Difference Between Hardwired and Microprogrammed Control Unit

The main difference between Hardwired and Microprogrammed Control Unit is that a Hardwired Control Unit is a sequential circuit that generates control signals while a Microprogrammed Control Unit is a unit with microinstructions in the control memory to generate control signals. 

The processor or the CPU is the main component of the computer that handles most of the tasks. It consists of main two subsystems: Arithmetic and Logic Unit (ALU) and the control unit (CU). ALU handles the mathematical and logical operations while CU sends timing and control signals to the other units to synchronize the tasks. The Control Unit can either be hardwired or microprogrammed. Hardwired Control Units are difficult to modify, decode and implement, but executes operations much faster. On the other hand, Microprogrammed Control Units are easier to modify, decode, implement and capable of handling complex instructions.

Key Areas Covered

1. What is a Hardwired Control Unit
     – Definition, Functionality
2. What is a Microprogrammed Control Unit
     – Definition, Functionality
3. Difference Between Hardwired and Microprogrammed Control Unit
     – Comparison of Key Differences

Key Terms

Hardwired, Microprogrammed Control Unit

What is a Hardwired Control Unit

Hardwired Control Unit is implemented using various electronic components such as combinational logic units and gates. The circuit uses a fixed architecture. If the instruction set is changed, the wiring should also be changed. As it is hardwired, the instruction set is constant and does not change. Therefore, a Hardwired Control Unit is used in processors that use simple instruction set known as the Reduced Instruction Set Computers (RISC).

Figure 1: Basic Structure of the Computer

Usually, these control units execute faster. However, Hardwired Control Units are difficult to modify and implement. It is also difficult to add new features to the existing design. Therefore, it has minimum flexibility.

What is a Microprogrammed Control Unit

Before talking about Microprogrammed Control Unit, it is important to understand some terms. A control variable refers to a binary variable that specifies micro-operations. A control word is a set of ones and zeros in a control variable. The control memory contains control words.

Microprogrammed Control Unit has a sequence of microinstructions stored in control memory. These microinstructions refer to a control word that resides in control memory, containing control signals for execution of micro-operations. Furthermore, a Microprogrammed Control Unit is used in processors based on complex instruction set known as Complex Instruction Set Computer (CISC).

Overall, these control units have a simple structure. Therefore, it is easier to design, implement and test. Moreover, Microprogrammed Control Units are easier to modify.

Difference Between Hardwired and Microprogrammed Control Unit

Definition

Hardwired Control Unit is a unit that uses combinational logic units, featuring a finite number of gates that can generate specific results based on the instructions that were used to invoke those responses. Microprogrammed Control Unit is a unit that contains microinstructions in the control memory to produce control signals.

Speed

The speed of operations in Hardwired Control Unit is fast. The speed of operations in Microprogrammed Control Unit is slow because it requires frequent memory accesses.

Modification

To do modifications in a Hardwired Control Unit, the entire unit should be redesigned. In Microprogrammed Control Unit, modifications can be implemented by changing the microinstructions in the control memory. Therefore, Microprogrammed Control Unit is more flexible.

Cost

Furthermore, Hardwired Control Unit are more costly to implement than a Microprogrammed Control Unit.

Handling Complex Instructions

Also, it is difficult for Hardwired Control Unit to handle complex instructions, but is easier for the Microprogrammed Control Unit to handle complex instructions.

Instruction Decoding

Moreover, it is difficult to perform instruction decoding in Hardwired Control Unit than in Microprogrammed Control Unit.

Instruction set Size

In additon to the above differences, the Hardwired Control Unit uses a small instruction set while theMicroprogrammed Control Unit uses a large instruction set.

Control Memory

Also, there is no control memory usage in Hardwired Control Unit but, on the other hand, Microprogrammed Control Unit uses control memory.

Applications

Considering the applications, the Hardwired Control Unit is used in processors that use a simple instruction set known as the Reduced Instruction Set Computers (RISC).  Microprogrammed Control Unit is used in processors based on a complex instruction set known as Complex Instruction Set Computer (CISC).

Conclusion

In conclusion, the main difference between Hardwired and Microprogrammed Control Unit is that a Hardwired Control Unit is a sequential circuit that generates control signals while a Microprogrammed Control Unit is a unit consisting of microinstructions in the control memory to generate control signals.  

Reference:

1. “Control Unit.” Wikipedia, Wikimedia Foundation, 27 July 2018, Available here.
2. Microprogrammed Control Unit | Introduction | COA, Education 4u, 1 Mar. 2018, Available here.

Image Courtesy:

1. “Von Neumann Architecture” By Kapooht – Own work (CC BY-SA 3.0) via Commons Wikimedia