my first (ARMv6) assembly program

README.md

ARMv6 assembly language study

to run

on Raspberry PI (Raspbian)

as -o test.o test.s && ld -o test test.o && ./test

on x86 Ubuntu

1. install tools:

sudo apt install qemu qemu-user-static gcc-arm-linux-gnueabi

2. assemble and link program

arm-linux-gnueabi-as -o test.o test.s && arm-linux-gnueabi-ld -o test test.o

3. run on qemu:

qemu-arm-static -L /usr/arm-linux-gnueabi ./test

echo.s

        .data

prompt:
        .string "input something... > "
prompt_len = . - prompt

buffer:
        .skip   256
buffer_len = . - buffer

        .text
        .global _start

sys_write:
        mov     r7, #4  // sys_write
        svc     #0
        bx      lr

sys_read:
        mov     r7, #3  // sys_read
        svc     #0
        bx      lr

_start:
        mov     r0, #1  // stdout
        ldr     r1, =prompt
        mov     r2, #prompt_len
        bl      sys_write

        mov     r0, #0  // stdin
        ldr     r1, =buffer
        mov     r2, #buffer_len
        bl      sys_read

        mov     r3, r0  // save the return value of sys_read

        mov     r2, r0
        mov     r0, #1  // stdout
        ldr     r1, =buffer
        bl      sys_write

        cmp     r3, #0  // unless sys_read returns EOF then go next loop
        blne    _start

end:
        mov     r7, #1  // sys_exit
        mov     r1, #0
        svc     #0

test.s

.data
hello:
        .byte   0x68  // h
        .byte   0x6f  // o
        .byte   0x67  // g
        .byte   0x65  // e
        .byte   0x0a  // newline
hello_len = . - hello

.text
.global _start

_start:
        ldr     r0, =hello
        mov     r1, #42
        strb    r1, [r0, #1]

        mov     r0, #1          // file discripter
        ldr     r1, =hello
        mov     r2, #hello_len  // character count
        mov     r7, #4          // sys_write
        svc     #0

        mov     r0, #0
        mov     r7, #1
        svc     #0              // sys_exit