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CMPSD             Compare String Doubleword (386)    Flags: O D I T S Z A P C
                                                            *       * * * * *
CMPSD

           Logic:          CMP DS:SI, ES:DI
                           if DF = 0
                               SI . SI + 4
                               DI . DI + 4
                           else
                               SI . SI - 4
                               DI . DI - 4

     This instruction compares the doubleword at DS:SI to the one at ES:DI.
     If the direction flag is set, SI and DI are decremented by 4; otherwise,
     SI and DI are incremented by 4.

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   Operands                   Clocks   Transfers  Bytes  Example
   CMPSD                                   -        1    CMPSD
  --------------------------------------------------------------------------

        Note:          This instruction is usually used with a REP prefix,
                       which allows you to compare a number of doublewords in
                       a string.