The document provides information about the ARM architecture including: 1. ARM started as a replacement for the 8-bit 6502 chip in 1985 and was later spun off as its own company focusing on embedded CPU cores. 2. ARM is a 32-bit architecture with a load/store design using 3-operand instructions. It has a large register set and pipelined execution, along with some CISC features like complex multiply and load/store instructions. 3. The ARM programming model includes 7 processor modes, 37 registers including general purpose and special registers like the program counter and status registers, and conditional execution of instructions.

1. 1
TM 1
Agenda
 Introduction
Architecture
Programmers Model
Instruction Set

2. 2
TM 2
History of ARM
• ARM (Acorn RISC Machine) started as a new, powerful, CPU design for the
replacement of the 8-bit 6502 in Acorn Computers (Cambridge, UK, 1985)
• First models had only a 26-bit program counter, limiting the memory space
to 64 MB (not too much by today standards, but a lot at that time).
• 1990 spin-off: ARM renamed Advanced RISC Machines
• ARM now focuses on Embedded CPU cores
• IP licensing: Almost every silicon manufacturer sells some microcontroller
with an ARM core. Some even compete with their own designs.
• Processing power with low current consumption
• Good MIPS/Watt figure
• Ideal for portable devices
• Compact memories: 16-bit opcodes (Thumb)
• New cores with added features
• Harvard architecture (ARM9, ARM11, Cortex)
• Floating point arithmetic
• Vector computing (VFP, NEON)
• Java language (Jazelle)

3. 3
TM 3
Facts
• 32-bit CPU
• 3-operand instructions (typical): ADD Rd,Rn,Operand2
• RISC design…
• Few, simple, instructions
• Load/store architecture (instructions operate on registers, not memory)
• Large register set
• Pipelined execution
• … Although with some CISC touches…
• Multiplication and Load/Store Multiple are complex instructions (many cycles
longer than regular, RISC, instructions)
• … And some very specific details
• No stack. Link register instead
• PC as a regular register
• Conditional execution of all instructions
• Flags altered or not by data processing instructions (selectable)
• Concurrent shifts/rotations (at the same time of other processing)
• …

4. 4
TM 4

5. 5
TM 5
Agenda
Introduction
 Architecture
Programmers Model
Instruction Set

6. Address Register
REGISTER
BANK
PC
Address
Incrementer
SHIFT
Multiplier
Write Data Reg.
translator
D[31:0]
INSTRUCCTION
DECODER
Control
Lines
ARM
Thumb to
Instruction Reg.
Read Data Reg.
B
bus
A
bus
ALU
bus
PC
bus
A[31:0]
A.L.U.
ARM7TDMI
Block Diagram

7. 7
TM 7
ARM Pipelining examples
• Fetch: Read Op-code from memory to internal Instruction Register
• Decode: Activate the appropriate control lines depending on Opcode
• Execute: Do the actual processing

8. 8
TM 8
ARM7TDMI Pipelining (I)
FETCH DECODE EXECUTE
FETCH DECODE EXECUTE
FETCH DECODE EXECUTE
time
3
2
1
instruction
• Simple instructions (like ADD) Complete at a rate of one per cycle

9. 9
TM 9
ARM7TDMI Pipelining (II)
FETCH DECODE EXECUTE
FETCH DECODE
FETCH EXECUTE
DECODE
FETCH DECODE EXECUTE
FETCH DECODE EXECUTE
Cal. ADDR
1
2
3
Data Xfer.
time
instruction
5
4
ADD
STR
ADD
ADD
ADD
stall
stall
• More complex instructions:
STR : 2 effective clock cycles (+1 cycle)

10. 10
TM 10
Agenda
Introduction
Architecture
 Programmers Model
Instruction Set

11. 11
TM 11
Data Sizes and Instruction Sets
 The ARM is a 32-bit architecture.
 When used in relation to the ARM:
 Byte means 8 bits
 Halfword means 16 bits (two bytes)
 Word means 32 bits (four bytes)
 Most ARM’s implement two instruction sets
 32-bit ARM Instruction Set
 16-bit Thumb Instruction Set

12. 12
TM 12
Processor Modes
 The ARM has seven operating modes:
 User : unprivileged mode under which most tasks run
 FIQ : entered when a high priority (fast) interrupt is raised
 IRQ : entered when a low priority (normal) interrupt is raised
 SVC : (Supervisor) entered on reset and when a Software Interrupt
instruction is executed
 Abort : used to handle memory access violations
 Undef : used to handle undefined instructions
 System : privileged mode using the same registers as user mode

13. 13
TM 13
The Registers
 ARM has 37 registers all of which are 32-bits long.
 1 dedicated program counter
 1 dedicated current program status register
 5 dedicated saved program status registers
 30 general purpose registers
 The current processor mode governs which of several banks is
accessible. Each mode can access
 a particular set of r0-r12 registers
 a particular r13 (the stack pointer, sp) and r14 (the link register, lr)
 the program counter, r15 (pc)
 the current program status register, cpsr
Privileged modes (except System) can also access
 a particular spsr (saved program status register)

14. 14
TM 14
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
r15 (pc)
cpsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
spsr
FIQ IRQ SVC Undef Abort
User Mode
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
r15 (pc)
cpsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
spsr
Current Visible Registers
Banked out Registers
FIQ IRQ SVC Undef Abort
r0
r1
r2
r3
r4
r5
r6
r7
r15 (pc)
cpsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
spsr
Current Visible Registers
Banked out Registers
User IRQ SVC Undef Abort
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
FIQ Mode
IRQ Mode
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
r15 (pc)
cpsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
spsr
Current Visible Registers
Banked out Registers
User FIQ SVC Undef Abort
r13 (sp)
r14 (lr)
Undef Mode
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
r15 (pc)
cpsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
spsr
Current Visible Registers
Banked out Registers
User FIQ IRQ SVC Abort
r13 (sp)
r14 (lr)
SVC Mode
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
r15 (pc)
cpsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
spsr
Current Visible Registers
Banked out Registers
User FIQ IRQ Undef Abort
r13 (sp)
r14 (lr)
Abort Mode
r0
r1
r2
r3
r4
r5
r6
r7
r8
r9
r10
r11
r12
r15 (pc)
cpsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r13 (sp)
r14 (lr)
spsr
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
spsr
Current Visible Registers
Banked out Registers
User,
SYS
FIQ IRQ SVC Undef
r13 (sp)
r14 (lr)
The ARM Register Set

15. 15
TM 15
Special Registers
 Special function registers:
 PC (R15): Program Counter. Any instruction with PC as its destination register
is a program branch
 LR (R14): Link Register. Saves a copy of PC when executing the BL instruction
(subroutine call) or when jumping to an exception or interrupt routine
- It is copied back to PC on the return from those routines
 SP (R13): Stack Pointer. There is no stack in the ARM architecture. Even so,
R13 is usually reserved as a pointer for the program-managed stack
 CPSR : Current Program Status Register. Holds the visible status register
 SPSR : Saved Program Status Register. Holds a copy of the previous status
register while executing exception or interrupt routines
- It is copied back to CPSR on the return from the exception or interrupt
- No SPSR available in User or System modes

16. 16
TM 16
Register Organization Summary
User
mode
r0-r7,
r15,
and
cpsr
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
spsr
FIQ
r8
r9
r10
r11
r12
r13 (sp)
r14 (lr)
r15 (pc)
cpsr
r0
r1
r2
r3
r4
r5
r6
r7
User,
SYS
r13 (sp)
r14 (lr)
spsr
IRQ
User
mode
r0-r12,
r15,
and
cpsr
r13 (sp)
r14 (lr)
spsr
Undef
User
mode
r0-r12,
r15,
and
cpsr
r13 (sp)
r14 (lr)
spsr
SVC
User
mode
r0-r12,
r15,
and
cpsr
r13 (sp)
r14 (lr)
spsr
Abort
User
mode
r0-r12,
r15,
and
cpsr
Note: System mode uses the User mode register set

17. 17
TM 17
Program Status Registers
 Condition code flags
 N = Negative result from ALU
 Z = Zero result from ALU
 C = ALU operation Carried out
 V = ALU operation oVerflowed
Interrupt Disable bits.
I = 1: Disables the IRQ.
F = 1: Disables the FIQ.
T Bit (Arch. with Thumb mode only)
T = 0: Processor in ARM state
T = 1: Processor in Thumb state
Never change T directly (use BX instead)
Changing T in CPSR will lead to
unexpected behavior due to pipelining
Tip: Don’t change undefined bits.
This allows for code compatibility with
newer ARM processors
 Mode bits
10000 User
10001 FIQ
10010 IRQ
10011 Supervisor
10111 Abort
11011 Undefined
11111 System
I F T
N Z C V
31 28 27 24 23 16 15 7
8 6 5 4
mode
0
undefined
f x c
s

18. 18
TM 18
 When the processor is executing in ARM state:
 All instructions are 32 bits wide
 All instructions must be word aligned
 Therefore the PC value is stored in bits [31:2] and bits [1:0] are zero
 Due to pipelining, the PC points 8 bytes ahead of the current instruction, or 12
bytes ahead if current instruction includes a register-specified shift
 When the processor is executing in Thumb state:
 All instructions are 16 bits wide
 All instructions must be halfword aligned
 Therefore the PC value is stored in bits [31:1] and bit [0] is zero
Program Counter (R15)

19. 19
TM 19
Vector Table
Exception Handling
 When an exception occurs, the ARM:
 Copies CPSR into SPSR_<mode>
 Sets appropriate CPSR bits:
 Changes to ARM state
 Changes to related mode
 Disables IRQ
 Disables FIQ (only on fast interrupts)
 Stores the return address in LR_<mode>
 Sets PC to vector address
 To return, exception handler needs to:
 Restore CPSR from SPSR_<mode>
 Restore PC from LR_<mode>
(more about this later…)
This can only be done in ARM state.
FIQ
IRQ
(Reserved)
Data Abort
Prefetch Abort
Software Interrupt
Undefined Instruction
Reset
0x1C
0x18
0x14
0x10
0x0C
0x08
0x04
0x00

20. 20
TM 20
Agenda
Introduction
Architecture
Programmers Model
 Instruction Set (for ARM state)

21. 21
TM 21
 ARM instructions can be made to execute conditionally by postfixing
them with the appropriate condition code field.
 This improves code density and performance by reducing the number of
forward branch instructions.
CMP r3,#0 CMP r3,#0
BEQ skip ADDNE r0,r1,r2
ADD r0,r1,r2
skip
 By default, data processing instructions do not affect the condition code
flags but the flags can be optionally set by using “S” (comparisons
always set the flags).
loop
…
SUBS r1,r1,#1
BNE loop if Z flag clear then branch
decrement r1 and set flags
Conditional Execution and Flags

22. 22
TM 22
Condition Codes
Not equal
Unsigned higher or same
Unsigned lower
Minus
Equal
Overflow
No overflow
Unsigned higher
Unsigned lower or same
Positive or Zero
Less than
Greater than
Less than or equal
Always
Greater or equal
EQ
NE
CS/HS
CC/LO
PL
VS
HI
LS
GE
LT
GT
LE
AL
MI
VC
Suffix Description
Z=0
C=1
C=0
Z=1
Flags tested
N=1
N=0
V=1
V=0
C=1 & Z=0
C=0 or Z=1
N=V
N!=V
Z=0 & N=V
Z=1 or N=!V
 The 15 possible condition codes are listed below:
 Note AL is the default and does not need to be specified

23. 23
TM 23
Advantages of conditional
execution

24. 24
TM 24
Examples of conditional
execution
 Use a sequence of several conditional instructions
if (a==0) func(1);
CMP r0,#0
MOVEQ r0,#1
BLEQ func
 Set the flags, then use various condition codes
if (a==0) x=0;
if (a>0) x=1;
CMP r0,#0
MOVEQ r1,#0
MOVGT r1,#1
 Use conditional compare instructions
if (a==4 || a==10) x=0;
CMP r0,#4
CMPEQ r0,#10
MOVEQ r1,#0

25. 25
TM 25
Data processing Instructions
 Consist of :
 Arithmetic: ADD ADC SUB SBC RSB RSC
 Logical: AND ORR EOR BIC
 Comparisons: CMP CMN TST TEQ
 Data movement: MOV MVN
 These instructions only work on registers, NOT memory.
L, Literal: 0: Operand 2 from register, 1: Operand 2 immediate
 Syntax:
<Operation>{<cond>}{S} Rd, Rn, Operand2
 {S} means that the Status register is going to be updated
 Comparisons always update the status register. Rd is not specified
 Data movement does not specify Rn
 Second operand is sent to the ALU via barrel shifter.
31 28 25 24 20 16 15 12 11 0
21 19
0 Rd Operand 2
Rn
S
op-code
L
0
cond.

26. 26
TM 26
Register, optionally with shift operation
 Shift value can be either be:
 5 bit unsigned integer
 Specified in bottom byte of another
register.
 Used for multiplication by a power of 2
Example: ADD R1, R2, R3, LSL #2
(R2 + R3*4) -> R1
Immediate value
 8 bit number, with a range of 0-255.
 Rotated right through even number of
positions
 Allows increased range of 32-bit
constants to be loaded directly into
registers
Result
Operand
1
Barrel
Shifter
Operand
2
ALU
Using the Barrel Shifter:
The Second Operand

27. 27
TM 27
The Barrel Shifter
Destination
CF 0 Destination CF
LSL : Logical Left Shift ASR: Arithmetic Right Shift
Multiplication by a power of 2 Division by a power of 2,
preserving the sign bit
Destination CF
...0 Destination CF
LSR : Logical Shift Right ROR: Rotate Right
Division by a power of 2 Bit rotate with wrap around
from LSB to MSB
Destination
RRX: Rotate Right Extended
Single bit rotate with wrap around
from CF to MSB
CF

28. 28
TM 28
 No ARM instruction can contain a 32 bit immediate constant
 All ARM instructions are fixed as 32 bits long
 The data processing instruction format has 12 bits available for operand2
 4 bit rotate value (0-15) is multiplied by two to give range 0-30 in steps of 2
 Rule to remember is “8-bits shifted by an even number of bit positions”.
0
7
11 8
immed_8
Shifter
ROR
rot
x2
Immediate constants (1)

29. 29
TM 29
 To allow larger constants to be loaded, the assembler offers a pseudo-
instruction:
 LDR rd, =const (notice the “=“ sign)
 This will either:
 Produce a MOV or MVN instruction to generate the value (if possible).
or
 Generate a LDR instruction with a PC-relative address to read the constant
from a literal pool (Constant data area embedded in the code).
 For example
 LDR r0,=0xFF => MOV r0,#0xFF
 LDR r0,=0x55555555 => LDR r0,[PC,#Imm12]
…
…
DCD 0x55555555
 This is the recommended way of loading constants into a register
Loading 32 bit constants

30. 30
TM 30
Data processing instr. FLAGS
 Flags are changed only if the S bit of the op-code is set:
Mnemonics ending with “s”, like “movs”, and comparisons: cmp, cmn, tst, teq
 N and Z have the expected meaning for all instructions
 N: bit 31 (sign) of the result
 Z: set if result is zero
 Logical instructions (AND, EOR, TST, TEQ, ORR, MOV, BIC, MVN)
 V: unchanged
 C: from barrel shifter if shift ≠ 0. Unchanged otherwise
 Arithmetic instructions (SUB, RSB, ADD, ADC, SBC, RSC, CMP, CMN)
 V: Signed overflow from ALU
 C: Carry (bit 32 of result) from ALU
 When PC is the destination register (exception return)
 CPSR is copied from SPSR. This includes all the flags.
 No change in user or system modes
Example: SUBS PC,LR,#4 @ return from IRQ

31. 31
TM 31
Arithmetic Operations
Operations are:
ADD operand1 + operand2
ADC operand1 + operand2 + carry
SUB operand1 - operand2
SBC operand1 - operand2 + carry -1
RSB operand2 - operand1
RSC operand2 - operand1 + carry - 1
Syntax:
<Operation>{<cond>}{S} Rd, Rn, Operand2
Examples
ADD r0, r1, r2
SUBGT r3, r3, #1
RSBLES r4, r5, #5

32. 32
TM 32
Comparisons
The only effect of the comparisons is to
UPDATE THE CONDITION FLAGS. Thus no need to set S bit.
Operations are:
CMP operand1 - operand2, but result not written
CMN operand1 + operand2, but result not written
TSToperand1 AND operand2, but result not written
TEQ operand1 EOR operand2, but result not written
Syntax:
<Operation>{<cond>} Rn, Operand2
Examples:
CMP r0, r1
TSTEQ r2, #5

33. 33
TM 33
Logical Operations
Operations are:
AND operand1 AND operand2
EOR operand1 EOR operand2
ORR operand1 OR operand2
BIC operand1 AND NOT operand2 [ie bit clear]
Syntax:
<Operation>{<cond>}{S} Rd, Rn, Operand2
Examples:
AND r0, r1, r2
BICEQ r2, r3, #7
EORS r1,r3,r0

34. 34
TM 34
Data Movement
Operations are:
MOV operand2
MVN NOT operand2
Note that these make no use of operand1.
Syntax:
<Operation>{<cond>}{S} Rd, Operand2
Examples:
MOV r0, r1
MOVS r2, #10
MVNEQ r1,#0

35. 35
TM 35
Multiply
 Syntax:
 MUL{<cond>}{S} Rd, Rm, Rs Rd = Rm * Rs
 MLA{<cond>}{S} Rd,Rm,Rs,Rn Rd = (Rm * Rs) + Rn
 [U|S]MULL{<cond>}{S} RdLo, RdHi, Rm, Rs RdHi,RdLo := Rm*Rs
 [U|S]MLAL{<cond>}{S} RdLo, RdHi, Rm, Rs RdHi,RdLo:=(Rm*Rs)+RdHi,RdLo
 Cycle time
 Basic MUL instruction
 2-5 cycles on ARM7TDMI
 1-3 cycles on StrongARM/XScale
 2 cycles on ARM9E/ARM102xE
 +1 cycle for ARM9TDMI (over ARM7TDMI)
 +1 cycle for accumulate (not on 9E though result delay is one cycle longer)
 +1 cycle for “long”
 Above are “general rules” - refer to the TRM for the core you are using
for the exact details

36. 36
TM 36
 Branch : B{<cond>} label
 Branch with Link : BL{<cond>} subroutine_label
 The processor core shifts the offset field left by 2 positions, sign-extends
it and adds it to the PC
 ± 32 Mbyte range
 How to perform longer branches or absolute address branches?
solution: LDR PC,…
28
31 24 0
Cond 1 0 1 L Offset
Condition field
Link bit 0 = Branch
1 = Branch with link
23
25
27
Branch instructions

37. 37
TM 37
Single register data transfer
LDR STR Word
LDRB STRB Byte
LDRH STRH Halfword
LDRSB Signed byte load
LDRSH Signed halfword load
 Memory system must support all access sizes
 Syntax:
 LDR{<cond>}{<size>} Rd, <address>
 STR{<cond>}{<size>} <address>, Rn
e.g. LDREQB

38. 38
TM 38
Address accessed
 Address accessed by LDR/STR is specified by a base register plus an
offset
 For word and unsigned byte accesses, offset can be
 An unsigned 12-bit immediate value (ie 0 - 4095 bytes).
LDR r0,[r1,#8]
 A register, optionally shifted by an immediate value
LDR r0,[r1,r2]
LDR r0,[r1,r2,LSL#2]
 This can be either added or subtracted from the base register:
LDR r0,[r1,#-8]
LDR r0,[r1,-r2]
LDR r0,[r1,-r2,LSL#2]
 For halfword and signed halfword / byte, offset can be:
 An unsigned 8 bit immediate value (ie 0-255 bytes).
 A register (unshifted).
 Choice of pre-indexed or post-indexed addressing

39. 39
TM 39
0x5
0x5
r1
0x200
Base
Register 0x200
r0
0x5
Source
Register
for STR
Offset
12 0x20c
r1
0x200
Original
Base
Register
0x200
r0
0x5
Source
Register
for STR
Offset
12 0x20c
r1
0x20c
Updated
Base
Register
Base-update form (‘!’): STR r0,[r1,#12]!
Pre or Post Indexed Addressing?
 Pre-indexed: STR r0,[r1,#12]
 Post-indexed: STR r0,[r1],#12
Base register always updated

40. 40
TM 40
LDM / STM operation
 Load/Store Multiple Syntax:
<LDM|STM>{<cond>}<addressing_mode> Rb{!}, <register list>
 4 addressing modes:
LDMIA / STMIA increment after
LDMIB / STMIB increment before
LDMDA / STMDA decrement after
LDMDB / STMDB decrement before
IA
r1 Increasing
Address
r4
r0
r1
r4
r0
r1
r4
r0 r1
r4
r0
r10
IB DA DB
LDMxx r10, {r0,r1,r4}
STMxx r10, {r0,r1,r4}
Base Register (Rb)
Base-update possible:
LDM r10!,{r0-r6}

41. 41
TM 41
Atomic data swap
 Exchanges a word or byte between a register and a
memory location
 This operation cannot be interrupted, not even by DMA
 Main use: Operating System semaphores
 Syntax:
 SWP {<cond>} Rd, Rm, [Rn]
 SWPB{<cond>} Rd, Rm, [Rn]
Rd=[Rn]; [Rn]=Rm (Rd and Rm can be the same)