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Member Since 16 Apr 2003
Offline Last Active Jan 04 2018 12:17 PM

#1066792 Engine/Game Remake

Posted by Blade on 15 September 2017 - 09:58 AM

It depends on which aspect you want to understand. For the actual dll injection where you make the the original binary load your new dll, there are a couple of ways of doing it. You can patch the original with an extra dll import (I believe OpenRCT2 did this), but then you need to ship a patched original binary. You can make your dll pretend to be a dll already loaded by the original and then forward the function calls it makes to the actual original as well as patching in your own code when the dll loads (I believe GenTool does this). Finally you can have some kind of loader process that starts the target exe, pauses it immediately and then patches in a dll load command in memory before resuming the exe.


This final option is what Thyme uses, a small launcher program is also compiled along side the dll to do the startup and force the load of the dll. To get this information I basically did a lot of googling and consolidated information from different sources. Fortunately the work is done now, so in general any contributors to thyme won't need to know this stuff.


Regarding the actual reimplementation, mostly what you need to know are C++ to write the implementations and some disassembler and ASM knowledge to examine the original binary to find and study the original functions. If you were to purchase or otherwise acquire a commercial version of IDA it can also generate pseudo C code to make understanding a function easier.


Once you know the address in the binary that the original function lives at and you have your reimplemented version, there are functions in thyme that run when the dll loads that you can add to that take the original address and a function pointer and patch the original function to jump straight into your new function at run time so when the original code tries to call say the open file function, it will actually jump into your open file function.


There are also special functions that go the other way. Say there is a complex function you haven't figured out yet that gets called everywhere, you could use its address to create a function pointer to it to call it where its needed in your own code. Similarly you can access global variables that are shared between functions in the same way, by making a pointer or reference to them.


I would suggest reading some tutorials on writing code in x86 assembly to get an idea of how a program is structured after it has been compiled and probably grab at least the free version of IDA and load the ZH binary. When you load it, it will probably only reliably find winmain, I've done a fair amount of work at my end identifying functions in the game which makes it look a little less daunting and confusing. For BFME games, I recommend you load no-cd versions that have had safedisc stripped out, otherwise disassemblers have a hard time working out what is what due to how the protection worked. Its always the game.dat file that is the actual exe BTW, bfme.exe or whatever its called is just a loader. I suspect the unofficial patches that ship new game.dat files to not required the mini cd images are no-cd exes that have had the safedisc removed, so they will probably be fine.

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#1066207 what i learned in game.dat

Posted by Blade on 07 September 2017 - 11:23 AM

I realise this is a big necro to this thread, but I've currently got a project, Thyme (https://github.com/T...blyArmada/Thyme) on the go attempting to reimplementing the game.dat from Zero Hour by modifying it in memory to run new code and replacing the original function by function until the entire game.dat is reimplemented in C++.


Although not of immediate use for BFME and later, if anyone has started poking around in the game.dat files since the last post here and mapping what functions do what, I might be able to help speed that along with what I know of the ZH binary to work out what is where. Also, When Thyme is further along towards no longer needing the original game.dat, well mapped BFME binaries would potentially allow back porting of support for the newer games to Thyme as well. Sister projects to reimplement and inject missing generals functions or entirely new functions into BFME and later might also be possible.

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