arm-asm-reference

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ARM assembler simulator

system call table per arch

GNU build command reference

Assemble assembly source file into object file

as -o program.o program.s

Assemble assembly source file into object file with debugging info

as -g -o program.o program.s

Link assemble object file into executable

ld -o program program.o

Exiting ASM program

The proper way to finish a program is by calling sys_exit.
System calls are called “Software Interrupt” command
The following code is an example of using exit system call:

    .global _start
_start:
    MOV R0,#5     ; Set the parameter for sys_exit
    MOV R7,#1
    SWI 0         ; Call sys_exit

You can observe the exit code of the last exited command by running:

echo $?

GNU vs Keil Directives

.req - Alias for register.
       foo .req R0
.equ - Define a constant.
      .equ BYTES_TO_ALLOCATE, 16

System-calls calling conventions

Interesting topics

Stack pointer and Frame pointer