0xbffff8a8: aam $-0x8 error when saving the base pointer

Question

I am currently following an introductory course in microelectronics and assembly programming in Uni. At the beginning of every function, I'm saving the caller's base pointer by pushing it onto the stack. Given the following function, I get an error:

        .globl my_func
        .globl _my_func

my_func:
_my_func:
    pushl   %ebp
    movl    %esp,%ebp

    movl    4(%esp),%ebx

    subl    $1,%ebx
    movl    %ebx,%eax
    ret

0xbffff8a8: aam $-0x8 <-EXC_BAD_ACCESS (code=2, address=0xbffff8a8)

I've figured out this is a memory exception, I just don't understand why it's being thrown. When I skip the first two instructions in the function (the base pointer saving), the function runs well. And before you point it out -- yes, I know the function is pointless and slow, I'm just trying to learn how the instructions work, and how to use the stack and registers.

I'm assembling it for IA32 on an Intel Mac with OSX10.9 using LLVM5.1

Answer 1

You need to reset the stack pointer at the end of the function, either explicitly or by popping a register to match what you pushed at the start of the function, otherwise when you return it will be to an invalid address:

popl %ebp    ; restore stack pointer to its original value
ret