.text
    .global main

main:
    push {lr}

    loop:
        ldr r0, stdin_label
        ldr r0, [r0]
        bl fgetc

        cmp r0, #-1
        popeq {lr}
        bxeq lr

        cmp r0, #48
        blt loop

        cmp r0, #57
        ldrle r1, stdout_label
        ldrle r1, [r1]
        blle fputc

        b loop

stdin_label:
    .word stdin

stdout_label:
    .word stdout