Intel® C++ Compiler XE 13.1 User and Reference Guides
MMX™ technology is an extension to the Intel architecture instruction set. The MMX™ instruction set adds 57 opcodes and a 64-bit quadword data type, and eight 64-bit registers. Each of the eight registers can be directly addressed using the register names MM0 to MM7.
The prototypes for MMX™ technology intrinsics are in the mmintrin.h header file.