SCAS instruction? How it checks null string?

Q-SCAS instruction? How it checks null string?

SCAS compares a source byte or word in register AL or AX with the destination string element addressed by ES:DI and updates the flag. We use SCASB with REPNE and a zero in AL to find a zero byte in the string. In CX we load the maximum possible size, which are 64K bytes.