386 instruction format

      Field                   Bytes
   a) Instruction prefix      0 or 1           F3  REP prefix (REPE,REPZ)
                                               F2  REPNE prefix (REPNZ)
                                               F0  LOCK
   b) Address-size prefix     0 or 1           67  Address-size operand
   c) Operand-size prefix     0 or 1           66  Operand-size override
   d) Segment override        0 or 1           2E  CS override
                                               36  SS -
                                               3E  DS -
                                               26  ES -
                                               64  FS - (386+)
                                               65  GS - (386+)
   e) Opcode                  1 or 2
   f) ModR/M                  0 or 1                Mod_  Reg/Op_  __R/M__
                                               Bit  7  6  5  4  3  2  1  0
   g) SIB (scale index base)  0 or 1                _SS_  _Index_  _Base__
   h) Displacement            0,1,2,or 4
   i) Immediate               0,1,2,or 4