◄Key to Encodings►  ◄Reg►  ◄Mod►  ◄R/m►  ◄Back►
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  This three-bit field is called "register/memory," and it helps specify
  addressing mode. If mod field is 11, r/m specifies the source register
  using the reg field codes. Otherwise, r/m has the following meaning:
 
  000      DS:[BX+SI+disp]
  001      DS:[BX+DI+disp]
  010      SS:[BP+SI+disp]
  011      SS:[BP+DI+disp]
  100      DS:[SI+disp]
  101      DS:[DI+disp]
  110      DS:[BP+disp]
  111      DS:[BX+disp]
 
  If mod is 00 and r/m is 110, the operand is treated as a direct
  memory operand. This means that the operand [BP] is encoded as [BP+0]
  rather than having a short-form like other register indirect operands.
  Encoding [BX] takes one byte, but encoding [BP] takes two.
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