For example:
MOV : data transfer operation from Register to Register.
Op-code is

       

10001000

11000001

88H C1H

REG = 0 0 0 indicates Register AL

REG = 0 0 1 indicates Register CL

indicates it is a byte operation (8 bit)

indicates AL is a source register.

This instruction indicates MOV CL , AL , i.e CL AL