X86 Disassembly/Functions and Stack Frame Examples

Example: Number of Parameters MASM Given the following disassembled function (in MASM syntax), how many 4-byte parameters does this function receive? How many variables are created on the stack? What does this function do? _Question1: push ebp mov ebp, esp sub esp, 4 mov eax, [ebp + 8] mov ecx, 2 mul ecx mov [esp + 0], eax mov eax, [ebp + 12] mov edx, [esp + 0] add eax, edx mov esp, ebp pop ebp ret The function above takes 2 4-byte parameters, accessed by offsets +8 and +12 from ebp. The function also has 1 variable created on the stack, accessed by offset +0 from esp. The function is nearly identical to this C code: int Question1(int x, int y) { int z; z = x * 2; return y + z; } Example: Standard Entry Sequences MASM Does the following function follow the Standard Entry and Exit Sequences? if not, where does it differ? _Question2: call _SubQuestion2 mov ecx, 2 mul ecx ret The function does not follow the standard entry sequence, because it doesn't set up a proper stack frame with ebp and esp. The function basically performs the following C instructions: int Question2() { return SubQuestion2() * 2; } Although an optimizing compiler has chosen to take a few shortcuts.