24. x86 Instruction Prefix Bytes

• x86 instruction can have up to 4 prefixes.

• Each prefix adjusts interpretation of the opcode:

  1. Repeat/lock prefix byte guarantees that instruction will have exclusive use of all shared memory, until the instruction completes execution:

             F0h = LOCK
     
     

  2. String manipulation instruction prefixes

             F3h = REP, REPE
             F2h = REPNE
     

     where

     • REP repeats instruction the number of times specified by iteration count ECX.

     • REPE and REPNE prefixes allow to terminate loop on the value of ZF CPU flag.

     Related string manipulation instructions are:

     • MOVS, move string

     • STOS, store string

     • SCAS, scan string

     • CMPS, compare string, etc.

     See also string manipulation sample program: rep_movsb.asm

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  3. Segment override prefix causes memory access to use specified segment instead of default segment designated for instruction operand.

             2Eh = CS
             36h = SS
             3Eh = DS
             26h = ES
             64h = FS
             65h = GS
     
     

  4. Operand override, 66h. Changes size of data expected by default mode of the instruction e.g. 16-bit to 32-bit and vice versa.

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  5. Address override, 67h. Changes size of address expected by the instruction. 32-bit address could switch to 16-bit and vice versa.