Calling C printf from ARM ASM

A while back I started to learn some ARM assembly on the Raspberry Pi. In my previous snippets, I was using SWI to do a system call to write to stdout, but it appears syscall 4 to write to stdout only writes Strings – if you have a byte value, this approach doesn’t work (without converting to a String first?)

From this question here, it appears you can call C functions like printf, which seems like cheating a bit, but I guess it gets the job done.

Here’s a quick snippet to add two numbers, and then print the result using printf with a string containing %d to substitute the result into the String:

.global main
.extern printf
push {ip, lr}
mov r3, #1 /* move 1 to r3 */
mov r4, #2 /* move 2 to r4 */
add r4, r3, r4 /* r4 = r3 + r4 */
ldr r0,=output /* load address of output string */
mov r1, r4 /* move r4 result to r1 to include as param in string */
bl printf /* call printf */
MOV R1, #0
MOV R7, #1
.asciz "Result: %d\n"

Where previously I was compiling and linking in two steps with as and ld, it seems this doesn’t work if you are referencing C functions too, so for this example, compile with: ‘gcc add.s -o add’

This snippet and a few others I’ve started to collect I’ve shared in a github repo here:

Raspberry Pi ARM ASM syscall notes

A few notes on useful syscalls:

SWI 0 : executes instruction based on value in R7

Write to screen: R7 = 4

MOV R7, #4 @ Syscall 4 = write to screen
MOV R0, #1 @ stdout: move 1 to R0
MOV R2, #4 @ length of string to R2
LDR R1, =string
    .ascii "testn"

Read from keyboard: R7 = 3

MOV R7, #3 @ Syscall 3 = read from keyboard
MOV R0, #0 @ stdin?: move 0 to R0
MOV R2, #4 @ length of string to ready to R2
LDR R1, =string @ load R1 value to string location 
    .ascii ""