycle ratio. A single wait state is inserted for
address decode when a non-sequential access occurs. Typical, externally generated
DRAM control signals are also shown.
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-7
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Figure 6-5: Typical system timing
MCLK
nMREQ
SEQ
A[31:0]
nRW
nWAIT
APE
D[31:0]
DBE
nRAS
nCAS
SRAM Cycles Decode DRAM Cycles Decode SRAM Cycles
S S S S S SN N
A A+4 A+8 B B+4 B+8 C C+4 C+8
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-8
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Previous ARM processors included the ALE signal, and this is retained for backwards
compatibility. This signal also allows the address timing to be modified to achieve the
same results as APE, but in an asynchronous manner. To obtain clean MCLK low
timing of the address bus by this mechanism,ALE must be driven HIGH with the falling
edge ofMCLK, and LOW with the rising edge of MCLK.ALE can simply be the inverse
of MCLK but the delay from MCLK to ALE must be carefully controlled such that the
Tald
timing constraint is achieved. ➲
Figure 6-6: SRAM compatible address timing
shows how ALE can be used to achieve SRAM compatible address timing. Refer to
➲
Chapter 12, AC Parameters
for details of the exact timing constraints.
Figure 6-6: SRAM compatible address timing
Note If ALE is to be used to change address timing, then APE must be tied HIGH. Similarly ,
if APE is to be used, ALE must be tied HIGH.
MCLK
APE
ALE
nMREQ
SEQ
A[31:0]
D[31:0]
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-9
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6.4 Data Transfer Size
In an ARM7TDMI system, words, halfwords or bytes may be transferred between the
processor and the memory. The size of the transaction taking place is determined by
the MAS[1:0] pins. These are encoded as follows:
MAS[1:0] 00 Byte
01 halfword
10 word
11 reserved
The processor always produces a byte address, but instructions are either words (4
bytes) or halfwords (2 bytes), and data can be any size. Note that when word
instructions are fetched from memory, A[1:0] are undefined and when halfword
instructions are fetched, A[0] is undefined. The MAS[1:0] outputs share the same
timing as the address bus and thus can be modified by the use of ALE and APE as
described in ➲
6.3 Address Timing
on page 6-4.
When a data read of byte or halfword size is performed (eg LDRB), the memory
system may safely ignore the fact that the request is for a sub-word sized quantity and
present the whole word. ARM7TDMI will always correctly extract the addressed byte
or halfword from the data. The memory system may also choose just to supply the
addressed byte or halfword. This may be desirable in order to save power or to simplify
the decode logic.
When a byte or halfword write occurs (eg STRH), ARM7TDMI will broadcast the byte
or halfword across the whole of the bus. The memory system must then decodeA[1:0]
to enable writing only to the addressed byte or halfword.
One way of implementing the byte decode in a DRAM system is to separate the 32-bit
wide block of DRAM into four byte wide banks, and generate the column address
strobes independently as shown in ➲
Figure 6-7: Decoding byte accesses to memory
on page 6-11.
When the processor is configured for Little Endian operation, byte 0 of the memory
system should be connected to data lines 7 through 0 (D[7:0]) and strobed bynCAS0.
nCAS1 drives the bank connected to data lines 15 though 8, and so on. This has the
added advantage of reducing the load on each column strobe driver, which improves
the precision of this time-critical-signal.
In the Big Endian case, byte 0 of the memory system should be connected to data lines
31 through 24.
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-10
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6.5 Instruction Fetch
ARM7TDMI will perform 32- or 16-bit instruction fetches depending on whether the
processor is in ARM or THUMB state. The processor state may be determined
externally by the value of the TBIT signal. When this is LOW , the processor is in ARM
state and 32-bit instructions are fetched. When TBIT is HIGH, the processor is in
THUMB state and 16-bit instructions are fetched. The size of the data being fetched is
also indicated on the MAS[1:0] bits, as described above.
When the processor is in ARM state, 32-bit instructions are fetched on D[31:0]. When
the processor is in THUMB state, 16-bit instructions are fetched from either the upper ,
D[31:16], or the lower D[15:0] half of the bus. This is determined by the endianism of
the memory system, as configured by theBIGEND input, and the state of A[1].➲
Table
6-2: Endianism effect on instruction position
shows which half of the data bus is
sampled in the different configurations.
When a 16-bit instruction is fetched, ARM7TDMI ignores the unused half of the data
bus.
➲
Table 6-2: Endianism effect on instruction position
describes instructions fetched
from the bidirectional data bus (i.e. BUSEN is LOW). When the unidirectional data
busses are in use (i.e. BUSEN is HIGH), data will be fetched from the corresponding
half of the DIN[31:0] bus.
Endianism
Little
BIGEND = 0 Big
BIGEND = 1
A[1] = 0 D[15:0] D[31:16]
A[1] = 1 D[31:16] D[15:0]
Table 6-2: Endianism effect on instruction position
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-11
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Figure 6-7: Decoding byte accesses to memory
A[0] A[1] MAS[0] MCLK CAS
NCAS0
NCAS1
NCAS2
NCAS3
G
D Q
Quad
Latch
[1]
MAS[0] [1]
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-12
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6.6 Memory Management
The ARM7TDMI address bus may be processed by an address translation unit before
being presented to the memory, and ARM7TDMI is capable of running a virtual
memory system. The ABORT input to the processor may be used by the memory
manager to inform ARM7TDMI of page faults. Various other signals enable different
page protection levels to be supported:
1nRW can be used by the memory manager to protect pages from being
written to.
2nTRANS indicates whether the processor is in user or a privileged mode, and
may be used to protect system pages from the user, or to support completely
separate mappings for the system and the user.
Address translation will normally only be necessary on an N-cycle, and this fact may
be exploited to reduce power consumption in the memory manager and avoid the
translation delay at other times. The times when translation is necessary can be
deduced by keeping track of the cycle types that the processor uses.
6.7 Locked Operations
The ARM instruction set of ARM7TDMI includes a data swap (SWP) instruction that
allows the contents of a memory location to be swapped with the contents of a
processor register. This instruction is implemented as an uninterruptable pair of
accesses; the first access reads the contents of the memory, and the second writes
the register data to the memory. These accesses must be treated as a contiguous
operation by the memory controller to prevent another device from changing the
affected memory location before the swap is completed. ARM7TDMI drives theLOCK
signal HIGH for the duration of the swap operation to warn the memory controller not
to give the memory to another device.
6.8 Stretching Access Times
All memory timing is defined by MCLK, and long access times can be accommodated
by stretching this clock. It is usual to stretch the LOW period of MCLK, as this allows
the memory manager to abort the operation if the access is eventually unsuccessful.
Either MCLK can be stretched before it is applied to ARM7TDMI, or the nWAIT input
can be used together with a free-running MCLK. Taking nWAIT LOW has the same
effect as stretching the LOW period of MCLK, and nWAIT must only change when
MCLK is LOW.
ARM7TDMI does not contain any dynamic logic which relies upon regular clocking to
maintain its internal state. Therefore there is no limit upon the maximum period for
which MCLK may be stretched, or nWAIT held LOW.
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-13
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6.9 The ARM Data Bus
To ease the connection of ARM7TDMI to sub-word sized memory systems, input data
and instructions may be latched on a byte by byte basis. This is achieved by use of the
BL[3:0] input signals where BL[3] controls the latching of the data present on
D[31:24] of the data bus and so on.
In a memory system containing word wide memory only, BL[3:0] may be tied HIGH.
For sub word wide memory systems, BL[3:0] are used to latch the data as it is read
out of memory . For example, a word access to halfword wide memory must take place
in two memory cycles. In the first cycle, the data for D[15:0] is obtained from the
memory and latched into the processor on the falling edge of MCLK whenBL[1:0] are
both HIGH. In the second cycle, the data for D[31:16] is latched into the processor on
the falling edge of MCLK when BL[3:2] are both HIGH.
A memory access like this is shown in ➲
Figure 6-8: Memory access
on page 6-14.
Here, a word access is performed from halfword wide memory in two cycles.In the first,
the data read is applied to the lower half of the bus, in the second cycle the read data
is applied to the upper half of the bus. Since two memory cycles were required,nWAIT
is used to stretch the internal processor clock. However, nWAIT does not effect the
operation of the data latches. In this way, data may be extracted from memory word,
halfword or byte at a time, and the memory may have as many wait states as required.
In any multi-cycle memory access, nWAIT is held LOW until the final quantum of data
is latched.
In this example, BL[3:0] were driven to value 0x3 in the first cycle so that only the
latches onD[15:0] were opened. In fact, BL[3:0] could have been driven to value 0xF
and all the latches opened. Since in the second cycle, the latches on D[31:16] were
written with the correct data, this would not have effected the processor's operation.
Note BL[3:0]
should all be HIGH during store cycles.
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-14
Open Access
Figure 6-8: Memory access
As a further example, a halfword load from 2-wait state byte wide memory is shown in
➲
Figure 6-9: Two-cycle Memory access
on page 6-15. Here, each memory access
takes two cycles. In the first, access, BL[3:0] are driven to value 0xF. The correct data
is latched from D[7:0] whilst unknown data is latched from D[31:8]. In the second
access, the byte for D[15:8] is latched and so the halfword on D[15:0] has been
correctly read from the memory. The fact that internally D[31:16] are unknown does
not matter because internally the processor will extract only the halfword it is
interested in.
MCLK
APE
nMREQ
SEQ
A[31:0]
nWAIT
D[15:0]
D[31:16]
BL[3:0]
0x3 0xC
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-15
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Figure 6-9: Two-cycle Memory access
6.10 The External Data Bus
ARM7TDMI has a bidirectional data bus, D[31:0]. However, since some ASIC design
methodologies prohibit the use of bidirectional buses, unidirectional data in,
DIN[31:0], and data out, DOUT[31:0], busses are also provided. The logical
arrangement of these buses is shown in➲
Figure 6-10: ARM7TDMI external bus
arrangement
on page 6-16
MCLK
APE
nMREQ
SEQ
A[31:0]
nWAIT
D[7:0]
D[15:8]
BL[3:0] 0xF 0x2
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ARM7TDMI Data Sheet
ARM DDI 0029E
6-16
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Figure 6-10: ARM7TDMI external bus arrangement
When the bidirectional data bus is being used, the unidirectional busses must be
disabled by driving BUSEN LOW. The timing of the bus for three cycles,
load-store-load, is shown in ➲
Figure 6-11: Bidirectional bus timing
.
Figure 6-11: Bidirectional bus timing
MCLK
D[31:0]
Read Cycle Store Cycle Read Cycle
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-17
Open Access
Figure 6-12: Unidirectional bus timing
6.10.1 The unidirectional data bus
When the unidirectional data busses are being used, (i.e. when BUSEN is HIGH), the
bidirectional bus, D[31:0], must be left unconnected.
When BUSEN is HIGH, all instructions and input data are presented on the input data
bus, DIN[31:0]. The timing of this data is similar to that of the bidirectional bus when
in input mode. Data must be set up and held to the falling edge ofMCLK. For the exact
timing requirements refer to ➲
Chapter 12, AC Parameters
.
In this configuration, all output data is presented on DOUT[31:0]. The value on this
bus only changes when the processor performs a store cycle. Again, the timing of the
data is similar to that of the bidirectional data bus. The value on DOUT[31:0] changes
off the falling edge of MCLK.
The bus timing of a read-write-read cycle combination is shown in ➲
Figure 6-12:
Unidirectional bus timing
on page 6-17.
When BUSEN is LOW, the buffer between DIN[31:0] and D[31:0] is disabled. Any
data presented on DIN[31:0] is ignored. Also, when BUSEN is low, the value on
DOUT[31:0] is forced to 0x00000000.
Typically, the unidirectional busses would be used internally in ASIC embedded
applications. Externally, most systems still require a bidirectional data bus to interface
to external memory. ➲
Figure 6-13: External connection of unidirectional busses
on
page 6-18, shows how the unidirectional busses may be joined up at the pads of an
ASIC to connect to an external bidirectional bus.
MCLK
DIN[31:0]
DOUT[31:0]
D[31:0]
Read Cycle Store Cycle Read Cycle
D1 D2
Dout
D1 Dout D2
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-18
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Figure 6-13: External connection of unidirectional busses
6.10.2 The bidirectional data bus
ARM7TDMI has a bidirectional data bus, D[31:0]. Most of the time, the ARM reads
from memory and so this bus is configured to input. During write cycles however, the
ARM7TDMI must output data. During phase 2 of the previous cycle, the signal nRW
is driven HIGH to indicate a write cycle. During the actual cycle, nENOUT is driven
LOW to indicate that the ARM7TDMI is driving D[31:0] as an output. ➲
Figure 6-14:
Data write bus cycle
shows this bus timing (DBE has been tied HIGH in this example).
➲
Figure 6-15: ARM7TDMI data bus control circuit
on page 6-21 shows the circuit
which exists in ARM7TDMI for controlling exactly when the external bus is driven out.
ARM7TDMI
nENOUT
DOUT[31:0]
DIN[31:0]
PAD
XDATA[31:0]
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-19
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Figure 6-14: Data write bus cycle
The ARM7TDMI macrocell has an additional bus control signal, nENIN, which allows
the external system to manually tristate the bus. In the simplest systems, nENIN can
be tied LOW and nENOUT can be ignored. However, in many applications when the
external data bus is a shared resource, greater control may be required. In this
situation, nENIN can be used to delay when the external bus is driven. Note that for
backwards compatibility ,DBE is also included. At the macrocell level,DBE andnENIN
have almost identical functionality and in most applications one can be tied off.
Section ➲
6.10.3 Example system: The ARM7TDMI Testchip
on page 6-21 describes
how ARM7TDMI may be interfaced to an external data bus, using ARM7TDMI
Testchip as an example.
ARM7TDMI has another output control signal called TBE. This signal is normally only
used during test and must be tied HIGH when not in use. When driven LOW, TBE
forces all three-stateable outputs to high impedance. It is as if both DBE and ABE
have been driven LOW, causing the data bus, the address bus, and all other signals
normally controlled by ABE to become high impedance. Note, however, that there is
no scan cell on TBE. Thus, TBE is completely independent of scan data and may be
used to put the outputs into a high impedance state while scan testing takes place.
➲
Table 6-3: Output enable control summary
, below, shows the tri-state control of
ARM7TDMI’s outputs.
MCLK
A[31:0]
nRW
nENOUT
D[31:0]
Memory Cycle
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-20
Open Access
Signals without ✔in the ABE,DBE or TBE column cannot be driven to the high
impedance state:
ARM7TDMI output ABE DBE TBE
A[31:0] ✔ ✔
D[31:0] ✔
nRW ✔ ✔
LOCK ✔ ✔
MAS[1:0] ✔ ✔
nOPC ✔ ✔
nTRANS ✔ ✔
DBGACK
ECLK
nCPI
nENOUT
nEXEC
nM[4:0]
TBIT
nMREQ
SDOUTMS
SDOUTDATA
SEQ
DOUT[31:0]
Table 6-3: Output enable control summary
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-21
Open Access
Figure 6-15: ARM7TDMI data bus control circuit
6.10.3 Example system: The ARM7TDMI Testchip
Connecting ARM7TDMI’s data bus, D[31:0] to an external shared bus requires some
simple additional logic. This will vary from application to application. As an example,
the following describes how the ARM7TDMI macrocell was connected to the
bi-directional data bus pads of the ARM7TDMI testchip.
In this application, care must be taken to prevent bus clash on D[31:0] when the data
bus drive changes direction. The timing of nENIN, and the pad control signals must be
arranged so that when the core starts to drive out, the pad drive onto D[31:0] switches
off before the core starts to drive. Similarly, when the bus switches back to input, the
core must stop driving before the pad switches on.
All this can be achieved using a simple non-overlapping clock generator. The actual
circuit implemented in the ARM7TDMI testchip is shown in ➲
Figure 6-16: The
ARM7TDMI Testchip data bus circuit
on page 6-22. Note that at the core level, TBE
andDBE are tied HIGH (inactive). This is because in a packaged part, there is no need
nENOUT
nENIN
D[31:0]
DBE
TBE
Core Control
Scan
Cell
Scan
Cell
Scan
Cell
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-22
Open Access
to ever manually force the internal buses into a high impedance state. Note also that
at the pad level, the signal EDBE is factored into the bus control logic. This allows the
external memory controller to arbitrate the bus and asynchronously disable
ARM7TDMI testchip if required.
Figure 6-16: The ARM7TDMI Testchip data bus circuit
➲
Figure 6-17: Data bus control signal timing
on page 6-23 shows how the various
control signals interact. Under normal conditions, when the data bus is configured as
input, nENOUT is HIGH, nEN1 is LOW, and nEN2/nENIN is HIGH. Thus the pads
drive XD[31:0] onto D[31:0].
When a write cycle occurs, nRW is driven HIGH to indicate a write during phase 2 of
the previous cycle, (ie, with the address). During phase 1 of the actual cycle,nENOUT
is driven LOW to indicate that ARM7TDMI is about to drive the bus. The falling edge
of this signal makes nEN1 go HIGH, which disables the input half pad from driving
D[31:0]. This in turn makes nEN2 go LOW, which enables the output half of the pad
so that the ARM7TDMI is now driving the external data bus, XD[31:0].nEN2 is then
buffered and driven back into the core on nENIN, so that finally the ARM7TDMI
macrocell drives D[31:0]. The delay between all the signals ensures that there is no
clash on the data bus as it changes direction from input to output.
Pad
nENOUT
nENIN
D[31:0]
nEN2
nEN1
XD[31:0]
ARM7TDMI testchip
EDBE
Vdd
DBE
SRL
SRL
SRL
Vdd
TBE
ARM7TDMI
Core
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-23
Open Access
Figure 6-17: Data bus control signal timing
When the bus turns around to the other direction at the end of the cycle, the various
control signals switch the other way. Again, the non-overlap ensures that there is
never a bus clash. This time, nENOUT is driven HIGH to denote that ARM7TDMI no
longer needs to drive the bus and the core’s output is immediately switched off. This
causes nEN2 to disable the output half of the pad which in turn causes nEN1 to switch
on the input half. Thus, the bus is back to its original input configuration.
Note that the data out time of ARM7TDMI is not directly determined by nENOUT and
nENIN, and so delaying exactly when the bus is driven will not affect the propagation
delay. Please refer to ➲
Chapter 11, DC Parameters
for timing details.
nENOUT
nEN1
nEN2 /
nENIN
D[31:0]
Memory Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
6-24
Open Access
ARM7TDMI Data Sheet
ARM DDI 0029E
7-1
11
1
Open Access
Coprocessor Interface
The functionality of the ARM7TDMI instruction set can be extended by adding external
coprocessors. This chapter describes the ARM7TDMI coprocessor interface.
7.1 Overview 7-2
7.2 Interface Signals 7-2
7.3 Register Transfer Cycle 7-3
7.4 Privileged Instructions 7-3
7.5 Idempotency 7-4
7.6 Undefined Instructions 7-4
7
Coprocessor Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
7-2
Open Access
7.1 Overview
The functionality of the ARM7TDMI instruction set may be extended by the addition of
up to 16 external coprocessors. When the coprocessor is not present, instructions
intended for it will trap, and suitable software may be installed to emulate its functions.
Adding the coprocessor will then increase the system performance in a software
compatible way. Note that some coprocessor numbers have already been assigned.
Contact ARM Ltd for up-to-date information.
7.2 Interface Signals
Three dedicated signals control the coprocessor interface, nCPI,CPA and CPB. The
CPA and CPB inputs should be driven HIGH except when they are being used for
handshaking.
7.2.1 Coprocessor present/absent
ARM7TDMI takes nCPI LOW whenever it starts to execute a coprocessor (or
undefined) instruction. (This will not happen if the instruction fails to be executed
because of the condition codes.) Each coprocessor will have a copy of the instruction,
and can inspect the CP# field to see which coprocessor it is for. Every coprocessor in
a system must have a unique number and if that number matches the contents of the
CP# field the coprocessor should drive the CPA (coprocessor absent) line LOW. If no
coprocessor has a number which matches the CP# field, CPA and CPB will remain
HIGH, and ARM7TDMI will take the undefined instruction trap. Otherwise ARM7TDMI
observes the CPA line going LOW, and waits until the coprocessor is not busy.
7.2.2 Busy-waiting
If CPA goes LOW, ARM7TDMI will watch the CPB (coprocessor busy) line. Only the
coprocessor which is driving CPA LOW is allowed to driveCPB LOW , and it should do
so when it is ready to complete the instruction. ARM7TDMI will busy-wait while CPB
is HIGH, unless an enabled interrupt occurs, in which case it will break off from the
coprocessor handshake to process the interrupt. Normally ARM7TDMI will return from
processing the interrupt to retry the coprocessor instruction.
When CPB goes LOW, the instruction continues to completion. This will involve data
transfers taking place between the coprocessor and either ARM7TDMI or memory,
except in the case of coprocessor data operations which complete immediately the
coprocessor ceases to be busy.
All three interface signals are sampled by both ARM7TDMI and the coprocessor(s) on
the rising edge of MCLK. If all three are LOW, the instruction is committed to
execution, and if transfers are involved they will start on the next cycle. If nCPI has
gone HIGH after being LOW , and before the instruction is committed, ARM7TDMI has
broken off from the busy-wait state to service an interrupt. The instruction may be
restarted later, but other coprocessor instructions may come sooner, and the
instruction should be discarded.
Coprocessor Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
7-3
Open Access
7.2.3 Pipeline following
In order to respond correctly when a coprocessor instruction arises, each coprocessor
must have a copy of the instruction. All ARM7TDMI instructions are fetched from
memory via the main data bus, and coprocessors are connected to this bus, so they
can keep copies of all instructions as they go into the ARM7TDMI pipeline. The nOPC
signal indicates when an instruction fetch is taking place, and MCLK gives the timing
of the transfer , so these may be used together to load an instruction pipeline within the
coprocessor.
7.2.4 Data transfer cycles
Once the coprocessor has gone not-busy in a data transfer instruction, it must supply
or accept data at the ARM7TDMI bus rate (defined by MCLK). It can deduce the
direction of transfer by inspection of the L bit in the instruction, but must only drive the
bus when permitted to by DBE being HIGH. The coprocessor is responsible for
determining the number of words to be transferred; ARM7TDMI will continue to
increment the address by one word per transfer until the coprocessor tells it to stop.
The termination condition is indicated by the coprocessor drivingCPA and CPB HIGH.
There is no limit in principle to the number of words which one coprocessor data
transfer can move, but by convention no coprocessor should allow more than 16
words in one instruction. More than this would worsen the worst case ARM7TDMI
interrupt latency, as the instruction is not interruptible once the transfers have
commenced. At 16 words, this instruction is comparable with a block transfer of 16
registers, and therefore does not affect the worst case latency.
7.3 Register Transfer Cycle
The coprocessor register transfer cycle is the one case when ARM7TDMI requires the
data bus without requiring the memory to be active. The memory system is informed
that the bus is required by ARM7TDMI taking both nMREQ and SEQ HIGH. When the
bus is free, DBE should be taken HIGH to allow ARM7TDMI or the coprocessor to
drive the bus, and an MCLK cycle times the transfer.
7.4 Privileged Instructions
The coprocessor may restrict certain instructions for use in privileged modes only. To
do this, the coprocessor will have to track the nTRANS output.
As an example of the use of this facility, consider the case of a floating point
coprocessor (FPU) in a multi-tasking system. The operating system could save all the
floating point registers on every task switch, but this is inefficient in a typical system
where only one or two tasks will use floating point operations. Instead, there could be
a privileged instruction which turns the FPU on or off. When a task switch happens,
the operating system can turn the FPU off without saving its registers. If the new task
attempts an FPU operation, the FPU will appear to be absent, causing an undefined
instruction trap. The operating system will then realise that the new task requires the
FPU, so it will re-enable it and save FPU registers. The task can then use the FPU as
normal. If, however , the new task never attempts an FPU operation (as will be the case
for most tasks), the state saving overhead will have been avoided.
Coprocessor Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
7-4
Open Access
7.5 Idempotency
A consequence of the implementation of the coprocessor interface, with the
interruptible busy-wait state, is that all instructions may be interrupted at any point up
to the time when the coprocessor goes not-busy. If so interrupted, the instruction will
normally be restarted from the beginning after the interrupt has been processed. It is
therefore essential that any action taken by the coprocessor before it goes not-busy
must be idempotent, ie must be repeatable with identical results.
For example, consider a FIX operation in a floating point coprocessor which returns
the integer result to an ARM7TDMI register. The coprocessor must stay busy while it
performs the floating point to fixed point conversion, as ARM7TDMI will expect to
receive the integer value on the cycle immediately following that where it goes not-
busy. The coprocessor must therefore preserve the original floating point value and
not corrupt it during the conversion, because it will be required again if an interrupt
arises during the busy period.
The coprocessor data operation class of instruction is not generally subject to
idempotency considerations, as the processing activity can take place after the
coprocessor goes not-busy. There is no need for ARM7TDMI to be held up until the
result is generated, because the result is confined to stay within the coprocessor.
7.6 Undefined Instructions
Undefined instructions are treated by ARM7TDMI as coprocessor instructions. All
coprocessors must be absent (ie CPA and CPB must be HIGH) when an undefined
instruction is presented. ARM7TDMI will then take the undefined instruction trap. Note
that the coprocessor need only look at bit 27 of the instruction to differentiate
undefined instructions (which all have 0 in bit 27) from coprocessor instructions (which
all have 1 in bit 27)
Note that when in THUMB state, coprocessor instructions are not supported but
undefined instructions are. Thus, all coprocessors must monitor the state of the TBIT
output from ARM7TDMI. When ARM7TDMI is in THUMB state, coprocessors must
appear absent (ie they must drive CPA and CPB HIGH) and the instructions seen on
the data bus must be ignored. In this way, coprocessors will not erroneously execute
THUMB instructions, and all undefined instructions will be handled correctly.
ARM7TDMI Data Sheet
ARM DDI 0029E
8-1
11
1
Open Access
Debug Interface
This chapter describes the ARM7TDMI advanced debug interface.
8.1 Overview 8-2
8.2 Debug Systems 8-2
8.3 Debug Interface Signals 8-3
8.4 Scan Chains and JTAG Interface 8-6
8.5 Reset 8-8
8.6 Pullup Resistors 8-9
8.7 Instruction Register 8-9
8.8 Public Instructions 8-9
8.9 Test Data Registers 8-12
8.10 ARM7TDMI Core Clocks 8-18
8.11 Determining the Core and System State 8-19
8.12 The PC’s Behaviour During Debug 8-23
8.13 Priorities / Exceptions 8-25
8.14 Scan Interface Timing 8-26
8
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-2
Open Access
8.1 Overview
The ARM7TDMI debug interface is based on IEEE Std. 1149.1- 1990, “
Standard Test
Access Port and Boundary-Scan Architecture”
. Please refer to this standard for an
explanation of the terms used in this chapter and for a description of the T AP controller
states.
ARM7TDMI contains hardware extensions for advanced debugging features. These
are intended to ease the user’s development of application software, operating
systems, and the hardware itself.
The debug extensions allow the core to be stopped either on a given instruction fetch
(breakpoint) or data access (watchpoint), or asynchronously by a debug-request.
When this happens, ARM7TDMI is said to be in
debug state
. At this point, the core’s
internal state and the system’s external state may be examined. Once examination is
complete, the core and system state may be restored and program execution
resumed.
ARM7TDMI is forced into debug state either by a request on one of the external debug
interface signals, or by an internal functional unit known as
ICEBreaker
. Once in debug
state, the core isolates itself from the memory system. The core can then be examined
while all other system activity continues as normal.
ARM7TDMI’s internal state is examined via a JT AG-style serial interface, which allows
instructions to be serially inserted into the core’s pipeline without using the external
data bus. Thus, when in debug state, a store-multiple (STM) could be inserted into the
instruction pipeline and this would dump the contents of ARM7TDMI’s registers. This
data can be serially shifted out without affecting the rest of the system.
8.2 Debug Systems
The ARM7TDMI forms one component of a debug system that interfaces from the
high-level debugging performed by the user to the low-level interface supported by
ARM7TDMI. Such a system typically has three parts:
1 The Debug Host
This is a computer, for example a PC, running a software debugger such as
ARMSD. The debug host allows the user to issue high level commands such
as “set breakpoint at location XX”, or “examine the contents of memory from
0x0 to 0x100”.
2 The Protocol Converter
The Debug Host will be connected to the ARM7TDMI development system via
an interface (an RS232, for example). The messages broadcast over this
connection must be converted to the interface signals of the ARM7TDMI, and
this function is performed by the protocol converter.
3 ARM7TDMI
ARM7TDMI, with hardware extensions to ease debugging, is the lowest level
of the system. The debug extensions allow the user to stall the core from
program execution, examine its internal state and the state of the memory
system, and then resume program execution.
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-3
Open Access
Figure 8-1: Typical debug system
The anatomy of ARM7TDMI is shown in ➲
Figure 8-3: ARM7TDMI scan chain
arrangement
on page 8-7. The major blocks are:
ARM7TDMI This is the CPU core, with hardware support for debug.
ICEBreaker This is a set of registers and comparators used to generate
debug exceptions (eg breakpoints). This unit is described in
➲
Chapter 9, ICEBreaker Module
.
TAP controller This controls the action of the scan chains via a JTAG serial
interface.
The Debug Host and the Protocol Converter are system dependent. The rest of this
chapter describes the ARM7TDMI’s hardware debug extensions.
8.3 Debug Interface Signals
There are three primary external signals associated with the debug interface:
•BREAKPT and DBGRQ
with which the system requests ARM7TDMI to enter debug state.
• DBGACK
which ARM7TDMI uses to flag back to the system that it is in debug state.
8.3.1 Entry into debug state
ARM7TDMI is forced into debug state after a breakpoint, watchpoint or debug-request
has occurred.
Conditions under which a breakpoint or watchpoint occur can be programmed using
ICEBreaker. Alternatively, external logic can monitor the address and data bus, and
flag breakpoints and watchpoints via the BREAKPT pin.
Host computer running ARMSD
Protocol
Converter
Development System
Containing ARM7TDMI
Debug
Host
Debug
Target
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-4
Open Access
The timing is the same for externally generated breakpoints and watchpoints. Data
must always be valid around the falling edge of MCLK. If this data is an instruction to
be breakpointed, the BREAKPT signal must be HIGH around the next rising edge of
MCLK. Similarly, if the data is for a load or store, this can be marked as watchpointed
by asserting BREAKPT around the next rising edge of MCLK.
When a breakpoint or watchpoint is generated, there may be a delay before
ARM7TDMI enters debug state. When it does, the DBGACK signal is asserted in the
HIGH phase of MCLK. The timing for an externally generated breakpoint is shown in
➲
Figure 8-2: Debug state entry
.
Figure 8-2: Debug state entry
Entry into debug state on breakpoint
After an instruction has been breakpointed, the core does not enter debug state
immediately. Instructions are marked as being breakpointed as they enter
ARM7TDMI's instruction pipeline.
Thus ARM7TDMI only enters debug state when (and if) the instruction reaches the
pipeline’s execute stage.
MCLK
A[31:0]
D[31:0]
BREAKPT
DBGACK
nMREQ
SEQ
Memory Cycles Internal Cycles
‘
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-5
Open Access
A breakpointed instruction may not cause ARM7TDMI to enter debug state for one of
two reasons:
• a branch precedes the breakpointed instruction.
When the branch is executed, the instruction pipeline is flushed and the
breakpoint is cancelled.
• an exception has occurred.
Again, the instruction pipeline is flushed and the breakpoint is cancelled.
However, the normal way to exit from an exception is to branch back to the
instruction that would have executed next. This involves refilling the pipeline,
and so the breakpoint can be re-flagged.
When a breakpointed conditional instruction reaches the execute stage of the pipeline,
the breakpoint is
always
taken and ARM7TDMI enters debug state, regardless of
whether the condition was met.
Breakpointed instructions
do not
get executed: instead, ARM7TDMI enters debug
state. Thus, when the internal state is examined, the state
before
the breakpointed
instruction is seen. Once examination is complete, the breakpoint should be removed
and program execution restarted from the previously breakpointed instruction.
Entry into debug state on watchpoint
Watchpoints occur on data accesses. A watchpoint is always taken, but the core may
not enter debug state immediately. In all cases, the current instruction will complete. If
this is a multi-word load or store (LDM or STM), many cycles may elapse before the
watchpoint is taken.
Watchpoints can be thought of as being similar to data aborts. The difference is
however that if a data abort occurs, although the instruction completes, all subsequent
changes to ARM7TDMI’s state are prevented. This allows the cause of the abort to be
cured by the abort handler, and the instruction re-executed. This is not so in the case
of a watchpoint. Here, the instruction completes and all changes to the core’s state
occur (ie load data is written into the destination registers, and base write-back
occurs). Thus the instruction does not need to be restarted.
Watchpoints are
always
taken. If an exception is pending when a watchpoint occurs,
the core enters debug state in the mode of that exception.
Entry into debug state on debug-request
ARM7TDMI may also be forced into debug state on debug request. This can be done
either through ICEBreaker programming (see➲
Chapter 9, ICEBreaker Module
), or by
the assertion of the DBGRQ pin. This pin is an asynchronous input and is thus
synchronised by logic inside ARM7TDMI before it takes effect. Following
synchronisation, the core will normally enter debug state at the end of the current
instruction. However, if the current instruction is a busy-waiting access to a
coprocessor, the instruction terminates and ARM7TDMI enters debug state
immediately (this is similar to the action of nIRQ and nFIQ).
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-6
Open Access
Action of ARM7TDMI in debug state
Once ARM7TDMI is in debug state, nMREQ and SEQ are forced to indicate internal
cycles. This allows the rest of the memory system to ignore ARM7TDMI and function
as normal. Since the rest of the system continues operation, ARM7TDMI must be
forced to ignore aborts and interrupts.
The BIGEND signal should not be changed by the system during debug. If it changes,
not only will there be a synchronisation problem, but the programmer’s view of
ARM7TDMI will change without the debugger’s knowledge. nRESET must also be
held stable during debug. If the system applies reset to ARM7TDMI (ie. nRESET is
driven LOW) then ARM7TDMI’s state will change without the debugger’s knowledge.
The BL[3:0] signals must remain HIGH while ARM7TDMI is clocked by DCLK in
debug state to ensure all of the data in the scan cells is correctly latched by the internal
logic.
When instructions are executed in debug state, ARM7TDMI outputs (except nMREQ
andSEQ) will change asynchronously to the memory system. For example, every time
a new instruction is scanned into the pipeline, the address bus will change. Although
this is asynchronous it should not affect the system, sincenMREQ andSEQ are forced
to indicate internal cycles regardless of what the rest of ARM7TDMI is doing. The
memory controller must be designed to ensure that this asynchronous behaviour does
not affect the rest of the system.
8.4 Scan Chains and JTAG Interface
There are three JTAG style scan chains inside ARM7TDMI. These allow testing,
debugging and ICEBreaker programming. The scan chains are controlled from a
JTAG style TAP (Test Access Port) controller. For further details of the JTAG
specification, please refer to IEEE Standard 1 149.1 - 1990
“Standard T est Access Port
and Boundary-Scan Architecture”
. In addition, support is provided for an optional
fourth scan chain. This is intended to be used for an external boundary scan chain
around the pads of a packaged device. The control signals provided for this scan chain
are described later.
Note The scan cells are not fully JTAG compliant. The following sections describe the
limitations on their use.
8.4.1 Scan limitations
The three scan paths are referred to as scan chain 0, 1 and 2: these are shown in
➲
Figure 8-3: ARM7TDMI scan chain arrangement
on page 8-7.
Scan chain 0
Scan chain 0 allows access to the entire periphery of the ARM7TDMI core, including
the data bus. The scan chain functions allow inter-device testing (EXTEST) and serial
testing of the core (INTEST).
The order of the scan chain (from SDIN to SDOUTMS) is: data bus bits 0 through 31,
the control signals, followed by the address bus bits 31 through 0.
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-7
Open Access
Scan chain 1
Scan chain 1 is a subset of the signals that are accessible through scan chain 0.
Access to the core’s data bus D[31:0], and the BREAKPT signal is available serially.
There are 33 bits in this scan chain, the order being (from serial data in to out): data
bus bits 0 through 31, followed by BREAKPT.
Scan Chain 2
This scan chain simply allows access to the ICEBreaker registers. Refer to ➲
Chapter
9, ICEBreaker Module
for details.
Figure 8-3: ARM7TDMI scan chain arrangement
ARM7TDMI
Processor
ARM7TDMI
ICEbreaker
ARM7TDMI
TAP Controller
•
•
Scan Chain 1
Scan Chain 0
Scan Chain 2
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-8
Open Access
8.4.2 The JTAG state machine
The process of serial test and debug is best explained in conjunction with the JTAG
state machine.➲
Figure 8-4: Test access port (TAP) controller state transitions
shows
the state transitions that occur in the TAP controller.
The state numbers are also shown on the diagram. These are output from ARM7TDMI
on the TAPSM[3:0] bits.
Figure 8-4: Test access port (TAP) controller state transitions
8.5 Reset
The boundary-scan interface includes a state-machine controller (the TAP controller).
In order to force the TAP controller into the correct state after power-up of the device,
a reset pulse must be applied to thenTRST signal. If the boundary scan interface is to
Select-IR-Scan
Capture-IR
tms=0
Shift-IR
tms=0
Exit1-IR
tms=1
Pause-IR
tms=0
Exit2-IR
tms=1
Update-IR
tms=1
tms=0
tms=0
tms=1
tms=1
tms=0
Select-DR-Scan
Capture-DR
tms=0
Shift-DR
tms=0
Exit1-DR
tms=1
Pause-DR
tms=0
Exit2-DR
tms=1
Update-DR
tms=1
Test-Logic Reset
Run-Test/Idle
tms=0
tms=1
tms=0
tms=0
tms=0
tms=1
tms=1
tms=0
tms=1 tms=1
tms=1
tms=1 tms=1tms=0 tms=0
0xF
0xC 0x7 0x4
0xE
0xA
0x9
0xB
0x8
0xD
0x5
0x0
0x3
0x1
0x2
0x6
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-9
Open Access
be used, nTRST must be driven LOW, and then HIGH again. If the boundary scan
interface is not to be used, the nTRST input may be tied permanently LOW. Note that
a clock on TCK is not necessary to reset the device.
The action of reset is as follows:
1 System mode is selected (ie the boundary scan chain cells do
not
intercept
any of the signals passing between the external system and the core).
2 The IDCODE instruction is selected. If the TAP controller is put into the Shift-
DR state and TCK is pulsed, the contents of the ID register will be clocked out
of TDO.
8.6 Pullup Resistors
The IEEE 1149.1 standard effectively requires that TDI and TMS should have internal
pullup resistors. In order to minimise static current draw, these resistors are
not
fitted
to ARM7TDMI. Accordingly, the 4 inputs to the test interface (the above 3 signals plus
TCK) must all be driven to good logic levels to achieve normal circuit operation.
8.7 Instruction Register
The instruction register is 4 bits in length.
There is no parity bit. The fixed value loaded into the instruction register during the
CAPTURE-IR controller state is 0001.
8.8 Public Instructions
The following public instructions are supported:
Instruction Binary Code
EXTEST 0000
SCAN_N 0010
INTEST 1100
IDCODE 1110
BYPASS 1111
CLAMP 0101
HIGHZ 0111
CLAMPZ 1001
SAMPLE/PRELOAD 0011
RESTART 0100
Table 8-1: Public instructions
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In the descriptions that follow, TDI and TMS are sampled on the rising edge of TCK
and all output transitions on TDO occur as a result of the falling edge of TCK.
8.8.1 EXTEST (0000)
The selected scan chain is placed in test mode by the EXTEST instruction.
The EXTEST instruction connects the selected scan chain between TDI and TDO.
When the instruction register is loaded with the EXTEST instruction, all the scan cells
are placed in their test mode of operation.
In the CAPTURE-DR state, inputs from the system logic and outputs from the output
scan cells to the system are captured by the scan cells. In the SHIFT-DR state, the
previously captured test data is shifted out of the scan chain via TDO, while new test
data is shifted in via the TDI input. This data is applied immediately to the system logic
and system pins.
8.8.2 SCAN_N (0010)
This instruction connects the Scan Path Select Register between TDI and TDO.
During the CAPTURE-DR state, the fixed value 1000 is loaded into the register . During
the SHIFT-DR state, the ID number of the desired scan path is shifted into the scan
path select register. In the UPDATE-DR state, the scan register of the selected scan
chain is connected between TDI and TDO, and remains connected until a subsequent
SCAN_N instruction is issued. On reset, scan chain 3 is selected by default. The scan
path select register is 4 bits long in this implementation, although no finite length is
specified.
8.8.3 INTEST (1100)
The selected scan chain is placed in test mode by the INTEST instruction.
The INTEST instruction connects the selected scan chain between TDI and TDO.
When the instruction register is loaded with the INTEST instruction, all the scan cells
are placed in their test mode of operation.
In the CAPTURE-DR state, the value of the data applied from the core logic to the
output scan cells, and the value of the data applied from the system logic to the input
scan cells is captured.
In the SHIFT-DR state, the previously captured test data is shifted out of the scan
chain via the TDO pin, while new test data is shifted in via the TDI pin.
Single-step operation is possible using the INTEST instruction.
8.8.4 IDCODE (1110)
The IDCODE instruction connects the device identification register (or ID register)
between TDI and TDO. The ID register is a 32-bit register that allows the
manufacturer, part number and version of a component to be determined through the
TAP. See ➲
8.9.2 ARM7TDMI device identification (ID) code register
on page 8-13 for
the details of the ID register format.
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When the instruction register is loaded with the IDCODE instruction, all the scan cells
are placed in their normal (system) mode of operation.
In the CAPTURE-DR state, the device identification code is captured by the ID
register. In the SHIFT-DR state, the previously captured device identification code is
shifted out of the ID register via the TDO pin, while data is shifted in via the TDI pin
into the ID register. In the UPDATE-DR state, the ID register is unaffected.
8.8.5 BYPASS (1111)
The BYP ASS instruction connects a 1 bit shift register (the BYP ASS register) between
TDI and TDO.
When the BYPASS instruction is loaded into the instruction register, all the scan cells
are placed in their normal (system) mode of operation. This instruction has no effect
on the system pins.
In the CAPTURE-DR state, a logic 0 is captured by the bypass register. In the SHIFT-
DR state, test data is shifted into the bypass register via TDI and out via TDO after a
delay of one TCK cycle. Note that the first bit shifted out will be a zero. The bypass
register is not affected in the UPDA TE-DR state. Note that all unused instruction codes
default to the BYPASS instruction.
8.8.6 CLAMP (0101)
This instruction connects a 1 bit shift register (the BYPASS register) between TDI and
TDO.
When the CLAMP instruction is loaded into the instruction register, the state of all the
output signals is defined by the values previously loaded into the currently loaded scan
chain.
Note This instruction should only be used when scan chain 0 is the currently selected scan
chain.
In the CAPTURE-DR state, a logic 0 is captured by the bypass register. In the SHIFT-
DR state, test data is shifted into the bypass register via TDI and out via TDO after a
delay of one TCK cycle. Note that the first bit shifted out will be a zero. The bypass
register is not affected in the UPDATE-DR state.
8.8.7 HIGHZ (0111)
This instruction connects a 1 bit shift register (the BYPASS register) between TDI and
TDO.
When the HIGHZ instruction is loaded into the instruction register, the Address bus,
A[31:0], the data bus, D[31:0], plus nRW,nOPC,LOCK,MAS[1:0] and nTRANS are
all driven to the high impedance state and the external HIGHZ signal is driven HIGH.
This is as if the signal TBE had been driven LOW.
In the CAPTURE-DR state, a logic 0 is captured by the bypass register. In the
SHIFT-DR state, test data is shifted into the bypass register via TDI and out via TDO
after a delay of one TCK cycle. Note that the first bit shifted out will be a zero. The
bypass register is not affected in the UPDATE-DR state.
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8.8.8 CLAMPZ (1001)
This instruction connects a 1 bit shift register (the BYPASS register) between TDI and
TDO.
When the CLAMPZ instruction is loaded into the instruction register, all the 3-state
outputs (as described above) are placed in their inactive state, but the data supplied
to the outputs is derived from the scan cells. The purpose of this instruction is to
ensure that, during production test, each output can be disabled when its data value
is either a logic 0 or a logic 1.
In the CAPTURE-DR state, a logic 0 is captured by the bypass register. In the SHIFT-
DR state, test data is shifted into the bypass register via TDI and out via TDO after a
delay of one TCK cycle. Note that the first bit shifted out will be a zero. The bypass
register is not affected in the UPDATE-DR state.
8.8.9 SAMPLE/PRELOAD (0011)
This instruction is included for production test only, and should never be used.
8.8.10 RESTART (0100)
This instruction is used to restart the processor on exit from debug state. The
RESTART instruction connects the bypass register between TDI and TDO and the
TAP controller behaves as if the BYP ASS instruction had been loaded. The processor
will resynchronise back to the memory system once the RUN-TEST/IDLE state is
entered.
8.9 Test Data Registers
There are 6 test data registers which may be connected between TDI andTDO. They
are: Bypass Register, ID Code Register, Scan Chain Select Register, Scan chain 0, 1
or 2. These are now described in detail.
8.9.1 Bypass register
Purpose: Bypasses the device during scan testing by providing a path
between TDI and TDO.
Length: 1 bit
Operating Mode: When the BYP ASS instruction is the current instruction in the
instruction register , serial data is transferred from TDI to TDO
in the SHIFT-DR state with a delay of one TCK cycle.
There is no parallel output from the bypass register.
A logic 0 is loaded from the parallel input of the bypass
register in the CAPTURE-DR state.
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8.9.2 ARM7TDMI device identification (ID) code register
Purpose: Reads the 32-bit device identification code. No
programmable supplementary identification code is provided.
Length: 32 bits. The format of the ID register is as follows:
Please contact your supplier for the correct Device Identification Code.
Operating mode:
When the IDCODE instruction is current, the ID register is selected as the serial path
between TDI and TDO.
There is no parallel output from the ID register.
The 32-bit device identification code is loaded into the ID register from its parallel
inputs during the CAPTURE-DR state.
8.9.3 Instruction register
Purpose: Changes the current TAP instruction.
Length: 4 bits
Operating mode: When in the SHIFT-IR state, the instruction register is
selected as the serial path between TDI and TDO.
During the CAPTURE-IR state, the value 0001 binary is loaded into this register. This
is shifted out during SHIFT-IR (lsb first), while a new instruction is shifted in (lsb first).
During the UPDATE-IR state, the value in the instruction register becomes the current
instruction. On reset, IDCODE becomes the current instruction.
8.9.4 Scan chain select register
Purpose: Changes the current active scan chain.
Length: 4 bits
Operating mode: After SCAN_N has been selected as the current instruction,
when in the SHIFT -DR state, the Scan Chain Select Register
is selected as the serial path between TDI and TDO.
During the CAPTURE-DR state, the value 1000 binary is loaded into this register . This
is shifted out during SHIFT-DR (lsb first), while a new value is shifted in (lsb first).
During the UPDA TE-DR state, the value in the register selects a scan chain to become
the currently active scan chain. All further instructions such as INTEST then apply to
that scan chain.
011112272831
1Manufacturer IdentityPart NumberVersion
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The currently selected scan chain only changes when a SCAN_N instruction is
executed, or a reset occurs. On reset, scan chain 3 is selected as the active scan
chain.
The number of the currently selected scan chain is reflected on the SCREG[3:0]
outputs. The TAP controller may be used to drive external scan chains in addition to
those within the ARM7TDMI macrocell. The external scan chain must be assigned a
number and control signals for it can be derived from SCREG[3:0],IR[3:0],
TAPSM[3:0],TCK1 and TCK2.
The list of scan chain numbers allocated by ARM are shown in➲
Table 8-2: Scan chain
number allocation
. An external scan chain may take any other number .The serial data
stream to be applied to the external scan chain is made present on SDINBS, the serial
data back from the scan chain must be presented to the TAP controller on the
SDOUTBS input. The scan chain present between SDINBS and SDOUTBS will be
connected between TDI and TDO whenever scan chain 3 is selected, or when any of
the unassigned scan chain numbers is selected. If there is more than one external
scan chain, a multiplexor must be built externally to apply the desired scan chain
output to SDOUTBS. The multiplexor can be controlled by decoding SCREG[3:0].
8.9.5 Scan chains 0,1 and 2
These allow serial access to the core logic, and to ICEBreaker for programming
purposes. They are described in detail below.
Scan chain 0 and 1
Purpose: Allows access to the processor core for test and debug.
Length: Scan chain 0: 105 bits
Scan chain 1: 33 bits
Each scan chain cell is fairly simple, and consists of a serial register and a multiplexer .
The scan cells perform two basic functions,
capture
and
shift
.
For input cells, the capture stage involves copying the value of the system input to the
core into the serial register . During shift, this value is output serially. The value applied
to the core from an input cell is either the system input or the contents of the serial
register, and this is controlled by the multiplexer.
Scan Chain Number Function
0 Macrocell scan test
1 Debug
2 ICEbreaker programming
3 External boundary scan
4 Reserved
8 Reserved
Table 8-2: Scan chain number allocation
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Figure 8-5: Input scan cell
For output cells, capture involves placing the value of a core’s output into the serial
register. During shift, this value is serially output as before. The value applied to the
system from an output cell is either the core output, or the contents of the serial
register.
All the control signals for the scan cells are generated internally by the TAP controller.
The action of the TAP controller is determined by the current instruction, and the state
of the TAP state machine. This is described below.
There are three basic modes of operation of the scan chains, INTEST, EXTEST and
SYSTEM, and these are selected by the various TAP controller instructions. In
SYSTEM mode, the scan cells are idle. System data is applied to inputs, and core
outputs are applied to the system. In INTEST mode, the core is internally tested. The
data serially scanned in is applied to the core, and the resulting outputs are captured
in the output cells and scanned out. In EXTEST mode, data is scanned onto the core's
outputs and applied to the external system. System input data is captured in the input
cells and then shifted out.
Note The scan cells are not fully JTAG compliant in that they do not have an
Update
stage.
Therefore, while data is being moved around the scan chain, the contents of the scan
cell is not isolated from the output. Thus the output from the scan cell to the core or to
the external system could change on every scan clock.
This does not affect ARM7TDMI since its internal state does not change until it is
clocked. However, the rest of the system needs to be aware that every output could
change asynchronously as data is moved around the scan chain. External logic must
ensure that this does not harm the rest of the system.
Shift
Register
Latch
System Data in
SHIFT Clock
Data to Core
Serial Data In
Serial Data Out
CAPTURE
Clock
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Scan chain 0
Scan chain 0 is intended primarily for inter-device testing (EXTEST), and testing the
core (INTEST). Scan chain 0 is selected via the SCAN_N instruction: see ➲
8.8.2
SCAN_N (0010)
on page 8-10.
INTEST allows serial testing of the core. The TAP Controller must be placed in
INTEST mode after scan chain 0 has been selected. During CAPTURE-DR, the
current outputs from the core’s logic are captured in the output cells. During SHIFT-
DR, this captured data is shifted out while a new serial test pattern is scanned in, thus
applying known stimuli to the inputs. During RUN-TEST/IDLE, the core is clocked.
Normally, the T AP controller should only spend 1 cycle in RUN-TEST/IDLE. The whole
operation may then be repeated.
For details of the core’s clocks during test and debug, see ➲
8.10 ARM7TDMI Core
Clocks
on page 8-18.
EXTEST allows inter-device testing, useful for verifying the connections between
devices on a circuit board. The TAP Controller must be placed in EXTEST mode after
scan chain 0 has been selected. During CAPTURE-DR, the current inputs to the core's
logic from the system are captured in the input cells. During SHIFT-DR, this captured
data is shifted out while a new serial test pattern is scanned in, thus applying known
values on the core’s outputs. During UPDATE-DR, the value shifted into the data bus
D[31:0] scan cells appears on the outputs. For all other outputs, the value appears as
the data is shifted round. Note, during RUN-TEST/IDLE, the core is not clocked. The
operation may then be repeated.
Scan chain 1
The primary use for scan chain 1 is for debugging, although it can be used for EXTEST
on the data bus. Scan chain 1 is selected via the SCAN_N TAP Controller instruction.
Debugging is similar to INTEST, and the procedure described above for scan chain 0
should be followed.
Note that this scan chain is 33 bits long - 32 bits for the data value, plus the scan cell
on the BREAKPT core input. This 33rd bit serves four purposes:
1Under normal INTEST test conditions, it allows a known value to be scanned
into the BREAKPT input.
2During EXTEST test conditions, the value applied to theBREAKPT input from
the system can be captured.
3 While debugging, the value placed in the 33rd bit determines whether
ARM7TDMI synchronises back to system speed before executing the
instruction. See➲
8.12.5 System speed access
on page 8-25 for further
details.
4 After ARM7TDMI has entered debug state, the first time this bit is captured
and scanned out, its value tells the debugger whether the core entered debug
state due to a breakpoint (bit 33 LOW), or a watchpoint (bit 33 HIGH).
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Scan chain 2
Purpose: Allows ICEBreaker's registers to be accessed. The order of
the scan chain, from TDI to TDO is: read/write, register
address bits 4 to 0, followed by data value bits 31 to 0. See
➲
Figure 9-2: ICEBreaker block diagram
on page 9-4.
Length: 38 bits.
To access this serial register , scan chain 2 must first be selected via the SCAN_N TAP
controller instruction. The TAP controller must then be place in INTEST mode. No
action is taken during CAPTURE-DR. During SHIFT-DR, a data value is shifted into
the serial register. Bits 32 to 36 specify the address of the ICEBreaker register to be
accessed. During UPDA TE-DR, this register is either read or written depending on the
value of bit 37 (0 = read). Refer to ➲
Chapter 9, ICEBreaker Module
for further details.
Scan chain 3
Purpose: Allows ARM7TDMI to control an external boundary scan
chain.
Length: User defined.
Scan chain 3 is provided so that an optional external boundary scan chain may be
controlled via ARM7TDMI. Typically this would be used for a scan chain around the
pad ring of a packaged device. The following control signals are provided which are
generated only when scan chain 3 has been selected. These outputs are inactive at
all other times.
DRIVEBS This would be used to switch the scan cells from system
mode to test mode. This signal is asserted whenever either
the INTEST, EXTEST, CLAMP or CLAMPZ instruction is
selected.
PCLKBS This is an update clock, generated in the UPDATE-DR state.
Typically the value scanned into a chain would be transferred
to the cell output on the rising edge of this signal.
ICAPCLKBS,ECAPCLKBS
These are capture clocks used to sample data into the scan
cells during INTEST and EXTEST respectively . These clocks
are generated in the CAPTURE-DR state.
SHCLKBS,SHCLK2BS
These are non-overlapping clocks generated in the SHIFT-
DR state used to clock the master and slave element of the
scan cells respectively. When the state machine is not in the
SHIFT-DR state, both these clocks are LOW.
nHIGHZ This signal may be used to drive the outputs of the scan cells
to the high impedance state. This signal is driven LOW when
the HIGHZ instruction is loaded into the instruction register,
and HIGH at all other times.
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In addition to these control outputs, SDINBS output and SDOUTBS input are also
provided. When an external scan chain is in use, SDOUTBS should be connected to
the serial data output and SDINBS should be connected to the serial data input.
8.10 ARM7TDMI Core Clocks
ARM7TDMI has two clocks, the memory clock, MCLK, and an internally TCK
generated clock, DCLK. During normal operation, the core is clocked by MCLK, and
internal logic holds DCLK LOW. When ARM7TDMI is in the debug state, the core is
clocked by DCLK under control of the TAP state machine, and MCLK may free run.
The selected clock is output on the signal ECLK for use by the external system. Note
that when the CPU core is being debugged and is running from DCLK,nWAIT has no
effect.
There are two cases in which the clocks switch: during debugging and during testing.
8.10.1 Clock switch during debug
When ARM7TDMI enters debug state, it must switch from MCLK to DCLK. This is
handled automatically by logic in the ARM7TDMI. On entry to debug state,
ARM7TDMI asserts DBGACK in the HIGH phase of MCLK. The switch between the
two clocks occurs on the next falling edge of MCLK. This is shown in ➲
Figure 8-6:
Clock Switching on entry to debug state
.
Figure 8-6: Clock Switching on entry to debug state
ARM7TDMI is forced to use DCLK as the primary clock until debugging is complete.
On exit from debug, the core must be allowed to synchronise back toMCLK. This must
be done in the following sequence. The final instruction of the debug sequence must
be shifted into the data bus scan chain and clocked in by assertingDCLK. At this point,
BYPASS must be clocked into the TAP instruction register. ARM7TDMI will now
automatically resynchronise back to MCLK and start fetching instructions from
memory at MCLK speed. Please refer also to ➲
8.11.3 Exit from debug state
on page
8-21.
MCLK
DBGACK
DCLK
ECLK
Multiplexer Switching
point
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8.10.2 Clock switch during test
When under serial test conditions—ie when test patterns are being applied to the
ARM7TM core through the JTAG interface—ARM7TDMI must be clocked using
DCLK. Entry into test is less automatic than debug and some care must be taken.
On the way into test, MCLK must be held LOW. The TAP controller can now be used
to serially test ARM7TDMI. If scan chain 0 and INTEST are selected, DCLK is
generated while the state machine is in the RUN-TEST/IDLE state. During EXTEST,
DCLK is not generated.
On exit from test, BYPASS must be selected as the TAP controller instruction. When
this is done, MCLK can be allowed to resume. After INTEST testing, care should be
taken to ensure that the core is in a sensible state before switching back. The safest
way to do this is to either select BYPASS and then cause a system reset, or to insert
MOV PC, #0 into the instruction pipeline before switching back.
8.11 Determining the Core and System State
When ARM7TDMI is in debug state, the core and system’s state may be examined.
This is done by forcing load and store multiples into the instruction pipeline.
Before the core and system state can be examined, the debugger must first determine
whether the processor was in THUMB or ARM state when it entered debug. This is
achieved by examining bit 4 of ICEbreaker’s Debug Status Register. If this is HIGH,
the core was in THUMB state when it entered debug.
8.11.1 Determining the core’s state
If the processor has entered debug state from THUMB state, the simplest course of
action is for the debugger to force the core back into ARM state. Once this is done, the
debugger can always execute the same sequence of instructions to determine the
processor's state.
To force the processor into ARM state, the following sequence of THUMB instructions
should be executed on the core:
STR R0, [R0] ; Save R0 before use
MOV R0, PC ; Copy PC into R0
STR R0, [R0] ; Now save the PC in R0
BX PC ; Jump into ARM state
MOV R8, R8 ; NOP
MOV R8, R8 ; NOP
Note Since all THUMB instructions are only 16 bits long, the simplest course of action when
shifting them into Scan Chain 1 is to repeat the instruction twice. For example, the
encoding for BX R0 is 0x4700. Thus if 0x47004700 is shifted into scan chain 1, the
debugger does not have to keep track of which half of the bus the processor expects
to read the data from.
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From this point on, the processor's state can be determined by the sequences of ARM
instructions described below.
Once the processor is in ARM state, typically the first instruction executed would be:
STM R0, {R0-R15}
This causes the contents of the registers to be made visible on the data bus. These
values can then be sampled and shifted out.
Note The above use of R0 as the base register for the STM is for illustration only, any
register could be used.
After determining the values in the current bank of registers, it may be desirable to
access the banked registers. This can only be done by changing mode. Normally, a
mode change may only occur if the core is already in a privileged mode. However,
while in debug state, a mode change from any mode into any other mode may occur.
Note that the debugger must restore the original mode before exiting debug state.
For example, assume that the debugger had been asked to return the state of the
USER mode and FIQ mode registers, and debug state was entered in supervisor
mode.
The instruction sequence could be:
STM R0, {R0-R15}; Save current registers
MRS R0, CPSR
STR R0, R0; Save CPSR to determine current mode
BIC R0, 0x1F; Clear mode bits
ORR R0, 0x10; Select user mode
MSR CPSR, R0; Enter USER mode
STM R0, {R13,R14}; Save register not previously visible
ORR R0, 0x01; Select FIQ mode
MSR CPSR, R0; Enter FIQ mode
STM R0, {R8-R14}; Save banked FIQ registers
All these instructions are said to execute at
debug speed
. Debug speed is much
slower than system speed since between each core clock, 33 scan clocks occur in
order to shift in an instruction, or shift out data. Executing instructions more slowly than
usual is fine for accessing the core’s state since ARM7TDMI is fully static. However,
this same method cannot be used for determining the state of the rest of the system.
While in debug state, only the following instructions may legally be scanned into the
instruction pipeline for execution:
• all data processing operations, except TEQP
• all load, store, load multiple and store multiple instructions
• MSR and MRS
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8.11.2 Determining system state
In order to meet the dynamic timing requirements of the memory system, any attempt
to access system state must occur synchronously to it. Thus, ARM7TDMI must be
forced to synchronise back to system speed. This is controlled by the 33rd bit of scan
chain 1.
Any instruction may be placed in scan chain 1 with bit 33 (the BREAKPT bit) LOW.
This instruction will then be executed at debug speed. To execute an instruction at
system speed, the instruction prior to it must be scanned into scan chain 1 with bit 33
set HIGH.
After the system speed instruction has been scanned into the data bus and clocked
into the pipeline, the BYPASS instruction must be loaded into the TAP controller. This
will cause ARM7TDMI to automatically synchronise back to MCLK (the system clock),
execute the instruction at system speed, and then re-enter debug state and switch
itself back to the internally generated DCLK. When the instruction has completed,
DBGACK will be HIGH and the core will have switched back to DCLK. At this point,
INTEST can be selected in the TAP controller, and debugging can resume.
In order to determine that a system speed instruction has completed, the debugger
must look at both DBGACK and nMREQ. In order to access memory, ARM7TDMI
drives nMREQ LOW after it has synchronised back to system speed. This transition is
used by the memory controller to arbitrate whether ARM7TDMI can have the bus in
the next cycle. If the bus is not available, ARM7TDMI may have its clock stalled
indefinitely. Therefore, the only way to tell that the memory access has completed, is
to examine the state of both nMREQ and DBGACK. When both are HIGH, the access
has completed. Usually, the debugger would be using ICEBreaker to control
debugging, and by reading ICEBreaker's status register, the state of nMREQ and
DBGACK can be determined. Refer to ➲
Chapter 9, ICEBreaker Module
for more
details.
By the use of system speed load multiples and debug speed store multiples, the state
of the system’s memory can be fed back to the debug host.
There are restrictions on which instructions may have the 33rd bit set. The only valid
instructions on which to set this bit are loads, stores, load multiple and store multiple.
See also ➲
8.11.3 Exit from debug state
. When ARM7TDMI returns to debug state
after a system speed access, bit 33 of scan chain 1 is set HIGH. This gives the
debugger information about why the core entered debug state the first time this scan
chain is read.
8.11.3 Exit from debug state
Leaving debug state involves restoring ARM7TDMI’s internal state, causing a branch
to the next instruction to be executed, and synchronising back to MCLK. After
restoring internal state, a branch instruction must be loaded into the pipeline. See
➲
8.12 The PC’s Behaviour During Debug
on page 8-23 for details on calculating the
branch.
Bit 33 of scan chain 1 is used to force ARM7TDMI to resynchronise back to MCLK.
The penultimate instruction of the debug sequence is scanned in with bit 33 set HIGH.
The final instruction of the debug sequence is the branch, and this is scanned in with
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bit 33 LOW. The core is then clocked to load the branch into the pipeline. Now, the
RESTART instruction is selected in the T AP controller . When the state machine enters
the RUN-TEST/IDLE state, the scan chain will revert back to system mode and clock
resynchronisation toMCLK will occur within ARM7TDMI. ARM7TDMI will then resume
normal operation, fetching instructions from memory. This delay, until the state
machine is in the RUN-TEST/IDLE state, allows conditions to be set up in other
devices in a multiprocessor system without taking immediate effect. Then, when the
RUN-TEST/IDLE state is entered, all the processors resume operation
simultaneously.
The function of DBGACK is to tell the rest of the system when ARM7TDMI is in debug
state. This can be used to inhibit peripherals such as watchdog timers which have real
time characteristics. Also, DBGACK can be used to mask out memory accesses
which are caused by the debugging process. For example, when ARM7TDMI enters
debug state after a breakpoint, the instruction pipeline contains the breakpointed
instruction plus two other instructions which have been prefetched. On entry to debug
state, the pipeline is flushed. Therefore, on exit from debug state, the pipeline must be
refilled to its previous state. Thus, because of the debugging process, more memory
accesses occur than would normally be expected. Any system peripheral which may
be sensitive to the number of memory accesses can be inhibited through the use of
DBGACK.
For example, imagine a fictitious peripheral that simply counts the number of memory
cycles. This device should return the same answer after a program has been run both
with and without debugging. ➲
Figure 8-7: Debug exit sequence
on page 8-22 shows
the behaviour of ARM7TDMI on exit from the debug state.
Figure 8-7: Debug exit sequence
ECLK
nMREQ
SEQ
A[31:0]
D[31:0]
DBGACK
Internal Cycles N S S
Ab Ab+4 Ab+8
Debug Interface
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It can be seen from ➲
Figure 8-2: Debug state entry
on page 8-4 that the final memory
access occurs in the cycle
after
DBGACK goes HIGH, and this is the point at which
the cycle counter should be disabled. ➲
Figure 8-7: Debug exit sequence
shows that
the first memory access that the cycle counter has not seen before occurs in the cycle
after DBGACK goes LOW, and so this is the point at which the counter should be re-
enabled.
Note that when a system speed access from debug state occurs, ARM7TDMI
temporarily drops out of debug state, and so DBGACK can go LOW. If there are
peripherals which are sensitive to the number of memory accesses, they must be led
to believe that ARM7TDMI is still in debug state. By programming the ICEBreaker
control register, the value on DBGACK can be forced to be HIGH. See ➲
Chapter 9,
ICEBreaker Module
for more details.
8.12 The PC’s Behaviour During Debug
In order that ARM7TDMI may be forced to branch back to the place at which program
flow was interrupted by debug, the debugger must keep track of what happens to the
PC. There are five cases: breakpoint, watchpoint, watchpoint when another exception
occurs, debug request and system speed access.
8.12.1 Breakpoint
Entry to the debug state from a breakpoint advances the PC by 4 addresses, or 16
bytes. Each instruction executed in debug state advances the PC by 1 address, or 4
bytes. The normal way to exit from debug state after a breakpoint is to remove the
breakpoint, and branch back to the previously breakpointed address.
For example, if ARM7TDMI entered debug state from a breakpoint set on a given
address and 2 debug speed instructions were executed, a branch of -7 addresses
must occur (4 for debug entry, +2 for the instructions, +1 for the final branch). The
following sequence shows the data scanned into scan chain 1. This is msb first, and
so the first digit is the value placed in the BREAKPT bit, followed by the instruction
data.
0 E0802000; ADD R2, R0, R0
1 E1826001; ORR R6, R2, R1
0 EAFFFFF9; B -7 (2’s complement)
Note that once in debug state, a minimum of two instructions must be executed before
the branch, although these may both be NOPs (MOV R0, R0). For small branches, the
final branch could be replaced with a subtract with the PC as the destination (SUB PC,
PC, #28 in the above example).
8.12.2 Watchpoints
Returning to program execution after entering debug state from a watchpoint is done
in the same way as the procedure described above. Debug entry adds 4 addresses to
the PC, and every instruction adds 1 address. The difference is that since the
instruction that caused the watchpoint has executed, the program returns to the next
instruction.
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8.12.3 Watchpoint with another exception
If a watchpointed access simultaneously causes a data abort, ARM7TDMI will enter
debug state in abort mode. Entry into debug is held off until the core has changed into
abort mode, and fetched the instruction from the abort vector.
A similar sequence is followed when an interrupt, or any other exception, occurs
during a watchpointed memory access. ARM7TDMI will enter debug state in the
exception's mode, and so the debugger must check to see whether this happened.
The debugger can deduce whether an exception occurred by looking at the current
and previous mode (in the CPSR and SPSR), and the value of the PC. If an exception
did take place, the user should be given the choice of whether to service the exception
before debugging.
Exiting debug state if an exception occurred is slightly different from the other cases.
Here, entry to debug state causes the PC to be incremented by 3 addresses rather
than 4, and this must be taken into account in the return branch calculation. For
example, suppose that an abort occurred on a watchpointed access and 10
instructions had been executed to determine this. The following sequence could be
used to return to program execution.
0 E1A00000; MOV R0, R0
1 E1A00000; MOV R0, R0
0 EAFFFFF0; B -16
This will force a branch back to the abort vector , causing the instruction at that location
to be refetched and executed. Note that after the abort service routine, the instruction
which caused the abort and watchpoint will be reexecuted. This will cause the
watchpoint to be generated and hence ARM7TDMI will enter debug state again.
8.12.4 Debug request
Entry into debug state via a debug request is similar to a breakpoint. However, unlike
a breakpoint, the last instruction will have completed execution and so must not be
refetched on exit from debug state. Therefore, it can be thought that entry to debug
state adds 3 addresses to the PC, and every instruction executed in debug state
adds 1.
For example, suppose that the user has invoked a debug request, and decides to
return to program execution straight away. The following sequence could be used:
0 E1A00000; MOV R0, R0
1 E1A00000; MOV R0, R0
0 EAFFFFFA; B -6
This restores the PC, and restarts the program from the next instruction.
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8.12.5 System speed access
If a system speed access is performed during debug state, the value of the PC is
increased by 3 addresses. Since system speed instructions access the memory
system, it is possible for aborts to take place. If an abort occurs during a system speed
memory access, ARM7TDMI enters abort mode before returning to debug state.
This is similar to an aborted watchpoint except that the problem is much harder to fix,
because the abort was not caused by an instruction in the main program, and the PC
does not point to the instruction which caused the abort. An abort handler usually looks
at the PC to determine the instruction which caused the abort, and hence the abort
address. In this case, the value of the PC is invalid, but the debugger should know
what location was being accessed. Thus the debugger can be written to help the abort
handler fix the memory system.
8.12.6 Summary of return address calculations
The calculation of the branch return address can be summarised as follows:
• For normal breakpoint and watchpoint, the branch is:
- (4 + N + 3S)
•For entry through debug request (DBGRQ), or watchpoint with exception, the
branch is:
- (3 + N + 3S)
where N is the number of debug speed instructions executed (including the final
branch), and S is the number of system speed instructions executed.
8.13 Priorities / Exceptions
Because the normal program flow is broken when a breakpoint or a debug request
occurs, debug can be thought of as being another type of exception. Some of the
interaction with other exceptions has been described above. This section summarises
the priorities.
8.13.1 Breakpoint with prefetch abort
When a breakpointed instruction fetch causes a prefetch abort, the abort is taken and
the breakpoint is disregarded. Normally, prefetch aborts occur when, for example, an
access is made to a virtual address which does not physically exist, and the returned
data is therefore invalid. In such a case the operating system’s normal action will be
to swap in the page of memory and return to the previously invalid address. This time,
when the instruction is fetched, and providing the breakpoint is activated (it may be
data dependent), ARM7TDMI will enter debug state.
Thus the prefetch abort takes higher priority than the breakpoint.
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8.13.2 Interrupts
When ARM7TDMI enters debug state, interrupts are automatically disabled. If
interrupts are disabled during debug, ARM7TDMI will never be forced into an interrupt
mode. Interrupts only have this effect on watchpointed accesses. They are ignored at
all times on breakpoints.
If an interrupt was pending during the instruction prior to entering debug state,
ARM7TDMI will enter debug state in the mode of the interrupt. Thus, on entry to debug
state, the debugger cannot assume that ARM7TDMI will be in the expected mode of
the user’s program. It must check the PC, the CPSR and the SPSR to fully determine
the reason for the exception.
Thus, debug takes higher priority than the interrupt, although ARM7TDMI
remembers
that an interrupt has occurred.
8.13.3 Data aborts
As described above, when a data abort occurs on a watchpointed access, ARM7TDMI
enters debug state in abort mode. Thus the watchpoint has higher priority than the
abort, although, as in the case of interrupt, ARM7TDMI remembers that the abort
happened.
8.14 Scan Interface Timing
Figure 8-8: Scan general timing
TCK
TMS
TDI
TDO
Data In
Data Out
T
bscl
T
bsch
T
bsis
T
bsih
T
bsoh
T
bsod
T
bsss
T
bssh
T
bsdh
T
bsdd
T
bsdh
T
bsdd
Debug Interface
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Notes
1For correct data latching, the I/O signals (from the core and the pads) must be
setup and held with respect to the rising edge of TCK in the CAPTURE-DR
state of the INTEST and EXTEST instructions.
2 Assumes that the data outputs are loaded with the AC test loads (see AC
parameter specification).
All delays are provisional and assume a process which achieves 33MHz MCLK
maximum operating frequency.
In the above table all units are ns.
Symbol Parameter Min Typ Max Notes
Tbscl TCK low period 15.1
Tbsch TCK high period 15.1
Tbsis TDI,TMS setup to [TCr] 0
Tbsih TDI,TMS hold from [TCr] 0.9
Tbsoh TDO hold time 2.4 2
Tbsod TCr to TDO valid 16.4 2
Tbsss I/O signal setup to [TCr] 3.6 1
Tbssh I/O signal hold from [TCr] 7.6 1
Tbsdh data output hold time 2.4 2
Tbsdd TCf to data output valid 17.1 2
Tbsr Reset period 25
Tbse Output Enable time 16.4 2
Tbsz Output Disable time 14.7 2
Table 8-3: ARM7TDMI scan interface timing
Debug Interface
ARM7TDMI Data Sheet
ARM DDI 0029E
8-28
Open Access
No Signal Type No Signal Type
1 D[0] I/O 29 D[28] I/O
2 D[1] I/O 30 D[29] I/O
3 D[2] I/O 31 D[30] I/O
4 D[3] I/O 32 D[31] I/O
5 D[4] I/O 33 BREAKPT I
6 D[5] I/O 34 NENIN I
7 D[6] I/O 35 NENOUT O
8 D[7] I/O 36 LOCK O
9 D[8] I/O 37 BIGEND I
10 D[9] I/O 38 DBE I
11 D[10] I/O 39 MAS[0] O
12 D[11] I/O 40 MAS[1] O
13 D[12] I/O 41 BL[0] I
14 D[13] I/O 42 BL[1] I
15 D[14] I/O 43 BL[2] I
16 D[15] I/O 44 BL[3] I
17 D[16] I/O 45 DCTL ** O
18 D[17] I/O 46 nRW O
19 D[18] I/O 47 DBGACK O
20 D[19] I/O 48 CGENDBGACK O
21 D[20] I/O 49 nFIQ I
22 D[21] I/O 50 nIRQ I
23 D[22] I/O 51 nRESET I
24 D[23] I/O 52 ISYNC I
25 D[24] I/O 53 DBGRQ I
26 D[25] I/O 54 ABORT I
27 D[26] I/O 55 CPA I
28 D[27] I/O 56 nOPC O
Table 8-4: Macrocell scan signals and pins
Debug Interface
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ARM DDI 0029E
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57 IFEN I 82 A[23] O
58 nCPI O 83 A[22] O
59 nMREQ O 84 A[21] O
60 SEQ O 85 A[20] O
61 nTRANS O 86 A[19] O
62 CPB I 87 A[18] O
63 nM[4] O 88 A[17] O
64 nM[3] O 89 A[16] O
65 nM[2] O 90 A[15] O
66 nM[1] O 91 A[14] O
67 nM[0] O 92 A[13] O
68 nEXEC O 93 A[12] O
69 ALE I 94 A[11] O
70 ABE I 95 A[10] O
71 APE I 96 A[9] O
72 TBIT O 97 A[8] O
73 nWAIT I 98 A[7] O
74 A[31] O 99 A[6] O
75 A[30] O 100 A[5] O
76 A[29] O 101 A[4] O
77 A[28] O 102 A[3] O
78 A[27] O 103 A[2] O
79 A[26] O 104 A[1] O
80 A[25] O 105 A[0] O
81 A[24] O
No Signal Type No Signal Type
Table 8-4: Macrocell scan signals and pins
Debug Interface
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ARM DDI 0029E
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Key I - Input
O - Output
I/O - Input/Output
Note DCTL
is not described in this datasheet.
DCTL
is an output from the processor used
to control the unidirectional data out latch,
DOUT[31:0]
. This signal is not visible from
the periphery of ARM7TDMI.
8.15 Debug Timing
Notes • All delays are provisional and assume a process which achieves 33MHz
MCLK maximum operating frequency.
• Assumes that the data outputs are loaded with the AC test loads (see AC
parameter specification).
• All units are ns.
Symbol Parameter Min Max
Ttdbgd TCK falling to DBGACK, DBGRQI changing 13.3
Ttpfd TCKf to TAP outputs 10.0
Ttpfh TAP outputs hold time from TCKf 2.4
Ttprd TCKr to TAP outputs 8.0
Ttprh TAP outputs hold time from TCKr 2.4
Ttckr TCK to TCK1, TCK2 rising 7.8
Ttckf TCK to TCK1, TCK2 falling 6.1
Tecapd TCK to ECAPCLK changing 8.2
Tdckf DCLK induced: TCKf to various outputs valid 23.8
Tdckfh DCLK induced: Various outputs hold from TCKf 6.0
Tdckr DCLK induced: TCKr to various outputs valid 26.6
Tdckrh DCLK induced: Various outputs hold from TCKr 6.0
Ttrstd nTRSTf to TAP outputs valid 8.5
Ttrsts nTRSTr setup to TCKr 2.3
Tsdtd SDOUTBS to TDO valid 10.0
Tclkbs TCK to Boundary Scan Clocks 8.2
Tshbsr TCK to SHCLKBS, SHCLK2BS rising 5.7
Tshbsf TCK to SHCLKBS, SHCLK2BS falling 4.0
Table 8-5: ARM7TDMI debug interface timing
ARM7TDMI Data Sheet
ARM DDI 0029E
9-1
11
1
Open Access
ICEBreaker Module
This chapter describes the ARM7TDMI ICEBreaker module.
Note The name ICEbreaker has changed. It is now known as the EmbeddedICE macrocell.
Future versions of the datasheet will reflect this change.
9.1 Overview 9-2
9.2 The Watchpoint Registers 9-3
9.3 Programming Breakpoints 9-6
9.4 Programming Watchpoints 9-8
9.5 The Debug Control Register 9-9
9.6 Debug Status Register 9-10
9.7 Coupling Breakpoints and Watchpoints 9-11
9.8 Disabling ICEBreaker 9-13
9.9 ICEBreaker Timing 9-13
9.10 Programming Restriction 9-13
9.11 Debug Communications Channel 9-14
9
ICEBreaker Module
ARM7TDMI Data Sheet
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9-2
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9.1 Overview
The ARM7TDMI-ICEBreaker module, hereafter referred to simply as
ICEBreaker
,
provides integrated on-chip debug support for the ARM7TDMI core.
ICEBreaker is programmed in a serial fashion using the ARM7TDMI TAP controller. It
consists of two real-time watchpoint units, together with a control and status register.
One or both of the watchpoint units can be programmed to halt the execution of
instructions by the ARM7TDMI core via itsBREAKPT signal. Execution is halted when
a match occurs between the values programmed into ICEBreaker and the values
currently appearing on the address bus, data bus and various control signals. Any bit
can be masked so that its value does not affect the comparison.
➲
Figure 9-1: ARM7TDMI block diagram
shows the relationship between the core,
ICEBreaker and the TAP controller.
Note Only those signals that are pertinent to ICEBreaker are shown.
Figure 9-1: ARM7TDMI block diagram
MAS[1:0]
A[31:0]
D[31:0]
nOPC
nRW
nTRANS
DBGACKI
BREAKPTI
DBGRQI
IFEN
ECLK
nMREQ
EXTERN1
EXTERN0
BREAKPT
DBGRQ
DBGACK
TCK
DBGEN
TAP
ICEBreaker
Processor
TMS
TDI
TDO
SDIN SDOUT
nTRST
TBIT RANGEOUT0
RANGEOUT1
ICEBreaker Module
ARM7TDMI Data Sheet
ARM DDI 0029E
9-3
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Either watchpoint unit can be configured to be a watchpoint (monitoring data
accesses) or a breakpoint (monitoring instruction fetches). Watchpoints and
breakpoints can be made to be data-dependent.
Two independent registers, Debug Control and Debug Status, provide overall control
of ICEBreaker's operation.
9.2 The Watchpoint Registers
The two watchpoint units, known as
Watchpoint 0
and
Watchpoint 1,
each contain
three pairs of registers:
1 Address Value and Address Mask
2 Data Value and Data Mask
3 Control Value and Control Mask
Each register is independently programmable, and has its own address: see
➲
Table 9-1: Function and mapping of ICEBreaker registers
.
Address Width Function
00000 3 Debug Control
00001 5 Debug Status
00100 6 Debug Comms Control Register
00101 32 Debug Comms Data Register
01000 32 Watchpoint 0 Address Value
01001 32 Watchpoint 0 Address Mask
01010 32 Watchpoint 0 Data Value
01011 32 Watchpoint 0 Data Mask
01100 9 Watchpoint 0 Control Value
01101 8 Watchpoint 0 Control Mask
10000 32 Watchpoint 1Address Value
10001 32 Watchpoint 1 Address Mask
10010 32 Watchpoint 1 Data Value
10011 32 Watchpoint 1 Data Mask
10100 9 Watchpoint 1 Control Value
10101 8 Watchpoint 1 Control Mask
Table 9-1: Function and mapping of ICEBreaker registers
ICEBreaker Module
ARM7TDMI Data Sheet
ARM DDI 0029E
9-4
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9.2.1 Programming and reading watchpoint registers
A register is programmed by scanning data into the ICEBreaker scan chain (scan
chain 2). The scan chain consists of a 38-bit shift register comprising a 32-bit data
field, a 5-bit address field and a read/write bit. This is shown in ➲
Figure 9-2:
ICEBreaker block diagram
.
Figure 9-2: ICEBreaker block diagram
The data to be written is scanned into the 32-bit data field, the address of the register
into the 5-bit address field and a 1 into the read/write bit.
A register is read by scanning its address into the address field and a 0 into the read/
write bit. The 32-bit data field is ignored.
The register addresses are shown in ➲
Table 9-1: Function and mapping of
ICEBreaker registers
.
Note A read or write actually takes place when the TAP controller enters the UPDATE-DR
state.
Address
Data
Address
Decoder
Update
32
r/w
TDI TDO
A[31:0]
D[31:0]
+
Watchpoint
BREAKPOINT
0
31
0
4
Control
Scan Chain Register
Registers and Comparators
Comparator
Value
Mask
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9.2.2 Using the mask registers
For each Value register in a register pair, there is a Mask register of the same format.
Setting a bit to 1 in the Mask register has the effect of making the corresponding bit in
the Value register disregarded in the comparison.
For example, if a watchpoint is required on a particular memory location but the data
value is irrelevant, the Data Mask register can be programmed to 0xFFFFFFFF (all
bits set to 1) to make the entire Data Bus field ignored.
Note The mask is an XNOR mask rather than a conventional AND mask: when a mask bit
is set to 1, the comparator for that bit position will always match, irrespective of the
value register or the input value.
Setting the mask bit to 0 means that the comparator will only match if the input value
matches the value programmed into the value register.
9.2.3 The control registers
The Control V alue and Control Mask registers are mapped identically in the lower eight
bits, as shown below.
Figure 9-3: Watchpoint control value and mask format
Bit 8 of the control value register is the ENABLE bit, which cannot be masked.
The bits have the following functions:
nRW: compares against the not read/write signal from the core in order to
detect the direction of bus activity.nRW is 0 for a read cycle and 1 for
a write cycle.
MAS[1:0]:compares against theMAS[1:0] signal from the core in order to detect
the size of bus activity.
The encoding is shown in the following table.
bit 1 bit 0 Data size
0 0 byte
0 1 halfword
1 0 word
1 1 (reserved)
Table 9-2: MAS[1:0] signal encoding
ENABLE RANGE CHAIN EXTERN nTRANS nOPC MAS[0] nRW
012345678
MAS[1]
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nOPC: is used to detect whether the current cycle is an instruction fetch
(nOPC = 0) or a data access (nOPC = 1).
nTRANS: compares against the not translate signal from the core in order to
distinguish between User mode (nTRANS = 0) and non-User mode
(nTRANS = 1) accesses.
EXTERN:is an external input to ICEBreaker which allows the watchpoint to be
dependent upon some external condition. The EXTERN input for
Watchpoint 0 is labelled EXTERN0 and the EXTERN input for
Watchpoint 1 is labelled EXTERN1.
CHAIN: can be connected to the chain output of another watchpoint in order
to implement, for example, debugger requests of the form “breakpoint
on address YYY only when in process XXX”.
In the ARM7TDMI-ICEBreaker , theCHAINOUT output of W atchpoint
1 is connected to the CHAIN input of Watchpoint 0. The CHAINOUT
output is derived from a latch; the address/control field comparator
drives the write enable for the latch and the input to the latch is the
value of the data field comparator. The CHAINOUT latch is cleared
when the Control Value register is written or when nTRST is LOW.
RANGE:can be connected to the range output of another watchpoint register.
In the ARM7TDMI-ICEBreaker, the RANGEOUT output of
Watchpoint 1 is connected to the RANGE input of W atchpoint 0. This
allows the two watchpoints to be coupled for detecting conditions that
occur simultaneously, eg for range-checking.
ENABLE: If a watchpoint match occurs, the BREAKPT signal will only be
asserted when the ENABLE bit is set. This bit only exists in the value
register: it cannot be masked.
For each of the bits 8:0 in the Control Value register , there is a corresponding bit in the
Control Mask register. This removes the dependency on particular signals.
9.3 Programming Breakpoints
Breakpoints can be classified as hardware breakpoints or software breakpoints.
Hardware breakpoints typically monitor the address value and can be set in any
code, even in code that is in ROM or code that is self-
modifying.
Software breakpoints monitor a particular bit pattern being fetched from any
address. One ICEBreaker watchpoint can thus be used
to support any number of software breakpoints. Software
breakpoints can normally only be set in RAM because an
instruction has to be replaced by the special bit pattern
chosen to cause a software breakpoint.
ICEBreaker Module
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ARM DDI 0029E
9-7
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9.3.1 Hardware breakpoints
To make a watchpoint unit cause hardware breakpoints (ie on instruction fetches):
1 Program its Address Value register with the address of the instruction to be
breakpointed.
2For a breakpoint in ARM state, program bits [1:0] of the Address Mask register
to 1. For a breakpoint in THUMB state, program bit 0 of the Address Mask to
1. In both cases the remaining bits are set to 0.
3 Program the Data Value register only if you require a data-dependent
breakpoint: ie only if the actual instruction code fetched must be matched as
well as the address. If the data value is not required, program the Data Mask
register to 0xFFFFFFFF (all bits to1), otherwise program it to0x00000000.
4 Program the Control Value register with nOPC = 0.
5 Program the Control Mask register with nOPC =0, all other bits to 1.
6 If you need to make the distinction between user and non-user mode
instruction fetches, program the nTRANS Value and Mask bits as above.
7 If required, program the EXTERN,RANGE and CHAIN bits in the same way.
9.3.2 Software breakpoints
To make a watchpoint unit cause software breakpoints (ie on instruction fetches of a
particular bit pattern):
1 Program its Address Mask register to 0xFFFFFFFF (all bits set to 1) so that
the address is disregarded.
2 Program the Data Value register with the particular bit pattern that has been
chosen to represent a software breakpoint.
If a THUMB software breakpoint is being programmed, the 16-bit pattern must
be repeated in both halves of the Data Value register. For example, if the bit
pattern is 0xDFFF, then 0xDFFFDFFF must be programmed. When a 16-bit
instruction is fetched, ICEbreaker only compares the valid half of the data bus
against the contents of the Data Value register. In this way, a single
Watchpoint register can be used to catch software breakpoints on both the
upper and lower halves of the data bus.
3 Program the Data Mask register to 0x00000000.
4 Program the Control Value register with nOPC = 0.
5 Program the Control Mask register with nOPC = 0, all other bits to 1.
6 If you wish to make the distinction between user and non-user mode
instruction fetches, program the nTRANS bit in the Control V alue and Control
Mask registers accordingly.
7 If required, program the EXTERN,RANGE and CHAIN bits in the same way.
Note The address value register need not be programmed.
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Setting the breakpoint
To set the software breakpoint:
1 Read the instruction at the desired address and store it away.
2 Write the special bit pattern representing a software breakpoint at the
address.
Clearing the breakpoint
To clear the software breakpoint, restore the instruction to the address.
9.4 Programming Watchpoints
To make a watchpoint unit cause watchpoints (ie on data accesses):
1Program its Address Value register with the address of the data access to be
watchpointed.
2 Program the Address Mask register to 0x00000000.
3 Program the Data Value register only if you require a data-dependent
watchpoint; i.e. only if the actual data value read or written must be matched
as well as the address. If the data value is irrelevant, program the Data Mask
register to 0xFFFFFFFF (all bits set to 1) otherwise program it to 0x00000000.
4 Program the Control Value register with nOPC = 1, nRW = 0 for a read or
nRW = 1 for a write,MAS[1:0] with the value corresponding to the appropriate
data size.
5 Program the Control Mask register with nOPC = 0, nRW = 0, MAS[1:0] = 0,
all other bits to 1. Note that nRW or MAS[1:0] may be set to 1 if both reads
and writes or data size accesses are to be watchpointed respectively.
6 If you wish to make the distinction between user and non-user mode data
accesses, program the nTRANS bit in the Control Value and Control Mask
registers accordingly.
7 If required, program the EXTERN,RANGE and CHAIN bits in the same way.
Note The above are just examples of how to program the watchpoint register to generate
breakpoints and watchpoints; many other ways of programming the registers are
possible. For instance, simple range breakpoints can be provided by setting one or
more of the address mask bits.
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9.5 The Debug Control Register
The Debug Control Register is 3 bits wide. If the register is accessed for a write (with
the read/write bit HIGH), the control bits are written. If the register is accessed for a
read (with the read/write bit LOW), the control bits are read.
The function of each bit in this register is as follows:
Figure 9-4: Debug control register format
Bits 1 and 0 allow the values on DBGRQ and DBGACK to be forced.
As shown in ➲
Figure 9-6: Structure of TBIT, NMREQ, DBGACK, DBGRQ and INTDIS
bits
on page 9-11, the value stored in bit 1 of the control register is synchronised and
then ORed with the external DBGRQ before being applied to the processor. The
output of this OR gate is the signal DBGRQI which is brought out externally from the
macrocell.
The synchronisation between control bit 1 and DBGRQI is to assist in multiprocessor
environments. The synchronisation latch only opens when the TAP controller state
machine is in the RUN-TEST/IDLE state. This allows an
enter debug
condition to be
set up in all the processors in the system while they are still running. Once the
condition is set up in all the processors, it can then be applied to them simultaneously
by entering the RUN-TEST/IDLE state.
In the case of DBGACK, the value of DBGACK from the core is ORed with the value
held in bit 0 to generate the external value of DBGACK seen at the periphery of
ARM7TDMI. This allows the debug system to signal to the rest of the system that the
core is still being debugged even when system-speed accesses are being performed
(in which case the internal DBGACK signal from the core will be LOW).
If Bit 2 (INTDIS) is asserted, the interrupt enable signal (IFEN) of the core is forced
LOW. Thus all interrupts (IRQ and FIQ) are disabled during debugging (DBGACK =1)
or if the INTDIS bit is asserted. The IFEN signal is driven according to the following
table:
DBGACK INTDIS IFEN
0 0 1
1 x 0
x 1 0
Table 9-3: IFEN signal control
INTDIS DBGRQ DBGACK
0
1
2
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9.6 Debug Status Register
The Debug Status Register is 5 bits wide. If it is accessed for a write (with the read/
write bit set HIGH), the status bits are written. If it is accessed for a read (with the read/
write bit LOW), the status bits are read.
Figure 9-5: Debug status register format
The function of each bit in this register is as follows:
Bits 1 and 0 allow the values on the synchronised versions of DBGRQ and
DBGACK to be read.
Bit 2 allows the state of the core interrupt enable signal (IFEN) to be
read. Since the capture clock for the scan chain may be
asynchronous to the processor clock, the DBGACK output from
the core is synchronised before being used to generate the IFEN
status bit.
Bit 3 allows the state of the NMREQ signal from the core (synchronised
to TCK) to be read. This allows the debugger to determine that a
memory access from the debug state has completed.
Bit 4 allows TBIT to be read. This enables the debugger to determine
what state the processor is in, and hence which instructions to
execute.
The structure of the debug status register bits is shown in ➲
Figure 9-6: Structure of
TBIT, NMREQ, DBGACK, DBGRQ and INTDIS bits
on page 9-11.
IFEN DBGRQ DBGACK
0123
nMREQ
4
TBIT
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Figure 9-6: Structure of TBIT, NMREQ, DBGACK, DBGRQ and INTDIS bits
9.7 Coupling Breakpoints and Watchpoints
Watchpoint units 1 and 0 can be coupled together via the CHAIN and RANGE inputs.
The use of CHAIN enables watchpoint 0 to be triggered only if watchpoint 1 has
previously matched. The use of RANGE enables simple range checking to be
performed by combining the outputs of both watchpoints.
DBGRQ DBGRQ
DBGACK
DBGACK
Bit 1
Bit 1
Debug Control
Register Debug Status
Register
(from ARM7TDMI
input)
(to ARM7TDMI output)
(to core and
(from core)
+
Bit 0
+
Bit 2 Bit 2
+IFEN
(to core)
DBGACK
(from core)
Synch
Bit 0
Synch
+
Bit 3
Synch
nMREQ
(from core)
Bit 4
Synch
TBIT
(from core)
Synch
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Example
Let
Av[31:0] be the value in the Address Value Register
Am[31:0] be the value in the Address Mask Register
A[31:0] be the Address Bus from the ARM7TDMI
Dv[31:0] be the value in the Data Value Register
Dm[31:0] be the value in the Data Mask Register
D[31:0] be the Data Bus from the ARM7TDMI
Cv[8:0] be the value in the Control Value Register
Cm[7:0] be the value in the Control Mask Register
C[9:0] be the combined Control Bus from the ARM7TDMI, other watchpoint
registers and the EXTERN signal.
CHAINOUT signal
The CHAINOUT signal is then derived as follows:
WHEN (({Av[31:0],Cv[4:0]} XNOR {A[31:0],C[4:0]}) OR
{Am[31:0],Cm[4:0]} == 0xFFFFFFFFF)
CHAINOUT = ((({Dv[31:0],Cv[6:4]} XNOR {D[31:0],C[7:5]}) OR
{Dm[31:0],Cm[7:5]}) == 0x7FFFFFFFF)
The CHAINOUT output of watchpoint register 1 provides the CHAIN input to
Watchpoint 0. This allows for quite complicated configurations of breakpoints and
watchpoints.
Take for example the request by a debugger to breakpoint on the instruction at location
YYY when running process XXX in a multiprocess system.
If the current process ID is stored in memory, the above function can be implemented
with a watchpoint and breakpoint chained together. The watchpoint address is set to
a known memory location containing the current process ID, the watchpoint data is set
to the required process ID and the ENABLE bit is set to “off”.
The address comparator output of the watchpoint is used to drive the write enable for
the CHAINOUT latch, the input to the latch being the output of the data comparator
from the same watchpoint. The output of the latch drives the CHAIN input of the
breakpoint comparator. The address YYY is stored in the breakpoint register and when
the CHAIN input is asserted, and the breakpoint address matches, the breakpoint
triggers correctly.
RANGEOUT signal
The RANGEOUT signal is then derived as follows:
RANGEOUT = ((({Av[31:0],Cv[4:0]} XNOR {A[31:0],C[4:0]}) OR
{Am[31:0],Cm[4:0]}) == 0xFFFFFFFFF) AND ((({Dv[31:0],Cv[7:5]}
XNOR {D[31:0],C[7:5]}) OR {Dm[31:0],Cm[7:5]}) == 0x7FFFFFFFF)
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The RANGEOUT output of watchpoint register 1 provides the RANGE input to
watchpoint register 0. This allows two breakpoints to be coupled together to form
range breakpoints. Note that selectable ranges are restricted to being powers of 2.
This is best illustrated by an example.
Example
If a breakpoint is to occur when the address is in the first 256 bytes of memory , but not
in the first 32 bytes, the watchpoint registers should be programmed as follows:
1 Watchpoint 1 is programmed with an address value of 0x00000000 and an
address mask of 0x0000001F. The ENABLE bit is cleared. All other
Watchpoint 1 registers are programmed as normal for a breakpoint. An
address within the first 32 bytes will cause the RANGE output to go HIGH but
the breakpoint will not be triggered.
2 Watchpoint 0 is programmed with an address value of 0x00000000 and an
address mask of 0x000000FF. The ENABLE bit is set and the RANGE bit
programmed to match a 0. All other Watchpoint 0 registers are programmed
as normal for a breakpoint.
If Watchpoint 0 matches but Watchpoint 1 does not (ie theRANGE input to Watchpoint
0 is 0), the breakpoint will be triggered.
9.8 Disabling ICEBreaker
ICEBreaker may be disabled by wiring the DBGEN input LOW.
When DBGEN is LOW, BREAKPT and DBGRQ to the core are forced LOW,
DBGACK from the ARM7TDMI is also forced LOW and the IFEN input to the core is
forced HIGH, enabling interrupts to be detected by ARM7TDMI.
When DBGEN is LOW, ICEBreaker is also put into a low-power mode.
9.9 ICEBreaker Timing
The EXTERN1 and EXTERN0 inputs are sampled by ICEBreaker on the falling edge
of ECLK. Sufficient set-up and hold time must therefore be allowed for these signals.
9.10 Programming Restriction
The ICEBreaker watchpoint units should only be programmed when the clock to the
core is stopped. This can be achieved by putting the core into the debug state.
The reason for this restriction is that if the core continues to run at ECLK rates when
ICEBreaker is being programmed at TCK rates, it is possible for the BREAKPT signal
to be asserted asynchronously to the core.
This restriction does not apply if MCLK and TCK are driven from the same clock, or if
it is known that the breakpoint or watchpoint condition can only occur some time after
ICEBreaker has been programmed.
Note This restriction does not apply in any event to the Debug Control or Status Registers.
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9.11 Debug Communications Channel
ARM7TDMI’s ICEbreaker contains a communication channel for passing information
between the target and the host debugger. This is implemented as coprocessor 14.
The communications channel consists of a 32-bit wide Comms Data Read register, a
32-bit wide Comms Data Write Register and a 6-bit wide Comms Control Register for
synchronised handshaking between the processor and the asynchronous debugger.
These registers live in fixed locations in ICEbreaker’s memory map (as shown in
➲
Table 9-1: Function and mapping of ICEBreaker registers
on page 9-3) and are
accessed from the processor via MCR and MRC instructions to coprocessor 14.
9.11.1 Debug comms channel registers
The Debug Comms Control register is read only and allows synchronised hanshaking
between the processor and the debugger.
Figure 9-7: Debug comms control register
The function of each register bit is described below:
Bits 31:28 contain a fixed pattern which denote the ICEbreaker version number ,
in this case 0001.
Bit 1 denotes whether the Comms Data Write register (from the
processor’s point of view) is free. From the processor’s point of view ,
if the Comms Data Write register is free (W=0) then new data may be
written. If it is not free (W=1), then the processor must poll until W=0.
From the debugger’s point of view, if W=1 then some new data has
been written which may then be scanned out.
Bit 0 denotes whether there is some new data in the Comms Data Read
register. From the processor’s point of view , if R=1, then there is some
new data which may be read via an MRC instruction. From the
debugger’s point of view, if R=0 then the Comms Data Read register
is free and new data may be placed there through the scan chain. If
R=1, then this denotes that data previously placed there through the
scan chain has not been collected by the processor and so the
debugger must wait.
From the debugger’s point of view , the registers are accessed via the scan chain in the
usual way. From the processor , these registers are accessed via coprocessor register
transfer instructions.
31
0
30
0
29
0
28
1
0
R
1
W
...
...
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The following instructions should be used:
MRC CP14, 0, Rd, C0, C0
Returns the Debug Comms Control register into Rd
MCR CP14, 0, Rn, C1, C0
Writes the value in Rn to the Comms Data Write register
MRC CP14, 0, Rd, C1, C0
Returns the Debug Data Read register into Rd
Since the THUMB instruction set does not contain coprocessor instructions, it is
recommended that these are accessed via SWI instructions when in THUMB state.
9.11.2 Communications via the comms channel
Communication between the debugger and the processor occurs as follows. When the
processor wishes to send a message to ICEbreaker, it first checks that the Comms
Data Write register is free for use. This is done by reading the Debug Comms Control
register to check that the W bit is clear . If it is clear then the Comms Data Write register
is empty and a message is written by a register transfer to the coprocessor . The action
of this data transfer automatically sets the W bit. If on reading the W bit it is found to
be set, then this implys that previously written data has not been picked up by the
debugger and thus the processor must poll until the W bit is clear.
As the data transfer occurs from the processor to the Comms Data Write register, the
W bit is set in the Debug Comms Control register. When the debugger polls this
register it sees a synchronised version of both the R and W bit. When the debugger
sees that the W bit is set it can read the Comms Data Write register and scan the data
out. The action of reading this data register clears the W bit of the Debug Comms
Control register. At this point, the communications process may begin again.
Message transfer from the debugger to the processor is carried out in a similar
fashion. Here, the debugger polls the R bit of the Debug Comms Control register . If the
R bit is low then the Data Read register is free and so data can be placed there for the
processor to read. If the R bit is set, then previously deposited data has not yet been
collected and so the debugger must wait.
When the Comms Data Read register is free, data is written there via the scan chain.
The action of this write sets the R bit in the Debug Comms Control register. When the
processor polls this register, it sees an MCLK synchronised version. If the R bit is set
then this denotes that there is data waiting to be collected, and this can be read via a
CPRT load. The action of this load clears the R bit in the Debug Comms Control
register. When the debugger polls this register and sees that the R bit is clear, this
denotes that the data has been taken and the process may now be repeated.
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Instruction Cycle Operations
This chapter describes the ARM7TDMI instruction cycle operations.
10.1 Introduction 10-2
10.2 Branch and Branch with Link 10-2
10.3 THUMB Branch with Link 10-3
10.4 Branch and Exchange (BX) 10-3
10.5 Data Operations 10-4
10.6 Multiply and Multiply Accumulate 10-6
10.7 Load Register 10-8
10.8 Store Register 10-9
10.9 Load Multiple Registers 10-9
10.10 Store Multiple Registers 10-11
10.11 Data Swap 10-11
10.12 Software Interrupt and Exception Entry 10-12
10.13 Coprocessor Data Operation 10-13
10.14 Coprocessor Data Transfer (from memory to coprocessor) 10-14
10.15 Coprocessor Data Transfer (from coprocessor to memory) 10-15
10.16 Coprocessor Register Transfer (Load from coprocessor) 10-16
10.17 Coprocessor Register Transfer (Store to coprocessor) 10-17
10.18 Undefined Instructions and Coprocessor Absent 10-18
10.19 Unexecuted Instructions 10-18
10.20 Instruction Speed Summary 10-19
10
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10.1 Introduction
In the following tables nMREQ and SEQ (which are pipelined up to one cycle ahead
of the cycle to which they apply) are shown in the cycle in which they appear, so they
predict the type of the
next
cycle. The address, MAS[1:0],nRW,nOPC, nTRANSand
TBIT (which appear up to half a cycle ahead) are shown in the cycle to which they
apply. The address is incremented for prefetching of instructions in most cases. Since
the instruction width is 4 bytes in ARM state and 2 bytes in THUMB state, the
increment will vary accordingly . Hence the letter L is used to indicate instruction length
(4 bytes in ARM state and 2 bytes in THUMB state). Similarly, MAS[1:0] will indicate
the width of the instruction fetch, i=2 in ARM state and i=1 in THUMB state
representing word and halfword accesses respectively.
10.2 Branch and Branch with Link
A branch instruction calculates the branch destination in the first cycle, whilst
performing a prefetch from the current PC. This prefetch is done in all cases, since by
the time the decision to take the branch has been reached it is already too late to
prevent the prefetch.
During the second cycle a fetch is performed from the branch destination, and the
return address is stored in register 14 if the link bit is set.
The third cycle performs a fetch from the destination + L, refilling the instruction
pipeline, and if the branch is with link R14 is modified (4 is subtracted from it) to
simplify return from SUB PC,R14,#4 to MOV PC,R14. This makes the
STM..{R14} LDM..{PC} type of subroutine work correctly. The cycle timings are
shown below in ➲
Table 10-1: Branch instruction cycle operations
:
pc is the address of the branch instruction
alu is an address calculated by ARM7TDMI
(alu) are the contents of that address
Note
This applies to branches in ARM and THUMB state, and to Branch with Link in ARM
state only.
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC
1 pc+2L i 0 (pc + 2L) 0 0 0
2 alu i 0 (alu) 0 1 0
3 alu+L i 0 (alu + L) 0 1 0
alu+2L
Table 10-1: Branch instruction cycle operations
Instruction Cycle Operations
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10.3 THUMB Branch with Link
A THUMB Branch with Link operation consists of two consecutive THUMB
instructions, see ➲
5.19 Format 19: long branch with link
on page 5-40.
The first instruction acts like a simple data operation, taking a single cycle to add the
PC to the upper part of the offset, storing the result in Register 14 (LR).
The second instruction acts in a similar fashion to the ARM Branch with Link
instruction, thus its first cycle calculates the final branch destination whilst performing
a prefetch from the current PC.
The second cycle of the second instruction performs a fetch from the branch
destination and the return address is stored in R14.
The third cycle of the second instruction performs a fetch from the destination +2,
refilling the instruction pipeline and R14 is modified (2 subtracted from it) to simplify
the return to MOV PC, R14. This makes the PUSH {..,LR} ; POP {..,PC} type
of subroutine work correctly.
The cycle timings of the complete operation are shown in ➲
Table 10-2: THUMB Long
Branch with Link
pc is the address of the first instruction of the operation.
10.4 Branch and Exchange (BX)
A Branch and Exchange operation takes 3 cycles and is similar to a Branch.
In the first cycle, the branch destination and the new core state are extracted from the
register source, whilst performing a prefetch from the current PC. This prefetch is
performed in all cases, since by the time the decision to take the branch has been
reached, it is already too late to prevent the prefetch.
During the second cycle, a fetch is performed from the branch destination using the
new instruction width, dependent on the state that has been selected.
The third cycle performs a fetch from the destination +2 or +4 dependent on the new
specified state, refilling the instruction pipeline. The cycle timings are shown in➲
Table
10-3: Branch and Exchange instruction cycle operations
on page 10-4.
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC
1 pc + 4 1 0 (pc + 4) 0 1 0
2 pc + 6 1 0 (pc + 6) 0 0 0
3 alu 1 0 (alu) 0 1 0
4 alu + 2 1 0 (alu + 2) 0 1 0
alu + 4
Table 10-2: THUMB Long Branch with Link
Instruction Cycle Operations
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Notes:
1W and w represent the instruction width before and after the BX respectively.
In ARM state the width equals 4 bytes and in THUMB state the width equals
2 bytes. For example, when changing from ARM to THUMB state, W would
equal 4 and w would equal 2.
2I and i represent the memory access size before and after the BX respectively .
In ARM state, the MAS[1:0] is 2 and in THUMB state MAS[1:0] is 1. When
changing from THUMB to ARM state, I would equal 1 and i would equal 2.
3 T and t represent the state of the TBIT before and after the BX respectively.
In ARM state TBIT is 0 and in THUMB state TBIT is 1. When changing from
ARM to THUMB state, T would equal 0 and t would equal 1.
10.5 Data Operations
A data operation executes in a single datapath cycle except where the shift is
determined by the contents of a register. A register is read onto the A bus, and a
second register or the immediate field onto the B bus. The ALU combines the A bus
source and the shifted B bus source according to the operation specified in the
instruction, and the result (when required) is written to the destination register.
(Compares and tests do not produce results, only the ALU status flags are affected.)
An instruction prefetch occurs at the same time as the above operation, and the
program counter is incremented.
When the shift length is specified by a register, an additional datapath cycle occurs
before the above operation to copy the bottom 8 bits of that register into a holding latch
in the barrel shifter. The instruction prefetch will occur during this first cycle, and the
operation cycle will be internal (ie will not request memory). This internal cycle can be
merged with the following sequential access by the memory manager as the address
remains stable through both cycles.
The PC may be one or more of the register operands. When it is the destination,
external bus activity may be affected. If the result is written to the PC, the contents of
the instruction pipeline are invalidated, and the address for the next instruction
prefetch is taken from the ALU rather than the address incrementer. The instruction
pipeline is refilled before any further execution takes place, and during this time
exceptions are locked out.
Cycle Address MAS [1:0] nRW Data nMREQ SEQ noPC TBIT
1 pc + 2W I 0 (pc + 2W) 0 0 0 T
2 alu i 0 (alu) 0 1 0 t
3 alu+w i 0 (alu+w) 0 1 0 t
alu + 2w
Table 10-3: Branch and Exchange instruction cycle operations
Instruction Cycle Operations
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PSR Transfer operations exhibit the same timing characteristics as the data
operations except that the PC is never used as a source or destination register. The
cycle timings are shown below ➲
Table 10-4: Data Operation instruction cycle
operations
.
Note
Shifted registed with destination equals PC is not possible in THUMB state.
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC
normal 1 pc+2L i 0 (pc+2L) 0 1 0
pc+3L
dest=pc 1 pc+2L i 0 (pc+2L) 0 0 0
2 alu i 0 (alu) 0 1 0
3 alu+L i 0 (alu+L) 0 1 0
alu+2L
shift(Rs) 1 pc+2L i 0 (pc+2L) 1 0 0
2 pc+3L i 0 - 0 1 1
pc+3L
shift(Rs) 1 pc+8 2 0 (pc+8) 1 0 0
dest=pc 2 pc+12 2 0 - 0 0 1
3 alu 2 0 (alu) 0 1 0
4 alu+4 2 0 (alu+4) 0 1 0
alu+8
Table 10-4: Data Operation instruction cycle operations
Instruction Cycle Operations
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10.6 Multiply and Multiply Accumulate
The multiply instructions make use of special hardware which implements integer
multiplication with early termination. All cycles except the first are internal.
The cycle timings are shown in the following four tables, where
m
is the number of
cycles required by the multiplication algorithm; see ➲
10.20 Instruction Speed
Summary
on page 10-19.
Cycle Address nRW MAS[1:0] Data nMREQ SEQ nOPC
1 pc+2L 0 i (pc+2L) 1 0 0
2 pc+3L 0 i - 1 0 1
• pc+3L 0 i - 1 0 1
m pc+3L 0 i - 1 0 1
m+1 pc+3L 0 i - 0 1 1
pc+3L
Table 10-5: Multiply instruction cycle operations
Cycle Address nRW MAS[1:0] Data nMREQ SEQ nOPC
1 pc+8 0 2 (pc+8) 1 0 0
2 pc+8 0 2 - 1 0 1
• pc+12 0 2 - 1 0 1
m pc+12 0 2 - 1 0 1
m+1 pc+12 0 2 - 1 0 1
m+2 pc+12 0 2 - 0 1 1
pc+12
Table 10-6: Multiply-Accumulate instruction cycle operations
Instruction Cycle Operations
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Note
Multiply-Accumulate is not possible in THUMB state.
Cycle Address nRW MAS[1:0] Data nMREQ SEQ nOPC
1 pc+2L 0 i (pc+2L) 1 0 0
2 pc+3L 0 i - 1 0 1
• pc+3L 0 i - 1 0 1
m pc+3L 0 i - 1 0 1
m+1 pc+3L 0 i - 1 0 1
m+2 pc+3L 0 i - 0 1 1
pc+3L
Table 10-7: Multiply Long instruction cycle operations
Cycle Address nRW MAS[1:0] Data nMREQ SEQ nOPC
1 pc+8 0 2 (pc+8) 1 0 0
2 pc+8 0 2 - 1 0 1
• pc+12 0 2 - 1 0 1
m pc+12 0 2 - 1 0 1
m+1 pc+12 0 2 - 1 0 1
m+2 pc+12 0 2 - 1 0 1
m+3 pc+12 0 2 - 0 1 1
pc+12
Table 10-8: Multiply-Accumulate Long instruction cycle operations
Instruction Cycle Operations
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10.7 Load Register
The first cycle of a load register instruction performs the address calculation. The data
is fetched from memory during the second cycle, and the base register modification is
performed during this cycle (if required). During the third cycle the data is transferred
to the destination register, and external memory is unused. This third cycle may
normally be merged with the following prefetch to form one memory N-cycle. The cycle
timings are shown below in ➲
Table 10-9: Load Register instruction cycle operations
.
Either the base or the destination (or both) may be the PC, and the prefetch sequence
will be changed if the PC is affected by the instruction.
The data fetch may abort, and in this case the destination modification is prevented.
b, h and w are byte, halfword and word as defined in ➲
Table 9-2: MAS[1:0] signal
encoding
on page 9-5.
c represents current mode-dependent value.
d will either be 0 if the T bit has been specified in the instruction (eg. LDR T), or c at all
other times.
Note
Destination equals PC is not possible in THUMB state.
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC nTRANS
normal 1 pc+2L i 0 (pc+2L) 0 0 0 c
2 alu b/h/w 0 (alu) 1 0 1 d
3 pc+3L i 0 - 0 1 1 c
pc+3L
dest=pc 1 pc+8 2 0 (pc+8) 0 0 0 c
2 alu 0 pc’ 1 0 1 d
3 pc+12 2 0 - 0 0 1 c
4 pc’ 2 0 (pc’) 0 1 0 c
5 pc’+4 2 0 (pc’+4) 0 1 0 c
pc’+8
Table 10-9: Load Register instruction cycle operations
Instruction Cycle Operations
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10.8 Store Register
The first cycle of a store register is similar to the first cycle of load register. During the
second cycle the base modification is performed, and at the same time the data is
written to memory. There is no third cycle.
The cycle timings are shown below in ➲
Table 10-10: Store Register instruction cycle
operations
.
b, h and w are byte, halfword and word as defined in ➲
Table 9-2: MAS[1:0] signal
encoding
on page 9-5.
c represents current mode-dependent value
d will either be 0 if the T bit has been specified in the instruction (eg. SDRT), or c at all
other times.
10.9 Load Multiple Registers
The first cycle of LDM is used to calculate the address of the first word to be
transferred, whilst performing a prefetch from memory. The second cycle fetches the
first word, and performs the base modification. During the third cycle, the first word is
moved to the appropriate destination register while the second word is fetched from
memory, and the modified base is latched internally in case it is needed to patch up
after an abort. The third cycle is repeated for subsequent fetches until the last data
word has been accessed, then the final (internal) cycle moves the last word to its
destination register. The cycle timings are shown in ➲
Table 10-11: Load Multiple
Registers instruction cycle operations
on page 10-10.
The last cycle may be merged with the next instruction prefetch to form a single
memory N-cycle.
If an abort occurs, the instruction continues to completion, but all register writing after
the abort is prevented. The final cycle is altered to restore the modified base register
(which may have been overwritten by the load activity before the abort occurred).
When the PC is in the list of registers to be loaded the current instruction pipeline must
be invalidated.
Note
The PC is always the last register to be loaded, so an abort at any point will prevent
the PC from being overwritten.
Note
LDM with destination = PC cannot be executed in THUMB state. However
POP{Rlist,PC}
equates to an LDM with destination=PC.
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC nTRANS
1 pc+2L i 0 (pc+2L) 0 0 0 c
2 alu b/h/w 1 Rd 0 0 1 d
pc+3L
Table 10-10: Store Register instruction cycle operations
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
10-10
Open Access
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC
1 register 1 pc+2L i 0 (pc+2L) 0 0 0
2 alu 2 0 (alu) 1 0 1
3 pc+3L i 0 - 0 1 1
pc+3L
1 register 1 pc+2L i 0 (pc+2L) 0 0 0
dest=pc 2 alu 2 0 pc’ 1 0 1
3 pc+3L i 0 - 0 0 1
4 pc’ i 0 (pc’) 0 1 0
5 pc’+L i 0 (pc’+L) 0 1 0
pc’+2L
n registers 1 pc+2L i 0 (pc+2L) 0 0 0
(n>1) 2 alu 2 0 (alu) 0 1 1
• alu+• 2 0 (alu+•) 0 1 1
n alu+• 2 0 (alu+•) 0 1 1
n+1 alu+• 2 0 (alu+•) 1 0 1
n+2 pc+3L i 0 - 0 1 1
pc+3L
n registers 1 pc+2L i 0 (pc+2L) 0 0 0
(n>1) 2 alu 2 0 (alu) 0 1 1
incl pc • alu+• 2 0 (alu+•) 0 1 1
n alu+• 2 0 (alu+•) 0 1 1
n+1 alu+• 2 0 pc’ 1 0 1
n+2 pc+3L i 0 - 0 0 1
n+3 pc’ i 0 (pc’) 0 1 0
n+4 pc’+L i 0 (pc’+L) 0 1 0
pc’+2L
Table 10-11: Load Multiple Registers instruction cycle operations
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
10-11
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10.10Store Multiple Registers
Store multiple proceeds very much as load multiple, without the final cycle. The restart
problem is much more straightforward here, as there is no wholesale overwriting of
registers. The cycle timings are shown in ➲
Table 10-12: Store Multiple Registers
instruction cycle operations
, below.
10.11Data Swap
This is similar to the load and store register instructions, but the actual swap takes
place in cycles 2 and 3. In the second cycle, the data is fetched from external memory.
In the third cycle, the contents of the source register are written out to the external
memory. The data read in cycle 2 is written into the destination register during the
fourth cycle. The cycle timings are shown below in ➲
Table 10-13: Data Swap
instruction cycle operations
on page 10-11.
The LOCK output of ARM7TDMI is driven HIGH for the duration of the swap operation
(cycles 2 and 3) to indicate that both cycles should be allowed to complete without
interruption.
The data swapped may be a byte or word quantity (b/w).
The swap operation may be aborted in either the read or write cycle, and in both cases
the destination register will not be affected.
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC
1 register 1 pc+2L i 0 (pc+2L) 0 0 0
2 alu 2 1 Ra 0 0 1
pc+3L
n registers 1 pc+8 i 0 (pc+2L) 0 0 0
(n>1) 2 alu 2 1 Ra 0 1 1
• alu+• 2 1 R• 0 1 1
n alu+• 2 1 R• 0 1 1
n+1 alu+• 2 1 R• 0 0 1
pc+12
Table 10-12: Store Multiple Registers instruction cycle operations
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC LOCK
1 pc+8 2 0 (pc+8) 0 0 0 0
2 Rn b/w 0 (Rn) 0 0 1 1
Table 10-13: Data Swap instruction cycle operations
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
10-12
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b and w are byte and word as defined in ➲
Table 9-2: MAS[1:0] signal encoding
on
page 9-5.
Note
Data swap cannot be executed in THUMB state.
10.12Software Interrupt and Exception Entry
Exceptions (and software interrupts) force the PC to a particular value and refill the
instruction pipeline from there. During the first cycle the forced address is constructed,
and a mode change may take place. The return address is moved to R14 and the
CPSR to SPSR_svc.
During the second cycle the return address is modified to facilitate return, though this
modification is less useful than in the case of branch with link.
The third cycle is required only to complete the refilling of the instruction pipeline. The
cycle timings are shown below in ➲
Table 10-14: Software Interrupt instruction cycle
operations
.
C represents the current mode-dependent value.
T represents the current state-dependent value
pc for software interrupts is the address of the SWI instruction.
for exceptions is the address of the instruction following the last one
to be executed before entering the exception.
for prefetch aborts is the address of the aborting instruction.
for data aborts is the address of the instruction following the one
which attempted the aborted data transfer.
Xn is the appropriate trap address.
3 Rn b/w 1 Rm 1 0 1 1
4 pc+12 2 0 - 0 1 1 0
pc+12
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC nTRANS Mode TBIT
1 pc+2L i 0 (pc+2L) 0 0 0 C old mode T
2 Xn 2 0 (Xn) 0 1 0 1 exception mode 0
3 Xn+4 2 0 (Xn+4) 0 1 0 1 exception mode 0
Xn+8
Table 10-14: Software Interrupt instruction cycle operations
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC LOCK
Table 10-13: Data Swap instruction cycle operations
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
10-13
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10.13Coprocessor Data Operation
A coprocessor data operation is a request from ARM7TDMI for the coprocessor to
initiate some action. The action need not be completed for some time, but the
coprocessor must commit to doing it before driving CPB LOW.
If the coprocessor can never do the requested task, it should leave CPA and CPB
HIGH. If it can do the task, but can’t commit right now, it should drive CPA LOW but
leave CPB HIGH until it can commit. ARM7TDMI will busy-wait until CPB goes LOW.
The cycle timings are shown in➲
Table 10-15: Coprocessor Data Operation instruction
cycle operations
.
Note
This operation cannot occur in THUMB state.
Cycle Address nRW MAS[1:0] Data nMREQ SEQ nOPC nCPI CPA CPB
ready 1 pc+8 0 2 (pc+8) 0 0 0 0 0 0
pc+12
not
ready 1 pc+8 0 2 (pc+8) 1 0 0 0 0 1
2 pc+8 0 2 - 1 0 1 0 0 1
• pc+8 0 2 - 1 0 1 0 0 1
n pc+8 0 2 - 0 0 1 0 0 0
pc+12
Table 10-15:
Coprocessor Data Operation instruction cycle operations
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
10-14
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10.14Coprocessor Data Transfer (from memory to coprocessor)
Here the coprocessor should commit to the transfer only when it is ready to accept the
data. When CPB goes LOW, ARM7TDMI will produce addresses and expect the
coprocessor to take the data at sequential cycle rates. The coprocessor is responsible
for determining the number of words to be transferred, and indicates the last transfer
cycle by driving CPA and CPB HIGH.
ARM7TDMI spends the first cycle (and any busy-wait cycles) generating the transfer
address, and performs the write-back of the address base during the transfer cycles.
The cycle timings are shown in ➲
Table 10-16: Coprocessor Data Transfer instruction
cycle operations
on page 10-14
.
Cycles Address MAS
[1:0] nRW Data nMREQ SEQ nOPC nCPI CPA CPB
1
register 1 pc+8 2 0 (pc+8) 0 0 0 0 0 0
ready 2 alu 2 0 (alu) 0 0 1 1 1 1
pc+12
1
register 1 pc+8 2 0 (pc+8) 1 0 0 0 0 1
not
ready 2 pc+8 2 0 - 1 0 1 0 0 1
• pc+8 2 0 - 1 0 1 0 0 1
n pc+8 2 0 - 0 0 1 0 0 0
n+1 alu 2 0 (alu) 0 0 1 1 1 1
pc+12
n
regis-
ters
1 pc+8 2 0 (pc+8) 0 0 0 0 0 0
(n>1) 2 alu 2 0 (alu) 0 1 1 1 0 0
ready • alu+• 2 0 (alu+•) 0 1 1 1 0 0
n alu+• 2 0 (alu+•) 0 1 1 1 0 0
n+1 alu+• 2 0 (alu+•) 0 0 1 1 1 1
pc+12
Table 10-16: Coprocessor Data Transfer instruction cycle operations
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
10-15
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Note
This operation cannot occur in THUMB state.
10.15Coprocessor Data Transfer (from coprocessor to memory)
The ARM7TDMI controls these instructions exactly as for memory to coprocessor
transfers, with the one exception that thenRW line is inverted during the transfer cycle.
The cycle timings are show in ➲
Table 10-17: Coprocessor Data Transfer instruction
cycle operations
.
m
regis-
ters
1 pc+8 2 0 (pc+8) 1 0 0 0 0 1
(m>1) 2 pc+8 2 0 - 1 0 1 0 0 1
not
ready • pc+8 2 0 - 1 0 1 0 0 1
n pc+8 2 0 - 0 0 1 0 0 0
n+1 alu 2 0 (alu) 0 1 1 1 0 0
• alu+• 0 (alu+•) 0 1 1 1 0 0
n+m alu+• 2 0 (alu+•) 0 1 1 1 0 0
n+m+1 alu+• 2 0 (alu+•) 0 0 1 1 1 1
pc+12
Cycle Address MAS
[1:0] nRW Data nMREQ SEQ nOPC nCPI CPA CPB
1 register 1 pc+8 2 0 (pc+8) 0 0 0 0 0 0
ready 2 alu 2 1 CPdata 0 0 1 1 1 1
pc+12
1 register 1 pc+8 2 0 (pc+8) 1 0 0 0 0 1
not ready 2 pc+8 2 0 - 1 0 1 0 0 1
• pc+8 2 0 - 1 0 1 0 0 1
n pc+8 2 0 - 0 0 1 0 0 0
n+1 alu 2 1 CPdata 0 0 1 1 1 1
Table 10-17: Coprocessor Data Transfer instruction cycle operations
Cycles Address MAS
[1:0] nRW Data nMREQ SEQ nOPC nCPI CPA CPB
Table 10-16: Coprocessor Data Transfer instruction cycle operations
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
10-16
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Note
This operation cannot occur in THUMB state.
10.16Coprocessor Register Transfer (Load from coprocessor)
Here the busy-wait cycles are much as above, but the transfer is limited to one data
word, and ARM7TDMI puts the word into the destination register in the third cycle. The
third cycle may be merged with the following prefetch cycle into one memory N-cycle
as with all ARM7TDMI register load instructions. The cycle timings are shown in
➲
Table 10-18: Coprocessor register transfer (Load from coprocessor)
.
pc+12
n registers 1 pc+8 2 0 (pc+8) 0 0 0 0 0 0
(n>1) 2 alu 2 1 CPdata 0 1 1 1 0 0
ready • alu+• 2 1 CPdata 0 1 1 1 0 0
n alu+• 2 1 CPdata 0 1 1 1 0 0
n+1 alu+• 2 1 CPdata 0 0 1 1 1 1
pc+12
m registers 1 pc+8 2 0 (pc+8) 1 0 0 0 0 1
(m>1) 2 pc+8 2 0 - 1 0 1 0 0 1
not ready • pc+8 2 0 - 1 0 1 0 0 1
n pc+8 2 0 - 0 0 1 0 0 0
n+1 alu 2 1 CPdata 0 1 1 1 0 0
• alu+• 2 1 CPdata 0 1 1 1 0 0
n+m alu+• 2 1 CPdata 0 1 1 1 0 0
n+m+1 alu+• 2 1 CPdata 0 0 1 1 1 1
pc+12
Cycle Address MAS
[1:0] nRW Data nMREQ SEQ nOPC nCPI CPA CPB
ready 1 pc+8 2 0 (pc+8) 1 1 0 0 0 0
2 pc+12 2 0 CPdata 1 0 1 1 1 1
3 pc+12 2 0 - 0 1 1 1 - -
Table 10-18: Coprocessor register transfer (Load from coprocessor)
Cycle Address MAS
[1:0] nRW Data nMREQ SEQ nOPC nCPI CPA CPB
Table 10-17: Coprocessor Data Transfer instruction cycle operations (Continued)
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
10-17
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Note
This operation cannot occur in THUMB state.
10.17Coprocessor Register Transfer (Store to coprocessor)
As for the load from coprocessor, except that the last cycle is omitted. The cycle
timings are shown in ➲
Table 10-19: Coprocessor register transfer (Store to
coprocessor)
on page 10-17.
Note
This operation cannot occur in THUMB state.
pc+12
not ready 1 pc+8 2 0 (pc+8) 1 0 0 0 0 1
2 pc+8 2 0 - 1 0 1 0 0 1
• pc+8 2 0 - 1 0 1 0 0 1
n pc+8 2 0 - 1 1 1 0 0 0
n+1 pc+12 2 0 CPdata 1 0 1 1 1 1
n+2 pc+12 2 0 - 0 1 1 1 - -
pc+12
Cycle Address MAS
[1:0] nRW Data nMREQ SEQ nOPC nCPI CPA CPB
ready 1 pc+8 2 0 (pc+8) 1 1 0 0 0 0
2 pc+12 2 1 Rd 0 0 1 1 1 1
pc+12
not ready 1 pc+8 2 0 (pc+8) 1 0 0 0 0 1
2 pc+8 2 0 - 1 0 1 0 0 1
• pc+8 2 0 - 1 0 1 0 0 1
n pc+8 2 0 - 1 1 1 0 0 0
n+1 pc+12 2 1 Rd 0 0 1 1 1 1
pc+12
Table 10-19: Coprocessor register transfer (Store to coprocessor)
Cycle Address MAS
[1:0] nRW Data nMREQ SEQ nOPC nCPI CPA CPB
Table 10-18: Coprocessor register transfer (Load from coprocessor)
Instruction Cycle Operations
ARM7TDMI Data Sheet
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10.18Undefined Instructions and Coprocessor Absent
When a coprocessor detects a coprocessor instruction which it cannot perform, and
this must include all undefined instructions, it must not drive CPA or CPB LOW . These
will remain HIGH, causing the undefined instruction trap to be taken. Cycle timings are
shown in ➲
Table 10-20: Undefined instruction cycle operations
.
C represents the current mode-dependent value.
T represents the current state-dependent value.
Note
Coprocessor Instructions cannot occur in THUMB state.
10.19Unexecuted Instructions
Any instruction whose condition code is not met will fail to execute. It will add one cycle
to the execution time of the code segment in which it is embedded (see ➲
Table 10-21:
Unexecuted instruction cycle operations
).
Cycle Address MAS
[1:0] nRW Data nMREQ SEQ nOPC nCPI CPA CPB nTRANS Mode TBIT
1 pc+2L i 0 (pc+2L) 1 0 0 0 1 1 C Old T
2 pc+2L i 0 - 0 0 0 1 1 1 C Old T
3 Xn 2 0 (Xn) 0 1 0 1 1 1 1 00100 0
4 Xn+4 2 0 (Xn+4) 0 1 0 1 1 1 1 00100 0
Xn+8
Table 10-20: Undefined instruction cycle operations
Cycle Address MAS[1:0] nRW Data nMREQ SEQ nOPC
1 pc+2L i 0 (pc+2L) 0 1 0
pc+3L
Table 10-21: Unexecuted instruction cycle operations
Instruction Cycle Operations
ARM7TDMI Data Sheet
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10-19
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10.20Instruction Speed Summary
Due to the pipelined architecture of the CPU, instructions overlap considerably. In a
typical cycle one instruction may be using the data path while the next is being
decoded and the one after that is being fetched. For this reason the following table
presents the incremental number of cycles required by an instruction, rather than the
total number of cycles for which the instruction uses part of the processor. Elapsed
time (in cycles) for a routine may be calculated from these figures which are shown in
➲
Table 10-22: ARM instruction speed summary
on page 10-20. These figures assume
that the instruction is actually executed. Unexecuted instructions take one cycle.
n is the number of words transferred
m is 1 if bits [32:8] of the multiplier operand are all zero or one.
2 if bits[32:16] of the multiplier operand are all zero or one.
3if bits[31:24] of the multiplier operand are all zero or all one.
4 otherwise.
b is the number of cycles spent in the coprocessor busy-wait loop.
If the condition is not met all the instructions take one S-cycle. The cycle types N, S,
I, and C are defined in ➲
Chapter 6, Memory Interface.
Instruction Cycle Operations
ARM7TDMI Data Sheet
ARM DDI 0029E
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Instruction Cycle count Additional
Data Processing 1S + 1I for SHIFT(Rs)
+ 1S + 1N if R15 written
MSR, MRS 1S
LDR 1S+1N+1I + 1S + 1N if R15 loaded
STR 2N
LDM nS+1N+1I + 1S + 1N if R15 loaded
STM (n-1)S+2N
SWP 1S+2N+1I
B,BL 2S+1N
SWI, trap 2S+1N
MUL 1S+mI
MLA 1S+(m+1)I
MULL 1S+(m+1)I
MLAL 1S+(m+2)I
CDP 1S+bI
LDC,STC (n-1)S+2N+bI
MCR 1N+bI+1C
MRC 1S+(b+1)I+1C
Table 10-22: ARM instruction speed summary
ARM7TDMI Data Sheet
ARM DDI 0029E
11-1
11
1
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DC Parameters
11.1 Absolute Maximum Ratings 11-2
11.2 DC Operating Conditions 11-2
11
DC Parameters
ARM7TDMI Data Sheet
ARM DDI 0029E
11-2
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11.1 Absolute Maximum Ratings
Note
These are stress ratings only. Exceeding the absolute maximum ratings may
permanently damage the device. Operating the device at absolute maximum ratings
for extended periods may affect device reliability.
11.2 DC Operating Conditions
Notes 1 Voltages measured with respect to VSS.
2 IC CMOS-level inputs.
Symbol Parameter Min Max Units
VDD Supply voltage VSS-0.3 VSS+7.0 V
Vin Input voltage applied to any pin VSS-0.3 VDD+0.3 V
Ts Storage temperature -50 150 deg C
Table 11-1: ARM7TDMI DC maximum ratings
Symbol Parameter Min Typ Max Units Notes
VDD Supply voltage 2.7 3.0 3.6 V
Vihc IC input HIGH voltage .8xVDD VDD V 1,2
Vilc IC input LOW voltage 0.0 .2xVDD V 1,2
Ta Ambient operating temperature -40 85 C
Table 11-2: ARM7TDMI DC operating conditions
ARM7TDMI Data Sheet
ARM DDI 0029E
12-1
11
1
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AC Parameters
The timing parameters given here are preliminary data and subject to change.
12.1 Introduction 12-2
12.2 Notes on AC Parameters 12-11
12
AC Parameters
ARM7TDMI Data Sheet
ARM DDI 0029E
12-2
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12.1 Introduction
The AC timing diagrams presented in this section assume that the outputs of the
ARM7TDMI have been loaded with the capacitive loads shown in the “Test Load”
column of ➲
Table 12-1: AC test loads
. These loads have been chosen as typical of
the type of system in which ARM7TDMI might be employed.
The output drivers of the ARM7TDMI are CMOS inverters which exhibit a propagation
delay that increases linearly with the increase in load capacitance. An “Output
derating” figure is given for each output driver, showing the approximate rate of
increase of output time with increasing load capacitance.
Output Signal Test Load (pF) Output derating (ns/pF)
D[31:0] TBD TBD
A[31:0] TBD TBD
LOCK TBD TBD
nCPI TBD TBD
nMREQ TBD TBD
SEQ TBD TBD
nRW TBD TBD
MAS[1:0] TBD TBD
nOPC TBD TBD
nTRANS TBD TBD
TDO TBD TBD
Table 12-1: AC test loads
AC Parameters
ARM7TDMI Data Sheet
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Figure 12-1: General timings
Note nWAIT, APE, ALE and ABE are all HIGH during the cycle shown. Tcdel is the delay
(on either edge) from MCLK changing to ECLK changing.
MCLK
ECLK
A[31:0]
nRW
MAS[1:0],
LOCK
nM[4:0],
nTRANS
TBIT
nOPC
nMREQ,
SEQ
nEXEC
T
cdel
T
cdel
T
ah
T
addr
T
rwh
T
rwd
T
blh
T
bld
T
mdh
T
mdd
T
opch
T
opcd
T
msh
T
msd
T
exh
T
exd
AC Parameters
ARM7TDMI Data Sheet
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Figure 12-2: ALE address control
Note Tald is the time by whichALE must be driven LOW in order to latch the current address
in phase 2. If ALE is driven low after Tald, then a new address will be latched.
Figure 12-3: APE address control
Figure 12-4: ABE address control
MCLK
ALE
A[31:0],
nRW, LOCK,
nOPC,
nTRANS,
MAS[1:0]
T
ald
T
ale
MCLK
APE
A[31:0],
nRW, LOCK,
nOPC,
nTRANS,
MAS[1:0]
T
aph
T
aps
T
ape
MCLK
ABE
A[31:0],
nRW, LOCK,
nOPC,
nTRANS,
MAS[1:0] T
abz
T
abe
T
addr
AC Parameters
ARM7TDMI Data Sheet
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Figure 12-5: Bidirectional data write cycle
Note DBE is HIGH and nENIN is LOW during the cycle shown.
Figure 12-6: Bidirectional data read cycle
Note DBE is HIGH and nENIN is LOW during the cycle shown.
MCLK
nENOUT
D[31:0]
T
nen
T
nenh
T
dout
T
doh
MCLK
nENOUT
D[31:0]
BL[3:0]
Tnen
Tdis Tdih
Tbylh Tbyls
AC Parameters
ARM7TDMI Data Sheet
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Figure 12-7: Data bus control
Note The cycle shown is a data write cycle since nENOUT was driven LOW during phase
1. Here, DBE has first been used to modify the behaviour of the data bus, and then
nENIN.
Figure 12-8: Output 3-state time
MCLK
nENOUT
DBE
D[31:0]
nENIN
T
dbnen
T
dbnen
T
dbz
T
dbe
T
dout
T
doh
T
dbz
T
dbe
MCLK
TBE
A[31:0],
D[31:0],
nRW, LOCK,
nOPC,
nTRANS
MAS[1:0] T
tbz
T
tbe
AC Parameters
ARM7TDMI Data Sheet
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Figure 12-9: Unidirectional data write cycle
Figure 12-10: Unidirectional data read cycle
Figure 12-11: Configuration pin timing
MCLK
nENOUT
DOUT[31:0]
T
nen
T
dohu
T
doutu
MCLK
nENOUT
DIN[31:0]
BL[3:0]
T
nen
T
disu
T
dihu
T
bylh
T
byls
MCLK
BIGEND
ISYNC
T
cth
T
cts
T
cts
T
cth
AC Parameters
ARM7TDMI Data Sheet
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Figure 12-12: Coprocessor timing
Note Normally, nMREQ and SEQ become valid Tmsd after the falling edge of MCLK. In this
cycle the ARM has been busy-waiting, waiting for a coprocessor to complete the
instruction. If CPA and CPB change during phase 1, the timing of nMREQ and SEQ
will depend on Tcpms. Most systems should be able to generateCPA and CPB during
the previous phase 2, and so the timing of nMREQ and SEQ will always be Tmsd.
Figure 12-13: Exception timing
Note Tis/Trs guarantee recognition of the interrupt (or reset) source by the corresponding
clock edge. T im/Trm guarantee non-recognition by that clock edge. These inputs may
be applied fully asynchronously where the exact cycle of recognition is unimportant.
MCLK
nCPI
CPA, CPB
nMREQ,
SEQ
T
cpi
T
cpih
T
cps
T
cph
T
cpms
MCLK
ABORT
nFIQ, nIRQ
nRESET
T
abts
T
abth
T
is
T
im
T
rs
T
rm
AC Parameters
ARM7TDMI Data Sheet
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Figure 12-14: Debug timing
Figure 12-15: Breakpoint timing
Note BREAKPT changing in the LOW phase of MCLK to signal a watchpointed store can
affect nCPI,nEXEC,nMREQ, and SEQ in the LOW phase of MCLK.
Figure 12-16: TCK-ECLK relationship
MCLK
DBGACK
BREAKPT
DBGRQ
EXTERN[1:0]
T
dbgh
T
dbgd
T
brks
T
brkh
T
rqs
T
rqh
T
exts
T
exth
MCLK
BREAKPT
nCPI, nEXEC
nMREQ, SEQ T
bcems
TCK
ECLK Tctdel Tctdel
AC Parameters
ARM7TDMI Data Sheet
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Figure 12-17: MCLK timing
Note The ARM core is not clocked by the HIGH phase ofMCLK enveloped by nWAIT. Thus,
during the cycles shown, nMREQ and SEQ change once, during the first LOW phase
of MCLK, and A[31:0] change once, during the second HIGH phase of MCLK. For
reference, ph2 is shown. This is the internal clock from which the core times all its
activity. This signal is included to show how the high phase of the external MCLK has
been removed from the internal core clock.
MCLK
AC Parameters
ARM7TDMI Data Sheet
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12.2 Notes on AC Parameters
All figures are provisional and assume a process which achieves 33MHz MCLK
maximum operating frequency.
Output load is 0.45pF.
Symbol Parameter Min Max
Tmckl MCLK LOW time 15.1
Tmckh MCLK HIGH time 15.1
Tws nWAIT setup to MCLKr 2.3
Twh nWAIT hold from CKf 1.1
Tale address latch open 7.5
Taleh Address latch hold time 2.1
Tald address latch time 3.4
Taddr MCLKr to address valid 14.0
Tah address hold time from MCLKr 2.4
Tabe address bus enable time 6.2
Tabz address bus disable time 5.3
Taph APE hold time from MCLKr 4.9
Taps APE set up time to MCLKf 0
Tape MCLKf to address valid 8.9
Tapeh Address group hold time from MCLKf 2.1
Tdout MCLKf to D[31:0] valid 14.9
Tdoh D[31:0] out hold from MCLKf 2.2
Tdis D[31:0] in setup time to MCLKf 0.9
Tdih D[31:0] in hold time from MCLKf 2.6
Tdoutu MCLKf to DOUT[31:0] valid 17
Tdohu DOUT[31:0] hold time from MCLKf 2.4
Tdisu DIN[31:0] set up time to MCLKf 1.8
Tdihu DIN[hold time to MCLKf 1.7
Tnen MCLKf to nENOUT valid 11.2
Tnenh nENOUT hold time from MCLKf 2.4
Table 12-2: Provisional AC parameters (units of nS)
AC Parameters
ARM7TDMI Data Sheet
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12-12
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Tbylh BL[3:0] hold time from MCLKf 0.7
Tbyls BL[3:0] set up to from MCLKr 0.1
Tdbe Data bus enable time from DBEr 15.2
Tdbz Data bus disable time from DBEf 14.5
Tdbnen DBE to nENOUT valid 5.5
Ttbz Address and Data bus disable time from TBEf 5.5
Ttbe Address and Data bus enable time from TBEr 7.8
Trwd MCLKr to nRW valid 14.0
Trwh nRW hold time from MCLKr 2.4
Tmsd MCLKf to nMREQ & SEQ valid 17.9
Tmsh nMREQ & SEQ hold time from MCLKf 2.4
Tbld MCLKr to MAS[1:0] & LOCK 18.9
Tblh MAS[1:0] & LOCK hold from MCLKr 2.4
Tmdd MCLKr to nTRANS, nM[4:0], and TBIT valid 19.5
Tmdh nTRANS & nM[4:0] hold time from MCLKr 2.4
Topcd MCLKr to nOPC valid 10.6
Topch nOPC hold time from MCLKr 2.4
Tcps CPA, CPB setup to MCLKr 5.1
Tcph CPA,CPB hold time from MCLKr 0.2
Tcpms CPA, CPB to nMREQ, SEQ 9.9
Tcpi MCLKf to nCPI valid 17.9
Tcpih nCPI hold time from MCLKf 2.4
Tcts Config setup time 2.1
Tcth Config hold time 3.4
Tabts ABORT set up time to MCLKf 0.6
Tabth ABORT hold time from MCLKf 1.5
Tis Asynchronous interrupt set up time to MCLKf for guaranteed
recognition (ISYNC=0) 0.1
Tim Asynchronous interrupt guaranteed non-recognition time
(ISYNC=0) 3.1
Symbol Parameter Min Max
Table 12-2: Provisional AC parameters (units of nS) (Continued)
AC Parameters
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Tsis Synchronous nFIQ, nIRQ setup to MCLKf (ISYNC=1) 9.0
Tsih Synchronous nFIQ, nIRQ hold from MCLKf (ISYNC=1) 1.1
Trs Reset setup time to MCLKr for guaranteed recognition 1.9
Trm Reset guaranteed non-recognition time 3.7
Texd MCLKf to nEXEC valid 17.9
Texh nEXEC hold time from MCLKf 2.4
Tbrks Set up time of BREAKPT to MCLKr 14.6
Tbrkh Hold time of BREAKPT from MCLKr 2.5
Tbcems BREAKPT to nCPI, nEXEC, nMREQ, SEQ delay 14.3
Tdbgd MCLKr to DBGACK valid 15.2
Tdbgh DGBACK hold time from MCLKr 2.4
Trqs DBGRQ set up time to MCLKr for guaranteed recognition 2.6
Trqh DBGRQ guaranteed non-recognition time 1.0
Tcdel MCLK to ECLK delay 2.9
Tctdel TCK to ECLK delay 10.4
Texts EXTERN[1:0] set up time to MCLKf 0
Texth EXTERN[1:0] hold time from MCLKf 3.8
Trg MCLKf to RANGEOUT0, RANGEOUT1 valid 15.2
Trgh RANGEOUT0, RANGEOUT1 hold time from MCLKf 2.4
Tdbgrq DBGRQ to DBGRQI valid 2.9
Trstd nRESETf to D[], DBGACK, nCPI, nENOUT, nEXEC,
nMREQ, SEQ valid 13.7
Tcommd MCLKr to COMMRX, COMMTX valid 9.3
Ttrstd nTRSTf to every output valid 13.7
Trstl nRESET LOW for guaranteed reset 2 MCLK
cycles
Symbol Parameter Min Max
Table 12-2: Provisional AC parameters (units of nS) (Continued)
AC Parameters
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11
1
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A
Abort
data 3-12
during block data transfer 4-44
prefetch 3-12
Abort mode 3-4
ADC
ARM instruction 4-11
THUMB instruction 5-3,5-11
ADD
ARM instruction 4-11
THUMB instruction 5-3,5-7,5-9,5-28,5-30
with Hi register operand 5-13
address bus
configuring 6-4
Advantages
of THUMB 1-3
AND
ARM instruction 4-11
THUMB instruction 5-3,5-11
ARM state.
See
operating state
ASR
ARM instruction 4-13
THUMB instruction 5-3,5-5,5-11
B
B (Branch)
ARM instruction 4-8
THUMB instruction
conditional 5-3,5-36,5-37
unconditional 5-3,5-39
BICARM instruction 4-11
THUMB instruction 5-3,5-12
big endian.
See
memory format
BL (Branch and Link)
ARM instruction 4-8
THUMB instruction 5-3,5-41
Branch instruction 10-2
branching
in ARM state 4-8
in THUMB state 5-3,5-36,5-37,5-39
to subroutine
in ARM state 4-8
in THUMB state 5-3,5-41
Breakpoints
entering debug state from 8-23
with prefetch abort 8-25
BX (Branch and Exchange)
ARM instruction 4-6
THUMB instruction 5-3,5-14
with Hi register operand 5-14
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BYPASS
public instruction 8-11
Bypass register 8-12
byte (data type) 3-3
loading and storing 4-29,5-3,5-4,5-19,5-20,
5-23
C
CDP
ARM instruction 4-51
CLAMP
public instruction 8-11
CLAMPZ
public instruction 8-12
Clock switching
debug state 8-18
test state 8-19
CMN
ARM instruction 4-11,4-16
THUMB instruction 5-3,5-12
CMP
ARM instruction 4-11,4-16
THUMB instruction 5-3,5-9,5-12
with Hi register operand 5-14
Concepts
of THUMB 1-2
condition code flags 3-8
condition codes
summary of 4-5
conditional execution
in ARM state 4-5
coprocessor
data operations 4-51
data transfer 4-53
action on data abort 4-54
passing instructions to 7-2
pipeline following 7-3
register transfer 4-57
coprocessor interface 7-2–7-4
Core state
determining 8-19
CP# (coprocessor number) field 7-2
CPSR (Current Processor Status Register) 3-8
format of 3-8
reading 4-18
writing 4-18
D
data bus
external 6-18
internal 6-13
Data operations 10-4
data transfer
block
in ARM state 4-40
in THUMB state 5-3,5-4,5-34
single
in ARM state 4-28
in THUMB state 5-3,5-4,5-16,5-17,5-18,
5-19,5-20,5-21,5-22,5-23,5-24,
5-26
specifying size of 6-9
data types 3-3
Debug request
entering debug state via 8-24
Debug state
exiting from 8-21
Debug systems 8-2,8-3
Device Identification Code register 8-13
E
EOR
ARM instruction 4-11
THUMB instruction 5-3,5-11
exception
entering 3-10
leaving 3-10
priorities 3-14
returning to THUMB state from 3-10
vectors 3-13
EXTEST 8-10
public instruction 8-10
F
FIQ mode 3-4
definition of 3-11
See also
interrupts
Index
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H
halfword
loading and storing 4-34
halfword (data type) 3-3,4-34
loading and storing 5-3,5-4,5-20,5-21,5-24
Hi register
accessing from THUMB state 3-7
description 3-7
operations
example code 5-15
operations on 5-13
HIGHZ
public instruction 8-11
I
ICEbreaker
Breakpoints 9-6
coupling with Watchpoints 9-11
hardware 9-7
software 9-7
BREAKPT signal 9-2
communications 9-14
Control registers 9-5
Debug Control register 9-9
Debug Status register 9-10
disabling 9-13
TAP controller 9-2,9-4
Watchpoint registers 9-3–9-4
Watchpoints
coupling with Breakpoints 9-11
IDCODE
public instruction 8-10
Instruction register 8-13
INTEST
public instruction 8-10
IRQ mode 3-4
definition of 3-12
See also
interrupts
J
Jtag state machine 8-8
L
LDCARM instruction 4-53
LDM
action on data abort 4-44
ARM instruction 4-40
LDMIA
THUMB instruction 5-3,5-34
LDRARM instruction 4-28
THUMB instruction5-3,5-16,5-17,5-19,5-22,
5-26
LDRB
THUMB instruction 5-3,5-19,5-23
LDRH
THUMB instruction 5-3,5-20,5-21,5-24
LDSB
THUMB instruction 5-3,5-20
LDSH
THUMB instruction 5-3
little endian.
See
memory format
Lo registers 3-7
LOCK output 4-47
LSLARM instruction 4-12,4-13
THUMB instruction 5-3,5-5,5-11
LSRARM instruction 4-13
THUMB instruction 5-3,5-5
M
memory
locking 6-12
protecting 6-12
memory access times 6-12
memory cycle timing 6-3
memory cycle types 6-2
memory format
big endian
description 3-3
single data transfer in 4-30
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little endian
description 3-3
single data transfer in 4-29
memory transfer cycle
non-sequential 6-12
memory transfer cycle types 6-2
MLA
ARM instruction 4-23
MLAL
ARM instruction 4-23,4-25
MOV
ARM instruction 4-11
THUMB instruction 5-3,5-9
with Hi register operand 5-14
MRS
ARM instruction 4-18
MSR
ARM instruction 4-18
MUL
ARM instruction 4-23
THUMB instruction 5-3,5-12
MULL
ARM instruction 4-23,4-25
MVN
ARM instruction 4-11
THUMB instruction 5-3,5-12
N
NEG
THUMB instruction 5-4,5-11
O
operating mode
reading 3-9
setting 3-9
operating state
ARM 3-2
reading 3-8
switching 3-2
to ARM 3-2,5-14,5-15
to THUMB 3-2,4-7
THUMB 3-2
ORR
ARM instruction 4-11
THUMB instruction 5-4,5-12
P
pipeline 7-3
POP
THUMB instruction 5-4,5-32
privileged instruction 7-3
Public instructions 8-9
PUSH
THUMB instruction 5-32
R
registers
ARM 3-4
THUMB 3-6
reset
action of processor on 3-15
Return address calculations 8-25
ROR
ARM instruction 4-14
THUMB instruction 5-4,5-11
rotate operations 4-14,4-15
RRX
ARM instruction 4-14
RSB
ARM instruction 4-11
RSC
ARM instruction 4-11
S
SAMPLE/PRELOAD
public instruction 8-12
SBC
ARM instruction 4-11
THUMB instruction 5-11
Scan Chain Select register 8-13
Scan Chains 8-14
Scan limitations 8-6
SCAN_N
public instruction 8-10
shift operations 4-12,4-15,5-5,5-11
Software Interrupt 3-13,4-49,5-4
SPSR (Saved Processor Status Register) 3-8
format of 3-8
reading 4-18
writing 4-18
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stack operations 5-32
STCARM instruction 4-53
STM
ARM instruction 4-40
STMIA
THUMB instruction 5-4,5-34
STRARM instruction 4-28
THUMB instruction 5-4,5-18,5-22,5-26
STRB
THUMB instruction 5-4,5-19,5-23
STRH
THUMB instruction 5-4,5-20,5-24
SUB
ARM instruction 4-11
THUMB instruction 5-4,5-7,5-9
Supervisor mode 3-4
SWI 3-13
ARM instruction 4-49
THUMB instruction 5-4,5-38
SWP
ARM instruction 4-47
System mode 3-4
System speed access
during debug state 8-25
system state
determining 8-21
T
T bit (in CPSR) 3-8
TEQ
ARM instruction 4-11,4-16
THUMB Branch with Link operation 10-3
THUMB state.
See
operating state
TSTARM instruction 4-11,4-16
THUMB instruction 5-4,5-11
U
undefined instruction 7-4
undefined instruction trap 3-13,4-2
Undefined mode 3-4
User mode 3-4
V
virtual memory systems 3-12
W
Watchpoints
entering debug state from 8-23
word (data type)
address alignment 3-3
loading and storing 4-29,5-3,5-4,5-16,5-18,
5-19,5-22,5-26
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