However you can tweak skyrim to use more than 3.1GB of Vram, but it needs some appropriate mods and a lot of ini tweaks. So, not for everyone...lol
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Also it depends entirely on how portable the code was written in the first place. Memory addresses in x64 are 64 bits wide, while x86 addresses are 32 bits wide.
It would require an entire rewrite of the code to make it 64 bit. Also it would require many hours of going over the code, to ensure it is done correctly since it was originally built for 32 bit to begin with.
If any 32 bit pointers are present in the program and you compile it and run it on x64 it will crash.
Additionally, they are using some 3rd party libraries. The thing about DLLs is, that they are also compiled against a certain platform.
So, A 32-bit DLL can be injected only into a 32-bit process, and a 64-bit DLL can be injected only into a 64-bit process. It is not possible to inject a 32-bit DLL into a 64-bit process or vice versa.
So whose wanna do that ???