CMPSW
Usage: CMPSW dest,src Modifies flags: AF CF OF PF SF ZF
Subtracts 'dest' value from 'src' without saving results. Updates flags based on the subtraction and the index registers (E)SI and (E)DI are incremented or decremented depending on the state of the Direction Flag. CMPSW increments or decrements by 2.
Compare Strings
CMPSB ; A6 [8086]
CMPSW ; o16 A7 [8086]
CMPSD ; o32 A7 [386]
CMPSB compares the byte at [DS:SI] or [DS:ESI] with the byte at [ES:DI] or [ES:EDI], and sets the flags accordingly. It then increments or decrements (depending on the direction flag: increments if the flag is clear, decrements if it is set) SI and DI (or ESI and EDI).
The registers used are SI and DI if the address size is 16 bits, and ESI and EDI if it is 32 bits. If you need to use an address size not equal to the current BITS setting, you can use an explicit a16 or a32 prefix.
The segment register used to load from [SI] or [ESI] can be overridden by using a segment register name as a prefix (for example, ES CMPSB). The use of ES for the load from [DI] or [EDI] cannot be overridden.
CMPSW and CMPSD work in the same way, but they compare a word or a doubleword instead of a byte, and increment or decrement the addressing registers by 2 or 4 instead of 1.
The REPE and REPNE prefixes (equivalently, REPZ and REPNZ) may be used to repeat the instruction up to CX (or ECX - again, the address size chooses which) times until the first unequal or equal byte is found.
Example:
push esi edi
mov ecx 0FF, eax 0 | inc ebx
repe cmpsw
mov al B$edi-1, ah B$esi-1
pop edi esi