A feature on newer intel pentium processors. Most processor functions use one opcode (instruction) to operate on one set of data, so the CPU reads the opcode, loads the data, performs the operation, and outputs the result. lather, rinse, repeat at 200MHz. MMX loads one opcode and performs the same operation on a whole pipeline full of data. Since MMX loads the instruction once instead of every time, it can increase CPU speed for repetative functions like 3D rendering and MP3 encoding or decoding. My quick and dirty test indicates that using MMX opcodes gives about a 10% speed up in MP3 encoding. YMMV