Calling Conventions for IA-32

• %esp contains the stack pointer,
• %ebp contains the frame pointer,
• %eax contains the return value, and
• %ebx, %esi, and %edi are callee-save registers.

Additionally:

• The stack must always be 16-byte aligned, i.e. %esp mod 16 == 0.

In order to ensure tha alignment is maintained at a 16-byte alignment, the stack may need to be manually aligned after function entry.

When a function is entered via call, the return address is implicitly pushed onto the stack, misaligning it by one word (4 bytes). When the return stack pointer is then pushed onto the stack in the function prologue, the stack then becomes misaligned by two words (8 bytes).

Further pushes should occur in pairs or should be resolved using a sub instruction to re-align the stack to 16 bytes.

It is also the responsibility of the caller to ensure that when a function is called that the stack remains aligned to 16-bytes up to the call instruction. As the callee removes arguments from the stack upon return, the stack may be misaligned when a function is returned from. It is the responsibility of the caller to re-align the stack if this occurs.

Prologue

1. push %ebp onto the stack,
2. read the current value of %esp into %ebp
3. Reserve the stack frame and re-align the stack to 16-bytes.

push %ebp
mov  %esp, %ebp
// re-align to 16-bytes
sub  $0x8, %esp

Epilogue

leave
ret

Argument Passing

Arguments are passed on the stack in %ebp+4+(4 * n) (between the return address and the return frame pointer).