There are a few articles about mixing c and assembly. Not very difficult to do, but the techniques may be a bit different from one c compiler to another due to c calling convention and how the c compiler modified function names. For example microsoft compiler add an underline before the function name while borland compilers typically add it after the function name. other compilers may not change the function names at all.

And then you have the problem is how to pass parameters to the function. Here again you have to know the calling convention used by the c compiler. If it is __stdcall parameters are pushed differently than __fortran or __fastcall calling conventions.

Here is a short article that might help you. If you google for "mixed c and assembly language programming" you will get more help.