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assemblymasmmasm32

How to compare esp in masm x86?

I have a function in MASM x86 without a prologue. I want to compare an argument on the stack that was passed to the function.

.MODEL flat
extern x:dword

foo proc
cmp esp+4, x
...
foo endp

I've tried DWORD PTR [esp+4],x as well but that doesn't work either. I don't want to mess with popping from the stack or anything else because this is just a forwarding function. My question is basically why can't I make a comparison directly with an arg on the stack against a global variable? If it's impossible please tell me a different way to solve it.

Solution

  • x86 dooesn't support direct comparison of two memory locations. You need to move one of them into a register (preferably eax) first and then compare using the register:

    MOV EAX, x
CMP DWORD PTR [ESP + 4], EAX