Basic Components of Computer System

What is a Computer?

The term computer is taken from the word compute which means to calculate. A computer is an electronic device, widely used for calculations and controlling operations that can be expressed either in logical or numerical terms.

In other words, a computer is an electronic device that can perform a variety of operations in accordance with a set of instructions called a program.

A computer can also be defined as an electronic machine for analyzing or storing data, making calculations, controlling other machinery, etc., in accordance with a set of instructions given to it.

Basic Components of Computer System:

Since a computer follows an input-process-output cycle, the first stage is performed in the computer by the input unit, the second stage is performed by its central processing unit and the third stage is performed by the output unit. Thus the basic structure of a computer is as shown below. The main memory holds the input and intermediate output during the processing.

Basic Structure of Computer

Following are the basic components of computer system:

  • Input Unit.
  • Central Processing Unit (CPU).
    1. Arithmetic Logic Unit (ALU).
    2. Registers.
    3. Control Unit (CU).
  • Output Unit.
  • Storage Unit.
    1. Primary Memory.
    2. Secondary Memory.

Input Unit:

The input unit is formed by attaching various input devices to a computer. An input device is an electromechanical device used to provide data to the central processing unit for processing. After processing, the input data is converted into meaningful information and this output is presented to the user with the help of an output device.

Examples of Input devices and media are:
(1) Keyboard.
(2) Pointing Devices (Mouse, Joystick, Light Pen, Trackball and Touch Screen).
(3) Punch Card Reader.
(4) Punched Paper Tape Reader.
(5) Magnetic Tape.
(6) Magnetic Disk.
(7) Floppy Disk.
(8) Teleprocessing Terminal (Teletypewriter Terminal).
(9) Optical Scanners [Optical Mark Reader (OMR) and Optical Character Reader (OCR)].
(10) Magnetic Ink Character Reader (MICR).
(11) Voice Data Entry Terminal (VDE).

The basic task of the input unit is to gather the data and convert it into a computer-understandable form (binary code). Since a computer operates on electricity, it can understand only the language of electricity i.e. either ON or OFF or high voltage or low voltage. That means a computer can understand two stages ON/OFF or HIGH/Low voltage or the binary languages that use just two symbols: 1 for ON and 0 for OFF.

Central Processing Unit (CPU):

The Central Processing Unit (CPU) or the Central Processor is the control centre for a computer. It carries out a variety of essential data manipulation (including arithmetic/logical calculations, comparisons, sorting, etc.) and controlling tasks in the computer. It is the brain of the computer. The CPU consists of three main sub-systems, the Arithmetic Logic Unit (ALU), registers and the Control Unit (CU). These subsystems work together to provide operational capabilities to the computer.

Arithmetic Logic Unit (ALU):

The Arithmetic Logic Unit (ALU) is a part of the CPU where various computing functions are performed on data. The ALU performs arithmetic operations such as addition, subtraction, multiplication, division and logical operations. When two numbers are required to be added, these numbers are sent from memory to ALU where addition takes place and the result is put back in the memory. In the same way, other arithmetic operations are performed.

For logical operations also, the numbers to be compared are sent from memory to ALU where the comparison takes place and the result is returned to the memory. The result of a logical operation can be either TRUE or FALSE. These operations provide the capability of decision-making to the computer.


Registers are special purpose, high-speed temporary memory units which hold various types of information such as data, instructions, addresses and the intermediate results of calculations. Essentially, they hold the information that the CPU is currently working on.

Registers can be considered as the working memory of the CPU. Registers work under the direction of the control unit to accept, hold and transfer instructions or data.

Control Unit (CU):

The control unit directs all operations inside the computer. It is known as the nerve centre of the computer because it controls and coordinates all hardware operations i.e. those of the CPU and input-output devices. Its actions are given below:

  • It gives a command to transfer data from the input device to the memory and from there to the arithmetic logic unit.
  • It also transfers the results from ALU to the memory and onto the output device for printing.
  • It stores the program in the memory, takes instructions one by one, understands them and issues appropriate commands to the other units.
  • It fetches the required instructions from the main storage via memory data register (MDR) and places them in the control instruction register (CIR). Then it interprets the instructions in CIR and gets it executed by sending a command signal to the concerned hardware device. A memory data register is a special register which holds all data instructions temporarily as they pass in and out of the main memory. A control instruction register is also a special register which holds machine instructions currently being interpreted by control units.

Output Unit:

The output unit is formed by the output devices attached to the computer. The output coming from the CPU is in the form of electronic binary signals which need conversion in some form which can be easily understood by human beings i.e. characters, graphical or audio-visual. This function of conversion is performed by output units. The physical form of output is known as the hard copy. The electronic version of an output which usually resides in the computer memory and/or on disk is known as the soft copy.

Examples of Output devices are:
(1) Printers.
(2) Tape Puncher.
(3) Magnetic Tape.
(4) Magnetic Disk.
(5) Floppy Disk.
(6) Plotter.
(7) Visual Display Unit (VDU).
(8) Speech Synthesizer and Coder.

Storage Unit:

A computer system incorporates a storage unit to store the input entered through the input unit before processing starts and to store the results produced by the computer before supplying them to the output unit. The storage unit of a computer comprises two types of memory/storage: primary and secondary. The primary memory holds the instructions and data currently being processed by the CPU, the intermediate results produced during the course of calculations, and the recently processed data. While the instructions and data remain in primary memory, the CPU can access them directly and quickly. Due to the limited size of primary memory, a computer employs secondary memory, which is extensively used for storing data and instructions. It supplies the stored information to other units of the computer as and when required.

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