Computer System — Study Notes

1. Introduction

A computer is an electronic machine that accepts data as input, processes it according to a set of instructions, and produces information as output. The word "computer" comes from the Latin word computare, which means "to calculate". However, a modern computer does far more than just calculate. It can store data, compare values, draw pictures, play music, connect to the Internet, and control machines. A complete setup of a computer together with its connected parts and software is called a computer system. A computer system has four main working parts: the input unit, the processing unit, the output unit, and the storage unit. Together they follow a cycle known as the Information Processing Cycle — Input, Process, Output and Storage.

2. Input Devices

Input devices are used to feed data and instructions into the computer. They convert the data given by the user into a form that the computer can understand. The keyboard and the mouse are the most common input devices. The keyboard is used to type letters, numbers and symbols, while the mouse is used to point and click on items on the screen. A scanner copies a printed page or a photograph into the computer. A microphone records sound. A webcam captures live images and video. A light pen and a touch screen also allow the user to give input by touching or pointing. An Optical Character Recognition (OCR) device reads printed text and converts it into editable text.

3. Output Devices

Output devices show the results of processing to the user. The monitor, also called the Visual Display Unit (VDU), displays text and pictures on a screen. The printer prints the output on paper. Printers can be impact printers, which strike the paper, or non-impact printers such as inkjet and laser printers which are quieter and faster. The speaker produces sound output, and the plotter draws large maps and engineering drawings with great accuracy. A computer can work without output devices, but it cannot show or share its results, so output devices are essential for communication with the user.

4. The CPU and the System Unit

The central part of a computer is called the Central Processing Unit (CPU), and it is often called the brain of the computer. The CPU is placed on a small chip called the microprocessor which fits inside the system unit. The CPU has three main parts:

• Arithmetic Logic Unit (ALU): performs all calculations such as addition, subtraction, multiplication and division, and all logical comparisons such as greater than, equal to and less than.
• Control Unit (CU): controls and coordinates all the parts of the computer. It reads instructions from memory, understands them, and directs other units to act.
• Memory Unit (MU): stores data and instructions before and after processing.

5. Memory and Storage

Memory is used to store data and programs. There are two kinds of memory. Primary memory is inside the CPU and works very fast. Random Access Memory (RAM) is volatile memory that loses its contents when the power is switched off. Read Only Memory (ROM) is non-volatile memory that keeps its contents permanently, and the manufacturer stores the start-up instructions in it. Cache memory is a very small, very fast memory that stores frequently used data. Secondary memory, such as the hard disk, pen drive, CD and DVD, is used for permanent storage. It is slower than primary memory but can store a huge amount of data.

6. Types of Computers

Computers are classified according to their size, speed and power. A supercomputer is the fastest and most powerful computer, used for weather forecasting, scientific research and simulation. A mainframe computer is a large computer that processes data for thousands of users at the same time and is used by banks, railways and airlines. A minicomputer is smaller than a mainframe and supports a small number of users. A microcomputer is a personal computer meant for a single user. Microcomputers include desktop computers, laptop computers, tablets and smartphones.

7. Characteristics of a Computer

A computer is described by its five great characteristics. It is fast, performing millions of calculations in a second. It is accurate, giving error-free results as long as the input and instructions are correct (GIGO — Garbage In Garbage Out). It has a huge storage capacity and can remember large amounts of data. It is diligent, working without getting tired for hours. It is versatile, being able to do completely different kinds of jobs. However, a computer has no intelligence of its own; it can only do what it is instructed to do by humans.

8. The Information Processing Cycle

The working of a computer follows a fixed sequence of four steps. In the first step, Input, raw data is fed into the machine using an input device. In the second step, Processing, the CPU performs calculations and comparisons on the data. In the third step, Output, the processed results are shown on an output device. In the fourth step, Storage, the data and results are saved permanently on a storage device so that they can be used again later. This continuous cycle is the basic principle on which every computer works.

Quick Revision Tables

Table 1: Devices and Their Functions

Table 2: CPU Parts and Memory Types

Mind Map

Important Diagrams (SVG)

Diagram 1: Parts of a Computer System

Diagram 2: The Information Processing Cycle

Common Mistakes

1. Students confuse RAM with ROM. Remember RAM is volatile and temporary, while ROM is non-volatile and permanent.
2. Many think the mouse is an output device. The mouse and keyboard are input devices; the monitor and printer are output devices.
3. Writing ALU as "Arithmetic Logic" without "Unit" is incomplete. The three parts are ALU, CU and MU.
4. Students often say a computer never makes mistakes. A computer is accurate, but if the input is wrong, the output is also wrong (GIGO).
5. Calling a pen drive "primary memory" is wrong. A pen drive is secondary memory used for permanent and portable storage.
6. Believing that a supercomputer is used for office typing. Supercomputers are used for weather forecasting, research and simulation.
7. Confusing cache memory with secondary storage. Cache is a very fast memory placed between the CPU and RAM.

Exam Tips

1. Always write the full forms: CPU — Central Processing Unit, ALU — Arithmetic Logic Unit, CU — Control Unit, VDU — Visual Display Unit.
2. Draw the Information Processing Cycle diagram and label Input, Process, Output and Storage clearly.
3. Learn at least two examples of each: two input devices, two output devices and two storage devices.
4. Explain RAM vs ROM with a one-line comparison table in descriptive answers to score full marks.
5. Memorise GIGO (Garbage In Garbage Out) as the keyword when the question asks about computer accuracy.
6. For the classification question, arrange in order: supercomputer, mainframe, minicomputer, microcomputer.
7. Practise one application of each computer type so that one-mark "uses" questions are never missed.

Conclusion

A computer system is a complete machine made of input, processing, output and storage units. The CPU is its brain, made of the ALU, CU and MU. It works through the Information Processing Cycle of Input, Process, Output and Storage, and its five characteristics are speed, accuracy, storage, diligence and versatility. With this basic understanding, we can move on to learn how the parts we can touch are classified as hardware and how the instructions that run them are called software.