Compare String (Byte, Word or Doubleword)
Subtracts destination value from source 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.
CMPSB inc/decrements the index registers by 1, CMPSW inc/decrements by 2, while CMPSD increments or decrements by 4. The REP prefixes can be used to process entire data items.