Binary to ASCII converter

Question

This is a program I wrote to convert binary to ASCII. I am still learning ARM assembly. How can I improve my program? Are there any bad programming habits in my code?

I've tested it on Raspberry Pi and Android.

@
@ ARM assembly programm which converts binary to ASCII
@ written by Kyle Kersey,  (C) 2015
@

.global main

main:
    adr r2, content     @ load address of binary string
    mov r3, #0          @ bit counter
    mov r4, #0          @ byte value
    b loop

loop:
    ldrb r0, [r2], #1   @ load next byte from memory
    cmp r0, #0          @ check for null terminator
    beq end_loop        @ quit loop

    sub r0, r0, #48     @ convert '0' or '1' to 0 or 1 to a bit
    lsl r4, r4, #1      @ byte<<1
    add r4, r4, r0      @ byte+=bit

    add r3, r3, #1      @ increment bit counter

    cmp r3, #8          @ if at 8th bit
    moveq r0, r4        @ move byte to r0 for printing        
    moveq r4, #0        @ clear byte
    moveq r3, #0        @ clear bit counter
    bleq printChar      @ print byte, branch with link

    b loop              @ continue loop, branch without link


end_loop:
    mov r0, #10         @ ascii new line
    bl putchar          @ print new line
    mov r7, #1          @ return 0
    swi 0               @ call software interupt 0

printChar:
    push {r2, lr}       @ store content and return address
    bl putchar          @ print ascii character stored in r0
    pop {r2,lr}         @ restore the content and return address
    mov pc, lr          @ set the return address

charFormat:
    .asciz "%c"
    .align 4

content:
    .ascii "0100100001100101011011000110110001101111001000000101011101101111011100100110110001100100\000"

Answer 1

XOR instead of MOV

In your main subroutine, you move the value 0 into two registers.

A more efficient way to do that is to XOR the register with itself, like so:

xor r3, r3

Unnecessary branching

In your main subroutine, you branch over to the loop subroutine.

Since the loop subroutine is right beneath the main subroutine, you do not need a branch there since execution will just "fall through" to the loop subroutine.

Labels

I'm not sure how this works in ARM, but you seem to be using only global labels in your program, when there should be some local labels.

Local labels exist inside global labels, and they act kind of like a subroutine for a subroutine.

A change I might make is make the end_loop label a local label.

For more information on local labels, see here

For information on the syntax of a local label, see here

Disclaimer: I am not too familiar with ARM assembly, although I am familiar in other assemblies; some things I tell you might not work