Control Unit In Computer Architecture

Control Unit

Control Unit In Computer Architecture

The control unit and its functions is an important topic in computer organization and architecture. The control unit is an important component of the processor architecture. The control unit performs some important functions.

The processor internally consist of three functional units. The CPU functional units include control unit ( CU ) , arithmetic and logic unit ( ALU ) and the memory unit ( MU ).

In computer architecture , the main function of the central processing unit ( CPU ) is to execute the program instructions. The processor ( CPU ) is also responsible to control all the operations performed by the computer system.

Control Unit And Computer Block Diagram

The processor ( CPU )  controls all the activities and operations of the computer with the help of control unit. The CU also decodes the machine instructions.

In this tutorial , you will learn what is control unit , how it works , CU block diagram and important functions performed by the control unit ( CU ).

Control Unit In Computer Architecture

Table Of Contents

What Is Control Unit ?

In computer architecture , the control unit is defined as an important component of the central processing unit ( CPU ) that controls and directs all the operations of the computer system.

The microprocessor is considered to be the brain of the computer system. The CPU internally consist of three main functional units  ( CU , ALU and MU ).

It is the control unit that generates the necessary control signals that directs the  various hardware components connected to the system.

Control Unit , CPU Control Unit

These control signals are essential part of the computer architecture to synchronize the operations of the various hardware system components and external peripheral devices connected to the system.

The control unit also internally contains a decoder unit . The main function of this decoder unit is to decode the machine instructions. The CU after decoding the instructions, direct the processor to perform the desired operation.

Control Unit Components

The control unit ( CU ) internally has four important components :

Control Unit

Timing And Control Logic

The central processing unit is driven by a stream of clock signals generated by the timing unit of the control unit.

The timing unit internally consist of quartz oscillating crystal that generates the analog signals . These analog signals are converted into digital sign wave by a analog to digital converter.

Control Unit , Timing And Control Logic
CPU clock pulse ,Control Unit , Timing And Control Logic

The  quartz  crystal  oscillator  is  used  to  generate a pulse of  analog  signals . However,  a computer is a digital machine and  designed to interpret only  digital clock pulse that  can be represented in the binary form  0  and  1 .

Therefore, a clock circuit converts  the  analog pulse generated by crystal  oscillator   to  a  steady stream  of digital  clock signals represented by a square wave.

For each clock signal , a CPU completes part of the instruction execution. And for this reason the CPU clock speed controls the processor speed.   

Control Unit

Program Counter Register ( PC )

The CPU initiates the program execution by placing the address of the first instruction into the program counter register to be fetched from the main memory ( RAM ).

The program counter register is automatically incremented each time the current instruction is fetched. And therefore, the program counter always contains the address of the next instruction to be fetched.

How CPU Works , Central Processing Unit

Control Unit

Instruction Register ( IR )

In computer architecture , the instruction register ( IR ) is also alternately referred to as current instruction register ( CIR ). The instruction register is a part of the control unit.

The instruction register actually holds the current instruction that is being decoded and executed by the processor.

During the program execution , the instruction register is used to store the instruction word. The IR is used to temporarily store the instruction that is fetched from the memory.

Instruction Cycle , Instruction Format
Instruction Cycle , Instruction Format

The instruction is a binary word or code that defines a specific operation to be performed. The instruction code is also called as operation code denoted by OPCODE in the machine instruction format.  

The CPU decodes the OPCODE from instruction format and then executes the desired operation as per the instruction set architecture ( ISA ).

Control Unit

Instruction Decoder

The control unit internally consist of instruction decoder circuit that decodes the machine instruction format.

The main function of the instruction decoder unit is to translate the operation code part of the machine instruction and direct the ALU. The ALU operates on the data as per the instruction set architecture ( ISA ) supported by the processor.

The instruction decoder circuit consist of number of sub decoders that decodes the bits pattern of the operation code ( OPCODE ).

Program Instruction Format , Instruction Cycle

Let us consider example of the 3 X 8 Decoder used in the 8085 architecture .The  3-to-8 Decoder  accepts three inputs and decodes into eight outputs.

This decoder has three inputs as A, B, and C and eight output from Y0 through Y7. Based on the combinations of the three inputs, only one of the eight outputs is selected.

Control Unit Decoder

Control Unit Functions

In computer architecture , The control unit performs following important function :

  1. The control unit controls the movement of data, memory read and write operations between the processor and the main memory RAM.
  2. The control unit generates the steady stream of clock pulses that regulates speed of the instruction cycle. The instruction cycle is performed by the CPU to execute the program instructions.
  3. The control unit generates the control signals for all hardware system components to regulate their activities.
  4. The control unit also decodes the machine instruction format and directs the ALU to perform the desired operation. The ALU operates on the data fetched into the CPU registers.

Control Unit And Instruction Cycle

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