Design of Control Unit

Design of Control Unit

Control unit generates timing and control signals for the operations of the computer. The control unit communicates with ALU and main memory. It also controls the transmission between processor, memory and the various peripherals. It also instructs the ALU which operation has to be performed on data.

CU functions are as follows:
  • Controls sequential instruction execution
  • Interprets instructions
  • Guides data flow through different computer areas
  • Regulates and controls processor timing
  • Sends and receives control signals from other computer devices
  • Handles multiple tasks, such as fetching, decoding, execution handling and storing results

Control unit can be designed by two methods which are given below:

Hardwired Control Unit

It is implemented with the help of gates, flip flops, decoders etc. in the hardware. The inputs to control unit are the instruction register, flags, timing signals etc. This organization can be very complicated if we have to make the control unit large.
If the design has to be modified or changed, all the combinational circuits have to be modified which is a very difficult task.

Microprogrammed Control Unit

It is implemented by using programming approach. A sequence of micro operations is carried out by executing a program consisting of micro-instructions. In this organization any modifications or changes can be done by updating the micro program in the control memory by the programmer.

Difference between Hardwired Control and Microprogrammed Control

Hardwired ControlMicroprogrammed Control
Technology is circuit based.Technology is software based.
It is implemented through flip-flops, gates, decoders etc.Microinstructions generate signals to control the execution of instructions.
Fixed instruction format.Variable instruction format (16-64 bits per instruction).
Instructions are register based.Instructions are not register based.
ROM is not used.ROM is used.
It is used in RISC.It is used in CISC.
Faster decoding.Slower decoding.
Difficult to modify.Easily modified.
Chip area is less.Chip area is large.

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