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CMPSW            Compare String Word                 Flags: O D I T S Z A P C
                                                            *       * * * * *
CMPSW

          Logic:    CMP (DS:SI), (ES:DI)           ; Sets flags only
                    if DF = 0
                        SI . SI + 2
                        DI . DI + 2
                    else
                        SI . SI - 2
                        DI . DI - 2

    This instruction compares two numbers by subtracting the word pointed
    to by ES:DI, from the word pointed to by DS:SI, and sets the flags
    according to the results of the comparison. The operands themselves
    are not altered. After the comparison, SI and DI are incremented (if
    the direction flag is cleared) or decremented (if the direction flag
    is set), in preparation for comparing the next element of the string.

  --------------------------------------------------------------------------
   Operands                  Clocks   Transfers  Bytes   Example
      -                        30         2        1     CMPSW
   (repeat)                9 + 30/rep   2/rep      1     REPE CMPSW

  -------------------------------- Example ---------------------------------

    The following example compares BUFFER1 against BUFFER2 for the first
    mismatch.

          cld                     ;Scan in the forward direction
          mov     cx, 50          ;Scanning 50 words (100 bytes)
          lea     si, buffer1     ;Starting address of first buffer
          lea     di, buffer2     ;Starting address of second buffer
  repe    cmpsw                   ;   ...and compare it.
          jne     mismatch        ;The Zero Flag will be cleared if there
                                  ;    is a mismatch
  match:          .               ;If we get here, buffers match
                  .
  mismatch:
          dec     si              ;If we get here, we found a mismatch.
          dec     si              ;Back up DI and SI so they point to the
          dec     di              ;   first mismatch
          dec     di

    Upon exit from the REPE CMPSW loop, the Zero Flag will be cleared if a
    mismatch was found, and set otherwise. If a mismatch was found, DI and
    SI will be pointing one word (two bytes) past the word that didn't
    match; the DEC DI and DEC SI pairs backup these registers so they
    point to the mismatched characters.