Computer Science Organization | Control Unit

In this article, we will learn about Hardwired control unit and Micro programmed control unit with their advantages and disadvantages. Need of a control unit in computer organization is also prescribed in this article.
Submitted by Abhishek Kataria, on July 22, 2018

Hardwired Control Unit

Hardware control unit can be viewed as a state machine which will change from one state to another in every clock cycle depending on the content of the instruction register, external inputs and the codes used in it.

The hardwired control unit is implemented as a sequential logic circuit or a finite state machine which will generate a specific sequence of the control signal to execute an instruction. The hardwired control unit uses the logic to interpret the instruction and generate control signals. While designing such type of control unit various factors are considered which are:

Cost of designing
Number of hardware used
Speed of the operation (performance)

Generally, there are four techniques for designing a Hardwired control unit they are as follows:

State table method: It is a simple method of sequential circuit design. Its attempt is to minimize the amount of hardware.
Delay element method: It is a method which is based on the use of D flip-flop for control signal timing.
Sequence counter method: It uses the counter for the timing purposes.
PLA method: It uses the programmable logic array.

Advantages of Hardwired Control unit:

It is faster than the microprogrammed control unit.
It can be optimized to produce the fast mode of operation.

Disadvantages of Hardwired control unit:

Instruction set, the control logic is directly implemented.
Requires change in wiring if the design has to be controlled.
An occurrence of an error is more.
Complex decoding and sequencing logic.
It requires a more chip area, therefore, it is a costlier control unit.

Micro-Programmed Control Unit

In the microprogrammed control unit, microinstructions are stored in a memory called control memory. In a response to a machine instruction, a set of a microinstruction is executed by which each microinstruction will generate a set of the control signals i.e. execution of a set of microinstructions will resemble the execution of a program.

A micro instruction consists of:

One or more microinstruction to be executed
Address of the next microinstruction to be executed

A microinstruction can cause the execution of one or more micro-operation and sequence of the microinstruction can cause execution of the instruction. The microprogram consists of the microinstruction which is generally nothing but a string of 0 and 1. In this if the content of the memory cell is 0, it will indicate, that the signal is not generated and is the content of the memory cell is 1 then it will indicate to generate the control signal at a time.

On execution of microinstructions at memory address 0000, C1, C2, C5, C7, C8 will be generated. Address of the next instruction is provided by the address field.

Functioning of micro programmed control unit:

The control unit can generate the control signal for any instruction by sequentially reading the control word of the corresponding microprogram from the memory.
To read the control word sequentially from the microprogram memory, a microprogram counter is needed.
The starting address block is responsible for the loading. The starting address of the microprogram into the PC provides a new instruction which is loaded into IR.
The PC is then automatically incremented by the clock, and it reads the successive microinstruction from the memory.
By this, each microinstruction provides the control signal and the microprogram counters ensure that the control signal will be delivered to the various parts of the CPU in the correct sequence.

Advantages of Micro Programmed Control unit:

It is both cheaper and the occurrence of an error is less.
More flexible to accommodate with new instructions.
Easier decoding and sequencing can be done.
Easier to handle complex instruction sets.
It requires a less chip area.

Disadvantages of Micro programmed control unit:

This is slower than the hardwired control unit because the microinstructions are to be fetched from the control memory which is time-consuming.

Need of control unit

The control unit is very essential as a state machine circuit in which input signal is transformed into a set of output logic signals which are termed as control signals. As the control unit manages the translation of instruction and the scheduling of the microinstruction between the execution units.