MMX capability was implemented in the later model Pentiums and in the Pentium® II processors. While MMX has the ability to perform matrix math and logical operations on multiple data items simultaneously (using SIMD instructions), it can only operate on integer numbers, not on FP numbers. Furthermore, the widest data items it can operate on are 64-bit items in the MMX registers (MM[7:0]).
The Pentium® III processor added 70 new instructions that, among other capabilities, can perform SIMD operations on multiple 32-bit, Single Precision (SP) FP numbers packed into the new 128-bit XMM registers (XMM[7:0]). They were originally referred to as the Katmai New Instructions (KNI) and are now referred to as the Streaming SIMD Extensions ...