Operation code is mandatory part - every instruction must contain it. Address part also usually exists, although some system processor's commands need not operands (halt instruction for example).

MMIX has several formats of instructions, presented below (for future reference we shall enumerate them). All instruction have 4-byte length.

Format for system instructions:

• format 0: only operation code essential

  OP

  X

  Y

  Z

  OP - operation code; X, Y, Z arbitrary

The most frequently used formats:

• format 1: register-register operation; result is placed into the first register
  (arithmetic and logical instructions, load from memory)

  OP

  $X

  $Y

  $Z

  $X = register, $Y = register, $Z = register

• format 2: register-constant operation; result is placed into the first register
  (arithmetic and logical instructions, load from memory)

  OP

  $X

  $Y

  Z

  $X = register, $Y = register, Z = unsigned immediate (constant)

Branch formats:

• format 3: format for branch and manipulation with registers wydes

  OP

  $X

  YZ

  $X = register, YZ = unsigned immediate

• format 4: jump instruction

  OP

  XYZ

  XYZ = unsigned immediate

• instructions that set register value
• instructions that load value from memory
• instructions that save value to memory
• arithmetic operations
• logical operations
• conditional branches
• unconditional jump