AMD Am186 Series, Instruction Set

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Instruction Set 4-195
REPE REPE
Examples
This example compares one string of bytes in memory with another string in the same
segment until it finds a mismatch or all bytes are compared. The microcontroller copies the
bytes, one by one, from first to last. If the strings are different, the following instructions
save the segment and offset of the first mismatch.
; defined in SEG_E segment
STRING1 DB 20h DUP (?)
STRING2 DB LENGTH STRING1 DUP (?)
; notify assembler: DS and ES specify
; the same segment
ASSUME DS:SEG_E, ES:SEG_E
; set up segment registers with same segment
MOV AX,SEG_E ; load segment into DS
MOV DS,AX ; DS points to SEG_E, source
MOV ES,AX ; ES points to SEG_E, destination
; compare one string for equality to another
; initialize and use both strings
...
; save ES
PUSH ES
; set up registers and flags
LEA SI,STRING1 ; load source offset (segment = DS)
LES DI,STRING2 ; load dest. offset (segment = ES)
MOV CX,LENGTH STRING1 ; set up counter
CLD ; process string low to high
; compare first string for equality to second string
REPE CMPSB
; if strings are identical, then jump
JE EQUAL
; else, load segment of mismatch into ES and offset into DI
DEC DI ; mismatch is back one byte
LES DI,STRING2[DI]
JMP CONTINUE
EQUAL:
...
CONTINUE:
...
; restore ES
POP ES