THIScall I understand, is an invention of Microsoft, where the pointer to THIS object is passed in ecx. The normal protocol is to push it onto the stack after all the other parameters.
THIS points to an object's data, the first entry of which is itself a pointer to its virtual table of functions. A sort of double pointering scheme.
It's standard C++ stuff, also used in COM. In the source code, THIS is always implicit and is never seen on the parameter list in member function calls.
