Of the three calling convention modes (STDCALL, FASTCALL, and CDECL) allowed on the x86-model compiler, the calling convention mode for the 64-bit compiler contained in Microsoft Windows Server 2003 Driver Development Kit (DDK) is limited to just the FASTCALL type. Essentially, the two primary benefits from the 64-bit architecture are the 64-bit addressing capability and a flat set of 16 64-bit registers for general use. Given the expanded register set, using a FASTCALL methodology of calling convention and a RISC-based exception-handling model makes much more sense. This is because the FASTCALL model requires that the first four arguments be in registers and that the stack frame is used to pass the other parameters. The following sections further describe the 64-bit x64 calling convention: