In computer programming, a x86 calling convention that specifies that arguments to a function are to be passed in processor registers, if available. This is the fastest way to call a function, other than inline. In Microsoft C/C++ compiler (using __fastcall), the first two arguments are passed in ECX and EDX registers, while the stack maintenance is handled by the called function. In the GNU C Compiler, you can use __attribute__((regparm(3))) to achieve a similar result.