I talked about calling conventions in an earlier post. There I mentioned that if the function is qualified with the __fastcall calling convention the system will try to push the parameters to the registers instead of the stack and there by tries to increase the speed of the function call. But this would just be an attempt. We are not assured that our parameters will always go to the registers. This is because of the availability of the registers.
Apart from this there is another issue associated with __fastcall. Since the parameters are put in the registers, those variables will not have any address location. So if we try to fetch/use the address of the parameters the system will be in a fix as it will not have any address to return. In such a case the params in the registers are copied to some temporary memory location and the address of that location is returned. This temporary location can be anything. It can be the regular stack itself.