I have an .exe that I would like to make some minor changes to. The author said he will not give me the c++ source, but if said if I can he doesn't mind if I make changes to it. My question is, what's the best/easiest way for me to edit an .exe which I do not have source to? Is there any way for me to convert the program to the fasm/masm .asm and work with that?

Thank you.
-Eric Springer

In theory, that's possible, if it's not packed or otherwise protected against RE. Just use a disassembler (IDA Pro is the best one!) to get an equivalent assembler source.

BTW, what kind of modifications do you want to perform?

that's the funniest post i've seen in months

I'm just modifying an American Option calculator program to handle Australian options (modifying shares per contract from 100 to 1000). If I am able to do that, I want to try add stuff like Australian frank dividend calculations - but that's probably a bit too much for me.

So if I use IDA Pro and get the assembly listing - will I be able to assembly that in MASM 8? I guess if I can get to that stage I should have a decent chance of getting my way through it.

Thanks!
-Eric Springer

you will only need to change one constant (2 or 4 bytes) in executable. finding this constant will be the problem.

After some minor changes and assuming IDA produced a 100% correct listing, which isn't true most of the time. You don't need to disassemble the whole program to change just a couple of bytes. Better use a debugger to find thouse bytes and patch 'em directly.

Ollydgb may help you find your patch points without availability of source code. www.ollydbg.de

Thanks everyone for your reply.
If I find the value I would like to change, does ollydbg allow me to make the change and test the modified exe?

-Eric Springer

Yes, this is exactly what OllyDbg does.

Attached is the Ollydbg "Quick Start Guide" (MS Word format). It's kinda fun stepping through the FASM "Beer" demo in ollydbg.