<<< General-Purpose Registers     Index     MOD R/M Byte and Addressing Modes >>>

8. REG Field of the MOD-REG-R/M Byte

  •   MOD-REG-R/M Byte

  • The REG field specifies an x86 register(*):

      x86 register encoding

  • Depending on the instruction , this can be either the source or the destination operand.

  • Many instructions have the d (direction) field in their opcode to choose REG operand role:

    1. If d=0, REG is the source,
      MOD R/M <- REG.

    2. If d=1, REG is the destination,
      REG <- MOD R/M.

<<< General-Purpose Registers     Index     MOD R/M Byte and Addressing Modes >>>