I am currently analyzing an old Win32 game from 1999 that was probably compiled with Visual C++ 6 and was programmed in C.
I noticed that there are almost no usercalls (i.e. calls that use registers to pass arguments) except for calls in the statically linked CRT library. Is this a reasonable assumption for a game of this age?
To identify registers used as function paramters I used an algorithm similar to the one described in this IDA blog post. The algorithm identifies PUSH/POP pairs and searches for registers usages before any assignment except in the PUSH/POP pairs.