ARM Assembly

From assembly to machine code

Directly on the ARM machine

Assemble the human-readable assembly code into an object file (program.o).

as program.s -o program.o

Link the object file into a fully executable program:

• Resolves symbol references

• Assigns final memory addresses

• Adds startup code (if specified)

• Produces an executable binary (program)

ld program.o -o program

Not on the ARM machine

arm-none-eabi-as program.s -o program.o
arm-none-eabi-ld program.o -o program

Assembly basics

ARM modes

• ARM mode: 4 bytes (32-bit) instruction

• Thumb mode: 2 bytes (16-bit) instruction

ARM datatypes

ARM Operation modes

Mode	Privileged	Purpose
User	No	Normal operating mode for most programs (tasks)
Fast Interrupt (FIQ)	Yes	Used to handle a high-priority (fast) interrupt
Interrupt (IRQ)	Yes	Used to handle a low-priority (normal) interrupt
Supervisor	Yes	Used when the processor is reset, and to handle the software interrupt instruction (SWI)
Abort	Yes	Used to handle memory access violations
Undefined	Yes	Used to handle undefined or unimplemented instructions
System	Yes	Uses the same registers as User mode

Registers use

CPSR

Flag	Description
N	Enabled if result of the instruction yields a negative number.
Z	Enabled if result of the instruction yields a zero value.
C	Enabled if result of the instruction yields a value that requires a 33rd bit to be fully represented.
V	Enabled if result of the instruction yields a value that cannot be represented in 32 bit two’s complement.
E	ARM can operate either in little endian, or big endian. This bit is set to 0 for little endian, or 1 for big endian
T	This bit is set if you are in Thumb state and is disabled when you are in ARM state.
M	These bits specify the current privilege mode (USR, SVC, etc.).
J	Third execution state that allows some ARM processors to execute Java bytecode in hardware.

Relationship between ARM and Intel processors

Instructions

Instruction	Description	Example Snippet	Explanation
MOV	Move data	MOV R0, R1	R0 = R1
MVN	Move and negate (bitwise NOT)	MVN R0, R1	R0 = ~R1
ADD	Addition	ADD R0, R1, R2	R0 = R1 + R2
SUB	Subtraction	SUB R0, R1, #5	R0 = R1 − 5
MUL	Multiplication	MUL R0, R1, R2	R0 = R1 × R2
LSL	Logical Shift Left	LSL R0, R1, #1	R0 = R1 << 1
LSR	Logical Shift Right	LSR R0, R1, #2	Shift right, fill with 0
ASR	Arithmetic Shift Right	ASR R0, R1, #1	Preserve sign
ROR	Rotate Right	ROR R0, R1, #4	Rotate bits right
CMP	Compare (sets flags)	CMP R0, #10	Compare R0 with 10
AND	Bitwise AND	AND R0, R1, R2	R0 = R1 & R2
ORR	Bitwise OR	ORR R0, R1, R2	R0 = R1 | R2
EOR	Bitwise XOR	EOR R0, R1, R2	R0 = R1 ^ R2
LDR	Load from memory	LDR R0, [R1]	R0 = *R1
STR	Store to memory	STR R0, [R1]	*R1 = R0
LDM	Load Multiple	LDMIA R0!, {R1-R3}
STM	Store Multiple	STMDB R0!, {R1-R3}
PUSH	Push on stack	PUSH {R0, R1}
POP	Pop off stack	POP {R0, R1}
B	Branch	B loop
BL	Branch with Link (function call)	BL myFunction
BX	Branch and eXchange	BX LR	Return from function
BLX	Branch with Link and eXchange	BLX R3
SWI / SVC	System call	SVC #0

Assembly sections

Section	Purpose
.text	Code
.data	Initialized variables
.bss	Uninitialized variables
.rodata	Constants
.init / .fini	Startup / shutdown
.symtab	Symbols
.debug_*	Debugging
.ARM.exidx	ARM exceptions
.got / .plt	Dynamic linking

Common Assembly snippets

Hello World

    .section .data
msg:
    .ascii  "Hello, World!\n"
len = . - msg

    .section .text
    .global _start

_start:
    /* write(1, msg, len) */
    MOV     R0, #1          @ file descriptor: stdout
    LDR     R1, =msg        @ address of message
    MOV     R2, #len        @ length of message
    MOV     R7, #4          @ syscall number: sys_write
    SVC     #0

    /* exit(0) */
    MOV     R0, #0          @ exit status
    MOV     R7, #1          @ syscall number: sys_exit
    SVC     #0

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Further details

• ARM tutorials