Advantages & Disadvantages of Hardwired Control Unit

In this article, we will discuss the overview of the control unit and their types. And our main focus will be on the Advantages & Disadvantages of the Hardwired Control Unit. Let’s discuss it one by one as follows.

Control Unit :
The unit which directs the operation of the processor & is a part of the CPU is known as Control Unit. It generates control signals for the operations of a computer.

Types of Control Unit :
There are two types of control units as follows.  

1. Hardwired control unit  
2. Micro-programmed control unit

We are going to discuss the Hardwired control unit in this article.

Hardwired control unit :
To interpret the instructions & generate control signals for them, this control unit uses fixed logic circuits. To generate signals, the fixed logic circuits use the contents of the control step counter, Instruction Register (IR) & code flag, and some external input signals such as interrupt signals. The figure below shows the architecture view of the Hardwired control unit as follows.

Typical Hardwired Control Unit

The fixed logic circuit in the diagram is a combinational circuit made from decoders & encoders. It generates the output based on the state of its input(s). The decoder decodes the instruction loaded in IR (Instruction Register) & generates the signal that serves as an input to the encoder. Also, external input & conditional codes act as an input to the encoder. The encoder then accordingly generates the control signals based on the inputs. After the execution of each instruction, another signal: the end signal is generated which resets the state of control step counter & makes it ready for the next instruction.

Advantages of Hardwired Control Unit :
Here, we will discuss the advantages of the Hardwired Control Unit as follows.

1. Because of the use of combinational circuits to generate signals, Hardwired Control Unit is fast.
2. It depends on number of gates, how much delay can occur in generation of control signals.
3. It can be optimized to produce the fast mode of operation.
4. Faster than micro- programmed control unit.
5. It does not require control memory.

Disadvantages of Hardwired Control Unit :
Here, we will discuss the disadvantages of the Hardwired Control Unit as follows.

1. The complexity of the design increases as we require more control signals to be generated (need of more encoders & decoders)
2. Modifications in the control signals are very difficult because it requires rearranging of wires in the hardware circuit.
3. Adding a new feature is difficult & complex.
4. Difficult to test & correct mistakes in the original design.
5. It is Expensive.