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

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


The following example compares BUFFER1 against BUFFER2 for the first

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
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.

See also: CMP
See also: CMPS
See also: CMPSB
See also: SCAS
See also: REP
See also: CLD
See also: STD
See also: Flags

CMPSW Compare String Word