@ My second ARM assembly program!
        .syntax unified
        .code 16
        .thumb_func
        .globl main
main:   push {r4, r5, r6, lr}
        @ r0 is argc; r1 is argv.  r2-r6 are available; r4-6 are callee-saved.
        @ So they are suitable for local vars.
        @ First validate argc.
        cmp r0, 3
        bne fail
        movs r4, r1        @ argv
        ldr r0, [r4, #4]   @ argv[1]
        bl atoi
        movs r5, r0        @ save result
        ldr r0, [r4, #8]
        bl atoi
        movs r1, r5
        movs r2, r0
        adds r3, r2, r5    @ third arg is sum
        ldr r0, =format
        .data
format: .asciz "%d + %d = %d\n"
        .text
        bl printf
        movs r0, #0
done:   pop {r4, r5, r6, pc}
fail:   movs r0, #53
        b done