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This instruction compares two values by subtracting the byte pointed to by ES:DI, from the byte 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.
Logic: CMP (DS:SI), (ES:DI) ; Sets flags only if DF = 0 SI SI + 1 DI DI + 1 else SI SI - 1 DI DI - 1
The following example compares BUFFER1 against BUFFER2 for the first mismatch.
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 cmpsb ; ...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 di ;Back up SI and DI so they point to the . ; first mismatch