Microprogrammed Control Unit

Microprogrammed control unit is used to design the CISC style processor. In this context, we will be discussing the microprogrammed control unit, its block diagram along with its advantages and disadvantages.

What is Microprogrammed Control Unit?

Basically, the control unit is a component in the processor that generates the control signals that are essential to control the proper functioning of the processor. To design the control unit there are two approaches i.e., using a hardwired control unit and using a microprogrammed control unit.

These two control units can be differentiated on the basis that the hardwired control unit is designed using hardware components such as flip-flops, logic gates, and other digital components. The hardwired control unit is used to design RISC style processor. It generates control signals faster than a microprogrammed control unit.

But the hardwired control unit was not well with the complex instructions and it was difficult to amend any modification in the hardwired control unit. Whereas the microprogrammed control unit doesn’t have these drawbacks.

The microprogrammed control unit uses the software approach to generate the control signals. The generation of the control signals is determined with the help of the program. This program is stored in the special memory of the processor which is smaller and faster. This memory is termed as microprogram memory or control store and the program is termed as the microprogram.

Block Diagram

The block diagram of the microprogrammed control unit is comprised of the components like an instruction register, a microinstruction address generator, and the control store or microprogram memory where the microprogram is stored.

Let us discuss the workflow of the microprogram counter with the help of successive steps.

Step 1: The instruction to executed is fetched and stored in the instruction register.

Step 2: The microinstruction address generator component fetches the instruction from the instruction register and decodes the instruction.

Step 3: On decoding the instruction, the microinstruction address generator generates the starting address of the corresponding micro routine in the control store.

Step 4: The microinstruction address generator loads the starting address of the microprogram routine into the microprogram counter. This helps the microinstruction address generator to track the addresses of successive microinstructions of the routine.

Step 5: The microinstruction address generator increments the microprogram counter to read the successive instruction in the micro routine present in the control store.

Step 6: The end of the micro routine can be determined with the help of a bit in the last instruction of the micro routine. This bit is termed the end bit. When the end bit is equal to 1 it defines the successful execution of the micro routine. This causes the new instruction to be fetched.

Advantages and Disadvantages

Advantages

• The microprogram control unit is flexible in controlling the execution instruction.
• It can be modified easily as it is easy to change the code.
• It easily handles complex instructions also.
• The implementation of a microprogram control unit is less costly.

Disadvantages

• The microprogram control unit is slow as compared to the hardwired control unit.

Nowadays the cost of logic circuitry is not considerable anymore so usually, the hardwired control unit is preferred over the microprogrammed control unit.