34. Intel Reference Opcode Bytes, Cont.

• /digit - A digit between 0 and 7 indicates that

  • The reg field of Mod R/M byte contains the instruction opcode extension.

  • The r/m (register or memory) operand of Mod R/M byte indicates

      R/M Addressing Mode
      === ===========================
      000 register ( al / ax / eax )
      001 register ( cl / cx / ecx )
      010 register ( dl / dx / edx )
      011 register ( bl / bx / ebx )
      100 register ( ah / sp / esp )
      101 register ( ch / bp / ebp )
      110 register ( dh / si / esi )
      111 register ( bh / di / edi )
  

• The size bit in the opcode specifies 8 or 32-bit register size.

• A 16-bit register requires a prefix byte:

      Opcode     Instruction
      --------   ------------
      41h        INC ECX
      66h 41h    INC CX