Example 1
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Welcome to the first example assembly1 If you've read and understood all the sections up to here, there won't be any surprises here.
That way you should approach this is to to do the following:
- Copy all the assembly code to your IDE or somewhere safe.
- Go through each line, and make a note of what it does (typically, by putting a ; at the end and adding a comment works well). Try and understand what the code is doing.
- Go through each line, and convert it to the equivalent C code (or Java, if you're more comfortable with that).
- Try and combine and reduce the code to make it as simple as possible.
I'll go through those steps here, to hopefully give you an idea of how to approach a function like this. I highly recommend you try it yourself first, though.
; Note: ecx is a pointer to a 13-digit Starcraft cdkey ; This is a function that returns 1 if it's a valid key, or 0 if it's invalid mov eax, 3 mov esi, ecx xor ecx, ecx Top: movsx edx, byte ptr [ecx+esi] sub edx, 30h lea edi, [eax+eax] xor edx, edi add eax, edx inc ecx cmp ecx, 0Ch jl short Top xor edx, edx mov ecx, 0Ah div ecx movsx eax, byte ptr [esi+0Ch] add edx, 30h cmp eax, edx jnz bottom mov eax, 1 ret bottom: xor eax, eax ret