Question

I have a simple function written in ARM assembler. The first time it’s ran, everything works as desired (it prints BOOT\n). However, the second time the function is executed, nothing is printed.

.globl __printTest
.text
.align 2

__printTest:
 sub sp, #64 /* yes, I know this is too much */

 mov r0, #66
 str r0, [sp]
 mov r0, #79
 str r0, [sp, #1]
 mov r0, #79
 str r0, [sp, #2]
 mov r0, #84
 str r0, [sp, #3]
 mov r0, #10
 str r0, [sp, #4]

 mov r0, #0
 mov r1, sp
 mov r2, #5

 bl _write
 add sp, #64

 bx lr

What could be the issue? I suspect that this somehow screws up the buffer that it no longer works. Write is a function that calls the write syscall on Linux using the svc instruction.

Answer 1

The problem is that you’re not saving lr.

     bl _write
     add sp, #64
     bx lr

bl _write will overwrite lr which then points to add sp, #64, so your bx lr will just result in an endless loop on the last two instructions.

It should work if you modify your code like this:

__printTest:
 push {lr}
 sub sp, #64 /* yes, I know this is too much */
 ....
 bl _write
 add sp, #64
 pop {pc}

As already stated in another answer, you should also use strb instead of str for byte-stores.