CMPS             Compare String (Byte or Word)       Flags: O D I T S Z A P C
*       * * * * *
CMPS destination-string,source-string

Logic:    CMP (DS:SI), (ES:DI)       ; Sets flags only
if DF = 0
SI   SI + n            ; n = 1 for byte, 2 for word
DI   DI + n
SI   SI - n
DI   DI - n

This instruction compares two values by subtracting the byte or word
pointed to by ES:DI, from the byte or 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
dest,source               22(30)       2        1     CMPS STR1,STR2
(repeat) dest,source  9 + 22(30)/rep  2/rep     1     REPE CMPS STR1,STR2

Note:          This instruction is always translated by the
assembler into either CMPSB, Compare String Byte, or
CMPSW, Compare String Word, depending upon whether
source refers to a string of bytes or words. In
either case, you must explicitly load the SI and DI
registers with the offset of the source and
destination strings.


Assuming the definition:

buffer1 db      100 dup (?)
buffer2 db      100 dup (?)

the following example compares BUFFER1 against BUFFER2 for the first

cld                     ;Scan in the forward direction
mov     cx, 100         ;Scanning 100 bytes (CX is used by REPE)
lea     si, buffer1     ;Starting address of first buffer
lea     di, buffer2     ;Starting address of second buffer
repe    cmps    buffer1,buffer2 ;...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     di              ;Back up SI and DI so they point to
.               ;the first mismatch

Upon exit from the REPE CMPS 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 byte past the byte that didn't match; the DEC
DI and DEC SI backup these registers so they point to the mismatched

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

CMPS Compare String (Byte or Word)