The controller is divided into a combinational logic controller and a micro-program controller, both of which have their own advantages and disadvantages. The combinational logic controller is troublesome to design and has a complex structure. Once the design is completed, it cannot be modified or expanded, but it is fast. The design of the microprogram controller is convenient, the structure is simple, and it is convenient to modify or expand. To modify the function of a machine instruction, only need to reprogram the corresponding microprogram; However, it does so by executing a shred. The specific comparison is as follows: Combinatorial logic controllers, also known as hard-wired controllers, are composed of logic circuits and rely entirely on hardware to realize the functions of instructions.