SCASW            Scan String Word                    Flags: O D I T S Z A P C
*       * * * * *

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

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

Operands                  Clocks   Transfers  Bytes   Example
-                        19         1        1     SCASW
(repeat)                9 + 19/rep   1/rep      1     REPNE SCASW

Notes:         SCAS is useful for searching a block for a given
byte or word value. Use CMPS, Compare String, if you
wish to compare two strings (or blocks) in memory,
element by element.


The following example searches the 100-byte block starting at LOST_A
for the character 'A' (65 decimal).

MOV     AX, DS
MOV     ES, AX          ;SCAS uses ES:DI, so copy DS to ES
CLD                     ;Scan in the forward direction
MOV     AL, 'A'         ;Searching for the lost 'A'
MOV     CX,50           ;Scanning 50 words (CX is used by REPNE)
LEA     DI, LOST_A      ;Starting address to DI
REPNE   SCASW                   ;   ...and scan it.
JE      FOUND           ;The Zero Flag will be set if we found
;   a match.
NOTFOUND:       .               ;If we get here, no match was found
FOUND:  DEC     DI              ;Back up DI so it points to the first
DEC     DI              ;   matching 'A'

Upon exit from the REPNE SCASW loop, the Zero Flag will be set if a
match was found, and cleared otherwise. If a match was found, DI will
be pointing two bytes (one word) past the match location; the DEC DI
pair at FOUND takes care of this problem.

See also: SCAS
See also: SCASB
See also: CMPS
See also: REP
See also: CLD
See also: STD
See also: Flags

SCASW Scan String Word