CMPS : Compare String Byte or String Word:-The CMPS instruction may be utilized to compare two strings of Words or byte. The length of the string ought to be stored in the CX. If both the word or byte strings are equal, zero flag is set. Flags are affected in the similar way as CMP instruction. The DS: SI and ES: DI point to the 2 strings. The REP instruction prefix is utilized to repeat the operation till register CX (counter) becomes zero or the condition specified by the REP prefix is false.

Following string of instructions describe the instruction. Comparison of the string begin from initial byte / word  of the string, after each comparison the index registers  are updated depending on the direction flag and the counter is decremented. This byte by byte / word by word comparison continues until a mismatch is found. When, a mismatch is found, the carry flag and zero flags are modified properly and the execution proceeds further.

Example :

If both strings are fully equal, for example the register CX becomes zero, the ZF is set, or else, ZF is reset.