MMX instructions are a set of 64-bit SIMD integer instructions.

There are 8 MMX registers; they are the floating point registers, just renamed. For this reason, mixing MMX instrucions and floating point instructions is verbotten. When the first MMX instruction of a sequence is executed, the floating point state is saved. After a sequence of MMX instructions, the EMMS instruction restores the floating point state.

The MMX instruction set can be divided into:

• pack and unpack instructions:
  PUNPCKLDQ, PUNPCKLWD, PUNPCKLBW,
  PUNPCKHDQ, PUNPCKHWD, PUNPCKHBW,
  and
  PACKSSWB, PACKSSDW, PACKUSWB
  
  
• Shift instructions:
  PSLLQ, PSLLD, PSLLW,
  PSRLQ, PSRLD, PSRLW,
  PSRAD, PSRAW
  
  
• Move instructions: MOVQ and MOVD.
  
  
• Addition and subraction operands:
  PADDB, PADDW, PADDD,
  PADDUSB, PADDUSW,
  PADDSSB, PADDSSW,
  and
  PSUBB, PSUBW, PSUBD,
  PSUBUSB, PSUBUSW,
  PSUBSSB, PSUBSSW
  
  
• Multiplication instructions:
  PMADDWD, PMULHW, PMULLW
  
  
• Comparison instructions:
  PCMPEQB, PCMPEQW, PCMPEQD,
  and
  PCMPGTB, PCMPGTW, PCMPGTD
  
  
• Logical operation instructions:
  PAND, PANDN, POR, PXOR
  
• EMMS is in a category of its own

Intel claims that the MMX instruction set consists of fifty-seven instructions. This is true on the machine level; but that counts moves to memory and moves to registers as different.