.arch armv7a
.data
msg: .asciz "Hello World!\n"

.text
.global _start
_start:
    ldr r0,=msg
    bl print
    mov r0,#0x0
    bl exit

exit: @ NORETURN void exit(int code)
    @ code already in r0
    mov r7,#0x1
    svc #0x0

print: @ void print(const char *str)
    stmdb sp!,{r0,lr}

    @ msg already in r0
    bl strlen

    mov r2,r0 @ len
    ldr r1,[sp] @ msg
    mov r7,#0x4 @ SYS_write
    mov r0,#1 @ FD_stdout
    svc #0x0

    ldmia sp!,{r0,lr}
    bx lr

strlen: @ u32 strlen(const char *str)
    stmdb sp!,{r0,lr}
strlen_loop:
    ldrb r1,[r0]
    cmp r1,#0
    beq strlen_end

    add r0,r0,#1
    b strlen_loop
strlen_end:
    ldmia sp!,{r1,lr}
    sub r0,r0,r1
    bx lr