Chapter 1 Computer System

Chapter 1: Computer System 1

Lecturer: Mr. Sandeep Sharma

The word ‘computer’ is derived from the Greek word ‘computare’ which means to calculate. It was used specially for fast and accurate computing in the past but nowadays it is

used for doing various kind of tasks.

What is computer?

A computer is an electronic device which can be reprogrammed to take input from user, process them by using CPU and give output in the human readable and understandable form and even store the output for future use.

Figure1: Computer system.

Input: Input is data and instruction entered into computer for processing.

Data: It is raw facts and figures about a person, place, thing etc. that can be recorded in the form of text, number, sound, image etc. e.g.: Ram, 25, old, years.

Instruction/ command: Command is the instruction given to the computer to perform the specific task. Commands are usually either typed at the keyboard or chosen from a menu.

Process: Processing is the manipulation of data and instruction given by the users and system. Processing is the actual task of computer for which it has been designed.

Output: Output is the processed data from computer which we get in desired from after processing.

Characteristics / Features of Computer

1. Speed: The computer has very fast processing speed due to advanced technology. It can perform billions of calculation within fractions of second. Modern computer can perform a

task in a picosecond (1/1012 parts of second). The processing speed of computer is measured in the unit of Hertz such as Hz, KHz, MHz, GHz and THz.

2. Accuracy: Accuracy means correctness. Computer never does any mistakes if the instruction, input data and hardware devices are correct. It always give 100% accurate result. But if we input wrong data and instruction it also gives wrong output. This phrase in known as "GIGO" (Garbage in Garbage Out).

3.Word Length: Word length or word size refers to the numbers of bits that the CPU can process in parallel at one time. Currently used word length are 32, 64 or higher bits. The longer the word length, the more powerful computer becomes.

4. .Automatic: Computer is a programmable machine, and it can be programmed in such a way that it can perform a series of tasks automatically after it has been started.

5. Diligence: Unlike the human beings, the computer never feels tired and bored to do a same task repeatedly. Computer can perform the same task with the same speed and accuracy for a long time in the suitable environment.

6. Versatility : Modern computer can used in bank, office, shopping center, cyber, school, home etc. for doing different types of jobs according to the program installed into it.

7. Reliability: Computer is a reliable machine because it can perform different types of jobs with 100% accuracy and with same speed for a long period of time.

8. Storage Capacity: A modern computer can store large amount of data, information and program for future use. Storage capacity in computer is measured in following units:

9.Electronic: Computer is an electronic devices so it work on the basis of electricity only. If there is no electricity it cannot work. It is made of semiconductor chips. 

Application/ Area of Computers

1.Education: Computer has been used in education for teaching and learning purpose. The teacher use to organize and prepare their teaching materials and students collect required information from the internet. The educational institute use computers for keeping the student record, preparing the result sheet, examination process, accounting and printing different documents etc.

2.Offices: In office the computers are used in preparing the reports, letter, notice salary sheet, income and expenditure reports etc. Generally Microsoft Office and Accounting Package are used in an office to handle the daily works.

3. Health and medical field: Computes have played a vital role in medical field as it has been used in hospital to help the doctors, nurses and technicians to diagnose diseases, getting information about patient, disease and drugs. Computer also used for CT scan, ECG (Electro-cardio grapy) and in virtual surgery.

4. Home : Computers used in home for storing the personal records, photos, videos , playing games, listening music, watching songs and movies etc. It can be used even to  connect the internet and use services like email, chatting, online TV streaming, downloading songs, movies etc.

5.Bank: A computer is used in bank to maintain ledger, to provide online service to the customers who need services from their home, to transfer amount etc. Due to computer with network facility, nowadays banks also provide ATM services and ABBS (any branch banking system) services etc.

6.Industries: Computers have been used in industries for various purpose such as keeping the employees record, income and expenditure record, salary sheet, measurement and display of physical quantity, quality control of production etc. The risky types of jobs are performed by robot controlled by computer.

7. Science and Technology: Since the invention of early computer, it has been a great aid for the mathematical and scientific for doing fast and correct calculations. Computers are used in science and technology for research work and simulations.

8. Recording and film studio: The modern computer is used for recording songs, movies and dialog. It is used to make movie more interesting, more popular and more attractive.

9. Communication: Due to invention of the internet, modern computers are massively used for www, email, voicemail, chat, internet phone etc. communication has been fast, reliable and cheaper. Text, photo, animation, email, voice etc. can be exchanged between the computers all around the world.

10.Library: We can use computer in library also to keep the records of books, magazines, reports, newspaper and other documents. We can keep the records of library members with which book they have borrowed with detailed information.

11.Weather Forecasting.

12.Reservation:

13.Military:

14.Desktop Publishing:

15.Hotel Management:

16.Air Corporation:

History of Computer

1. Mechanical Era

Mechanical Era means the development of computer devices without using the electricity.

a. Abacus

 Inventors Chinese, Babylonians, Egyptian and Japanese.

 Developed around 3000 B.C. . . .

 Made up of wooden frames, rods wires etc.

 It is divided into two unequal parts called heaven and earth.

 Heaven contains two beads in each rod each with the value 5.

 Earth contains 5 beads in each rod.

 It can calculate addition and subtraction. And is done by moving the beads.

b.Napier’s bone

 Inventor John Napier who a Scottish mathematician.

 Developed in 1617 A.D.

 Made up of bones, hard card boards etc.

 It contains 10 rods marked with the numbers.

 Used for multiplication and division.

c. Slide Rule

 Inventor William Oughtred who was an English Mathematician.

 Developed in 1624 A.D.

 Made up of two long ruler moving over one smaller ruler.

 Ruler was marked with numbers.

 Calculation was done by moving the smaller ruler.

 Used for faster multiplication and division.

d. Pascaline

 Inventor Blaise Pascal in 1642 A.D.

 It is made up of cogs and gears rotating in complement to each other.

 It could do addition and subtraction up to 8 digits.

 Multiplication and division are done by repeat the addition and subtraction respectively.

e. Stepped Reckoner/ Leibniz's Machine

 Inventor Gottfried Wilhelm von Leibniz who was a great mathematician.

 Developed in 1671 A.D.

 Made up of fluted drums having ten flutes arranged around their circumference in a stair–step fashion instead of gears.

 It can do addition, subtraction, multiplication, division and square root.

f. Jacquard's Loom

 Inventor Joseph Marie Jacquard who was a French textile

manufacturer.

 Developed in 1801 A.D.

 Jacquard loom's work on base of its weave upon a pattern

automatically read from punched wooden cards, held together in a long row by rope.

g. Difference Engine and Analytical Engine

 Inventor Charles Babbage who was an English mathematician at Cambridge University.

 Developed difference engine in 1822. A.D. and Analytical Engine in 1833 A.D.

 Difference Engine is made up mills, gears and shaft etc.

 It can do automatic calculations.

 Later developed Analytical Engine which is based on idea of modern electronic computer. I.e. Input, Process, Output and Storage.

h.Lady Augusta Ada Lovelace

 Student/Follower of Charles Babbage.

 First Computer Programmer because she suggested Babbage to use binary number system for computer program and data.

i. Tabulating Machine

 Inventor Dr. Herman Hollerith

 Developed in 1886 A.D.

 It could process on the punch cards perform the census calculation faster and accurately.

 It is called tabulating machine because it is in tabular shape.

 Hollerith established his own company called Tabulating Machine Company (TMC).

 Later, it merged with other company and established IBM Company in 1923 A.D.

2. Electro-Mechanical Era

 Mark I

Inventor Howard Aiken who develop Mark I in early 1940s.

• It became operable in May 1944.
• It was also called ASCC (Automatic Sequence Control Calculator).
• This was the first programmable digital computer.
• It is made using switches, relays rotating shafts, and clutches.
• It contain about 750000 parts and about 500 miles of wires.
• It was 51 feet long, 8 feet tall and 3 feet wide and use 5 horsepower electric motor to rotate the shaft.

 ABC (Atanasoff Berry Computer)

• Inventor John Vincent Atanasoff and his student Clifford Berry.
• Developed in 1942 A.D.
• This machine was the first to store data as a charge on a capacitor.
• It stores information in their main memory (RAM).

3. Electronic Era

I. ENIAC

• ENIAC stands for Electronic Numerical Integrator and Calculator
• It was built at University of Pennsylvania between 1943 and 1946 by two professors, John W. Mauchly and J. Presper Eckert.
• ENIAC filled a 20 by 40 foot room, weighed 30 tons, and used more than 18000 vacuum tubes.
• The basic clock speed was 100 000 cycles per second.

II. EDSAC (Electronic Delay Storage Automatic

Computer)

• Maurice Wilkes designed EDSAC at Cambridge University, mathematical laboratory in 1947.
• It used a mercury delay line storage system where the access time was one milliseconds.
• It contains 3000 vacuum tubes and operated on 30 KW.
• The time required for addition was 1500 microseconds and 4000 microseconds for multiplication.

III. EDVAC (Electronic Discrete Variable Automatic Computer)

• John W. Mauchly and J. Presper Eckert built at Pennsylvania University in 1950.
• It used vacuum tubes and some internal storage such as RAM memory.
• It is used for more school and the Ballistics Research Laboratory of the US Army.
• J.V. Neumann was a great mathematician and he discovered the principle of stored program concept in 1945 A.D.
• He is also called the ‘Father of Stored Program’.
• Von Neumann designed the Electronic Discrete Variable Automatic Computer.

IV. UNIVAC

• UNIVAC stands for UNIVersal Automatic Computer.
• Inventors J.P Eckert and John William Mauchly in 1951.
• It was the first computer manufactured for commercial use and general purpose.
• The machine was 25 feet by 50 feet in length, contained 5,600 tubes, 18000 crystal diodes, and 300 relays.
• It has an internal storage capacity of 1,000 words or 12000 characters and processing speed was up to 2.25MHz.

Generations of Computer

The generation of computer means the development of computer according to the time period. In the other words, the changes in the processing speed , size , storage capacity, cost, power consumption etc. is also known as generation of computer. There are five generations of computer. They are as follows:

a. First Generation Computer

b. Second Generation Computer

c. Third Generation Computer

d. Fourth Generation Computer

e. Fifth Generation Computer

Computer Speed and Measurement Unit

Classification of computer

A. On the Basis of size

a.Super Computer

 As the name suggest, super computer is physically the largest computer on size and it is the fastest, most expensive and most powerful computer.

 The fastest super computer can perform more than one trillion calculations per second.

 It has multiples CPUs hence it supports fully parallel processing.

 Application Areas:

o It can be used to forecast the weather and global climate.

o It is used in military research and defense system.

o It is used for encoding and decoding the sensitive

information.

o It is used in seismograph, plasma and nuclear research.

o It is used for the study of DNA and genetic engineering.

 Example: CRAY XT5 Jucar, IBM Roadrunner, IBM Blue Gene/L, NEC Earth Simulator, IBM ASCI White, IBM Deep Blue etc.

b. Mainframe Computer

It is also large computer which covers about 1000 sq. feet.

It is general purpose computer which has been designed for large scale data processing.

It is also very expensive and high speed computer.

It supports large numbers of computer terminal so that about 100 or more persons can work in this computer at a time.

• Application Area:

o It is used large organization such as insurance company and banks.

• Examples: IBM 1401, IBM 7090, ICL 2950/10 etc.

c. Mini Computer

It is the medium sized general purpose computer which was first released in 1960s.

The capabilities of a minicomputer are somewhere between those of main frame and personal computers, so it is called midrange computers.

It covers about 10 sq. ft.

Its CPU can be connected up to 50 terminals so that up to 50 persons can work in this computer at a time.

It is medium size organization and corporation like bank, insurance company, reservation center etc.

Examples: Prime series, AS/400, AP-3 etc.

d.Micro Computer

It is a smallest general purpose computer on the basis of size and it is called microcomputer because it has microprocessor as central processing unit.

It is also called personal computer because it can be used only by a single person at a time.

It is used in different application such as schools, college home, cyber etc.

Example: Desktop Computer, Laptop Computer, Notebook computer, Palmtop computer, Pocket PC etc.

Mobile Computing

 Taking a computer and all necessary files and software out into the field as a mobile device is called Mobile Computing.

 The computer carried out in the field is called mobile computing.

 These computers have facilities to transfer from one place to other easily.

 Mobile computing usually requires a portable computer that is battery powered, rather than a desktop.

 Many types of mobile computers have been introduced since 1990s, including the:

i. Personal Digital Assistant / Enterprise Digital Assistant:

 A lightweight palmtop computer design to provide specific functions like calendar, note taking, calculator,  communication etc.

 These devices are also called handheld computers or

pocket PC.

 These devices are not powerful as laptops and desktops

but handy to operate person’s day-to-day personal notes.

ii. Wearable Computer:

 A portable personal computer that its user wears like eye glasses, clothing or a wrist watch but which, unlike those items, is interactive responds to commands and carries out instructions.

 Eg; calculator watch.

iii. Carputers:

 Computer specifically designed and made to operate within an automobile.

 The functions of Carputers to operate internal electronic system of the automobiles.

 These may include in fuel injection, provide temperature of engine, speed and status of engine etc.

iv. Smart Phones:

 The smart phones are mobile phones that have some capabilities of computers.

 The Smart Phones can be used to make telephone calls, access the Internet, organize contact information, send and receive e-mails , text messages, play games, record photographs and videos via camera, with little larger screen than normal phone and manageable keyboard.

v. Tablet Computers:

 Tablet computers are mobile computers that combine the features of laptops and hand held computers.

 These devices are powerful and built in touch screen to operate directly through screen.

 They have features of converting handwriting characters to corresponding typed text.

Features of Mobile Computing

o Easy to handle and carry these small devices.

o Data can be transferred easily between users.

o Having fast processor speed.

o Huge memory capacity.

o Digital Camera system.

o Global positioning system.

o Durability

Mobile Computing Application

o Mobile computing used for traffic control and management. It helps to know road situation and traffic condition using GPS (Global Positioning System) and mobile apps.

o It helps to manage the emergency works like medical treatment, hire ambulance, locate the hospital etc. using the mobile apps.

o Mobile apps used for business applications. Concept of e-business management can be applied using mobile computing.

o Mobile computing is used for online payment processing.

o Mobile computing is used for anytime and any point computing applications.

Advantage of Mobile Computing

o It reduces the time and cost for business processing.

o It can be used as entertainment purposes using Internet connections.

o The main advantages of mobile computing is to access data at any time from where.

o Always connected system that helps to access the global resources.

Disadvantages of Mobile Computing

o Network problem due to the limited bandwidth in transmission.

o Chances of connection losses over the network.

o Chances of hacking and data destructions.

o Some mobile devices may have battery consumptions burden.

o Modern technology may not be compatible for all types of mobile connections.

1.2 Computer system and Input Output devices

1.2.1.1 Concept of computer architecture

 Computer system is made of various components and by connecting various peripherals such as mouse, keyboard, speaker, printer, monitor etc.

 Hence, the term ‘Computer Architecture’ is the conceptual design and fundamental operational structure of computer system.

 It forms a blueprint of functional description of requirements and design implementations for the various parts of a computer.

1.2.1.2 Concept of Computer Organization

 Computer Organization is concerned with the structure and behavior of a computer system as seen by the user.

 It acts as the interface between hardware and software. It deals with the components of a connection in a system.

 Computer Organizations helps us to understand the functionalities of a system.

Figure: Basic Organization of computer system.

1.2.2 Component of Computer System

A computer system is built up of components such as Hardware, software, programs, data, and networking. Without these components, no system can run.

In addition to these elements, a computer system consists of four major components. And they are:

central processing unit, input/output devices and

Memory as shown in figure below:

Major components of a Computer System

Input Unit

The input unit consist of input devices. Input is any data or instruction that a computer collects from the outside world. The input can be in the form of text, programs,

commands, and user responses.

The device that collects the input and sends it to the computer is called an input device. Input devices include the keyboard, mouse, scanners, digital cameras, microphone, etc. The input device is used for entering data or instruction to the central processing unit.

Some major functions of input devices are:

 It accepts data from the user or outside the environment.

 It converts user understandable form of data into binary code that is understandable to the computer.

 It sends data & instructions in binary form to the computer for further processing.

Central Processing unit (CPU)

The central processing unit (CPU) is the main information processor in a digital computer capable of executing a program. The manipulation of the raw data based on the instruction is called processing.

A computer processes data in a device called the central processing unit. It is a very complex integrated circuit, containing millions of miniaturized electronic components.

The CPU serves as a “brain” of the computer. The speed of the CPU is measured in Hertz (MHz, GHz etc.). The various types of CPU chips are Intel Celeron, Intel core i3, i5, i7, etc.

Output Unit

The output device is any peripheral device that receives or displays output from a computer. Output is any computer-generated information displayed on-screen or

printed on paper. It can be in the form of words, numbers, graphics, sound, videos, and animations.

An output device performs the following functions:

 It accepts results produced by the computer in binary coded form and converts them into human acceptable form.

 It supplies converted results to the display unit or outside world.

The monitor, printer, and graphic plotter are some of the popular output devices.

Output sent to a monitor is called softcopy while output sent to a printer is called

hardcopy.

Storage/Memory Unit

    A computer has a huge data storage capacity. Storage is the place where data is held in an electromagnetic or optical form for access by a computer processor.

Storage has been divided into:

Primary Storage:

    Primary storage holds data in memory. The primary storage (also known as main memory, internal memory, or primary memory), also known as memory, is the only one that is directly accessible to the CPU. E.g. Random Access Memory or RAM.

The CPU constantly reads instructions from memory and executes them as needed.

Secondary Storage:

Secondary storage holds data on hard disks, tapes, and other devices requiring input-output operations. Secondary storage (also known as external memory or auxiliary storage) differs from primary storage where it is inaccessible to the CPU directly.

The computer’s input/output channels are typically used to access secondary storage and transfer data to primary storage.

1.2.3 Microprocessor

            Figure: Microprocessor Internal Organization

 A microprocessor is a single chip in which millions of components like transistors are integrated together in different layers and it performs all the operations of the computers processor, since it is so small, it is called a microprocessor.

 It is a complete CPU built on a single chip by

using VLSI or ULSI technology.

 Intel Corporation of USA developed the first microprocessor named ‘Intel 4004’ in 1971 A.D.

 The modern processor are available in 32, 64 or higher bits word lengths.

The important characteristics of a microprocessor are clock speed, instruction set and word size. Processor are also often classified as being either RISC (Reduced Instruction Set Computer) or CISC (Complex Instruction Set Computer).

Clock Speed

Every microprocessor has an internal clock that regulates the speed at which it executes instructions and also synchronize it with other components.

The speed at which the microprocessor executes instructions is called clock speed.

Clock speeds are measured in Mega Hertz (MHz) or Giga Hertz (GHz) where one MHz means one million cycles per second whereas one GHz equals to one billion cycles per second.

Word Size

Number of bits that can be processed by a processor in a single instruction is called its word size.

Word size determines the amount of RAM that can be accessed at one go and total number of pins on the microprocessor.

Total number of input and output pins in turn determines the architecture of the microprocessor.

First commercial microprocessor Intel 4004 was a 4-bit processor.

Number of output pins is always equal to the number of input pins.

Currently most microprocessors use 32-bit or 64-bit architecture.

Instruction Set

A command given to digital machine to perform an operation on a piece of data is called an instruction.

Basic set of machine level instructions that a microprocessor is designed to execute is called its instruction set.

These instructions do carry out these the data transfer, arithmetic operations, logical operations, control flow and input and output mechanism of the system.

The microprocessor is responsible for the following function:

 To control and coordinate all the operation and other component of the computer system.

 To carry out processing

 To give commands to all the other components in the computer system

 To control the sequence of operation in the computer system

 To interpret the data, instruction and information in the computer system

Internally the microprocessor has three components:

1. Control unit (CU): The control unit of microprocessor coordinates and control the various operation and initiates the appropriate sequence of micro-operation for each task. It control and guides the interpretation, flow and manipulation of all data.

 Data exchange of CPU with the memory or input /output components

 Internal operation in the CPU such as moving data between register

 Making ALU perform a particular operation on the given data

 Regulation other internal operation

 Generate control mechanism to hardware and software

2. Arithmetic and logic unit (ALU): It is responsible for doing all type of arithmetic operation like addition subtraction, multiplication and division on integer and real operand. It can also perform the logical operation or comparison like equal to, greater than, less than etc. between two operand. It receives its operands from the register section of the CPU and stores its results back in the register section.

3. Register: Register are electronics memory chips made from transistors and flip-flops which serve as temporary storage location in the CPU. A PC normally uses the followings types of register:

 Memory Address Register (MAR)

 Memory Buffer Register (MBR)

 Program Counter (PC)

 Instruction Register (IR)

Concept of computer system Buses

As we need a highway to travel from one pace to another place. Similarly, the input and output of computer system also need a path to transmit data, information and control from one device to another device.

The path through which data and instruction flow is called bus of computer. It is a collection of wires, chips and slots inside the computer through which data and instruction are transmitted from one parts of computer to another part. In general electrical pulses are transmitted from source to destination through the system buses.

In the above figure, the system bus is shown with three types:

1.Address Bus: The pathway (circuit/chips/wires/slots) through which address of memory location is transmit is called Address Bus. When the CPU reads data or instruction from or writes data to memory, it must specify the address of the memory location it is going to access. Unlike the other buses, the address bus way always receives memory location from the CPU. There is one way flow in address bus.

2.Data Bus: The pathway (circuit/chip/wire/slots) through which transmit of data from one memory location to other is called Data bus. Data bus is the path through which data transfer takes place from one device to another. When the CPU fetches data from memory, it first output the memory address on its address bus. Then memory transfer the data onto the data bus; the CPU can

then read the data from the data bus. The data flows in the bidirectional way.

3.Control Bus: The pathway (circuit/chips/wires/slots) which transmit the control to operate and control device and software is called Control Bus. The control bus is the collection of individual controls signal for timing and controlling function sent by the control unit to other units of system. These signals indicates whether data is to be read or written, whether the CPU is accessing memory or an input/output devices, and whether the I/O devices or memory is ready to transfer data.

MEMORY

The memory is the working area of the computer. This is the place where our data and instructions get stored before processing and the result also gets stored before to its output is called memory. Data and instructions are stored in the memory in the binary form. Modern computer system has various types of memory which has been mentioned below.

Caches (L1, L2, L3)

Beside primary memory, a computer also has another memory which is called caches memory. The access time of main memory (RAM) is not very high in comparison to the processing speed of CPU.

    CPU has wait for a long period if it should fetch data and instruction from main memory for execution which cause the slowdown in performance. This problem is overcome by using a high speed memory called cache memory. Cache memory is the fastest memory inside computer system which is a memory chip placed between CPU and main memory whose access time is closer to the processing speed of the CPU. 

    It act as a high speed buffer between CPU and memory. It is used to temporarily store currently active data and instruction during processing. Since the cache memory is faster than main memory, the processing speed is increasing by making data and instruction necessary for current processing available to the CPU at a rapid time. The modern computer system are also designed to have multiple levels of caches (such as Level 1, Level 2, Level 3 caches ) and often they are

referred to as L1, L2, and L3 caches.

Virtual Memory (not in syllabus)

    Storage allocation has always been an important consideration in computer system. The cost of main memory is higher than the cost of secondary storage. Every program during execution must reside in main memory. But problem is, the main memory may not be large enough to accommodate the contents of all running programs.

    It is necessary to share a smaller amount of physical memory among many processes. These process being given the appearance of having their own executive address space. To overcome memory related conditions the concept of virtual memory emerges. Memory that no longer physically exists within computer system is called virtual memory. Virtual memory is a technique that allows the execution of process that may not be completely in memory. 

  

    Virtual memory automatically manages two levels of the memory hierarchy, representing the main memory (RAM and ROM) and secondary storage (Hard  Disk). The operating system automatically manages the required spaces from secondary storage in the case of main memory is not sufficient to run particular program.

Buffer Memory

    As we know that the execution speed of the CPU has been always faster than any other input/ output devices. Unless any special techniques has been applied the CPU remains idol most of the time as it has to wait for other slow input/ output devices. The technique which is used to keep both the CPU and input/output device busy all the time is called buffering.

    Many peripherals have their own buffers. The computer transfer the data to printer from memory into the printer buffer, and the printer then processes that data directly from its own buffer, freeing the computer to do other tasks.

Random Access Memory (RAM)

    RAM stands for Random Access Memory and is often called the memory or main, memory of computer. It is made up of semiconductor chips, hence also called primary memory. It is the working space used by the computer to hold the program that is currently running along with the necessary data and instructions. It is fast and expensive memory which allows the computer to access the data and instruction very quickly.

    It is called RAM because its contents can be accessed randomly. We can read from RAM as well as write into it. Hence, it is also called ‘read-write’ memory. The main drawback of RAM is that it is volatile memory so the contents RAM are lost when the computer is switched off.

Types of RAM

1. DRAM: stands for Dynamic Random Access Memory. It is the most common cheapest type of memory chip. 

    It is made of capacitors which is capable storing the electric charge. Due to leakage of charges, the capacitors discharge gradually and the

memory cells lose their contents. DRAM has to be refreshed periodically by recharging to the capacitors to retain its memory contents.

2.SRAM: stands for ‘Static Random Access Memory’ and it is made of transistors. It is called static because it can remember or retain its memory contents without being refreshed or recharged as long as there is power.

Read Only Memory

    ROM stands for ‘Read Only Memory’ and it is called ROM because only read operation can be performed on it. The user cannot write anything into ROM. 

    The program such as start up or booting instructions which does not need modification are stored in ROM. ROM is non-volatile memory because it can retain its contents even after the computer is turned off. It is also made semiconductor. 

    The program stored permanently in ROM is called firmware. Hence, firmware is immediately available when a device is powered on to start up the PC or other electronic equipment like Mobile, PDA and others.

Types of Read Only Memory (ROM)

I. PROM: PROM stands for ‘Programmer Read Only Memory’. Initially it is the blank chip which can be written or programmed only one time by using a special machine called ROM programmer or ROM burner. Once the PROM is written, it cannot be modified and becomes ROM.

ii. EPROM: EPROM stands for Erasable Programmable Read Only Memory. It is a special chip which can be re-programmed to record different information. The data and information are erased by exposing it to intense ultra violet light. EPROM chips are used in product development and experimental projects.

iii. EEPROM: EEPROM stands for ‘Electrically Erasable Programmable Read Only Memory’. This type of chip can be erased and reprogrammed repeatedly with special electrical pulses. It does not require a special device to write into it. EEPROM can be reprogrammed without removing it from the computer.

Storage Device

As we know that we cannot store any data and instruction into ROM because modification is unavailable. RAM cannot hold any information after the computer is shut down. 

    To overcome the problems of the primary memory, the computer has separate memory place and it is called secondary storage. It is called auxiliary or backing storage. The secondary storage is used to permanently store system programs, application program, large data files etc.

I) Hard Disk

Hard disk is a secondary storage device of computer. It is an air-tight sealed unit consisting of number of magnetic disks (platters) mounted on a spindle. It was

introduced by IBM 1956 A.D.

Input Devices

The devices which are used to give data and information to the computer system are called input device. Modern computer system can take input in the various forms such as text, number, sound, pictures, video etc. The various input devices used in modern computer system are as follows:

I) Keyboard

It is one of the most widely used input devices of the computer devices. It is similar to the keyboard of typewriter and enables to user to enter data and instruction into computer for processing. It contains alphabets (A-Z, a-z),digit (0-9),special character(!,@,#,% etc.) and some controls (Ctrl, Enter, Alt, Del, Shift, etc.) and function(F1,F2,F3,..F12) keys. When a key is pressed, an electronic signal is produced which is detected by the electronic circuit called keyboard encoder and converts it into binary code. Then the microprocessor transfers binary code into the memory and at the same time, the processor instructs the monitor to display the character on the screen.

    The mostly used keyboard is the QWERTY keyboard and it is called QEWRTY as the first 6 keys arrangement of left-top characters are the letters Q,W,E,R,T and Y and it was designed during 1800s A.D. for mechanical typewriter.

II) Mouse

It is a small hand held pointing input device which is used to point to a place or icon on the monitor and to give input to the computer. A mouse consists of a metal or plastic casing that contains 2, 3, or 4 buttons at the top. When the mouse is rolled over a flat surface, the cursor on the screen also moves in the direction of the mouse movement.

    The idea of mouse was developed by Doughlas C. Engerlbart of Stanford Research Institute in 1963 and the first Mouse was manufactured by Xerox Corporation.

Types of mouse:

    Mouse can be classified on the basis of following     technology.

    It can be Mechanical or Optical mouse.

a) Mechanical Mouse: It has a ball made from rough rubbery material and the rotation of the rubber effects sensors that are perpendicular to each other.

    Thus, the motion of the ball axes it detected and reflected as the motion of the pointer on the screen.

b) Optical Mouse : It can detect movement without any moving parts like a ball. The typical optical mouse used LEDs (Light Emitting Diodes) and photo-detection.

III) Scanner: It is an input device of computer and it allows you to capture drawings or photographs or text fro, tangible sources like paper, slides, etc. into electronic form. Scanners work by detecting differences in brightness of reflections from an image or object using light sensors. Scanners come in various types such as drum scanner, flatbed scanner, hand scanner and video scanner.

IV) Light pen: It is appointing input device of computer which is used to select screen position or icon by detecting the light coming from the screen. It is made up of a photocell mounted in a pen shaped tube. The light coming from the screen causes the photocell to respond by generating electrical pulse and it is transmitted to the processor. It enables the user to write words or draw pictures directly into computer screen. It is very useful to identify the specific location and drawing images on the screen.

v) OMR (Optical Mark Disk): OMR is also an input device which can detect the presence of the mark of dark pencil made in the boxes of special pre- printed forms. Then OMR, converts the marks into electrical pulses and transmitted to the computer for processing. It is widely used for the following purposes.

    i) To check objective type answer papers in examinations.

    ii) Various types of surveys where responses can be made restricted to one or more out of a few possibilities.

    iii) Order forms containing a small choice of items.

The main advantage of OMR is that the information is entered at its source and no further translation is required.

vi) OCR (optical character Reader): OCR is a type of scanner and used as an input device of computer. It is used to read character from the paper printed in special type font and converts the characters into ASCII form which the computer can process. It uses the light and sensor system to determine which character is being examined and convert into electric signal. OCR can understand the characters of only a few special fonts. It is widely used in airlines, banks, postal offices etc.

vii) BCR (Bar Code Reader): Bar code is a printed series of vertical lines or bars of varying thickness to indicate some specific information. Bar code reader is such input device of computer which can read the bar code from many items and convert them into electrical pulses for processing by computer. Bar code assigned on each item is the UPC (Universal Product

Code) and it is unique from other. Bar code is decoded and data is sent directly to the computer so that the computer can print the name of item, its price and other necessary information on the bill especially on the supermarket or pharmacies.

Viii) MICR: MICR stands for Magnetic Ink Character Reader. It is also an input device. This device is used to read characters, which is typed by using magnetic ink. The device coverts its equivalent text after reading from magnetic ink printed paper.     This process is used to implement security management to critical documents like some certificates, bank cheque and other critical documents.

ix) Touch Pad Kiosk: It is a pointing input device of computer device. It is also used to select the screen position and icons on screen and give command. It consists of specialized plain surface that can translate the motion and position of a user’s fingers to a relative position on screen. It is a common features of laptop computers. It is also used in Personal Digital Assistance (PDA), some portable media players and phones.

x) Microphones: A microphone is an input device for modern computer system. It is used to give output in form of audio into the computer system so that we can record voice on computer. It is an acoustic-to-electric transducer that converts sound into an electric signal. Sometimes it is also called ‘mike’ or ‘mic’. 

xi) Digital Camera: A digital camera is a camera that captures and stores still images and video as digital data instead of on photographic film. The first digital cameras became available in the early 1990s. Since the images are in digital form, they can be directly sent to a computer or printed on a printer.

CCD is a collection of tiny light-sensitive diodes called photosite, which converts photons (light) to electrons (electrical charges). Each photosite is proportionally sensitive to light- the brighter the light that hits a single photosites, the greater the electrical charge that will accumulated at that site.

Output Devices

a) Monitor or VDU (Video Display Unit)

    It is the standard output device of computer. It consists of a television like screen which is called a monitor and provides softcopy output in the form of

lines of texts and graphics. There are two types of display unit.

a) CRT monitors: CRT stands for ‘Cathode Ray Tube’ and it is the most common type output device of modern digital computer. It has an electron gun located at the back of tube which produces the electron beams. The screen has a layer of phosphor which emits light when it is struck by electron beam.

The CRT screen can be classified into 2 types in terms of color capabilities.

i) Monochrome: The monitor which displays output only in 2 color i.e. one for background and the other for foreground are called monochrome monitors. These monitors are also called black and white monitor or gray scale monitors.

ii)Color: The screen which can display output in more colors by striking Red, Green and Blue electron beam in the color screen are called color monitors. It is also called RGB monitor.

b) LCD: LCD stands for ‘Liquid Crystal Display’ and it is a flat panel screen especially used in portable computers like laptops, notebooks etc. because they are not heavy and bulky like CRT monitors. It is light weighted monitor, hence it is used in most of the portable devices like laptops, PDAs, mobile phones etc. A special liquid is sandwiched between two plates. The plate is transparent and the bottom plate is reflective. LCDs do not emit their own light hence it does not harm our eyes. The liquid crystal layer is used to block and allow polarized light from its source. And, polarized filter is used to filter and display appropriate color to particular pixel.

b)Printer:

It is also a common output of devices of computer which is used for producing hardcopy output like text and graphics on the paper. So, the printers provides information in a permanent readable form. Printers can be classified into different groups on the basis of different factors.

Types of printer on the basis of how they print

i) Impact printer

    Impact printers use the electromechanical mechanism. They print by striking the printer head or pin against an inked ribbon and paper .Dot-matrix printer, daisy wheel printer, line printer etc. are the example of impact printers. Impact printers are noisy printer because they produce sound while printing. Impact printers can print multiple copies of output at a time by using carbon between the papers. They are slow and noisy and their print quality is also not so high.

ii) Non-Impact

The printers which do not make the mechanical contact between the print head and paper are called non- impact printers. Ink -jet, Laser etc. are the example of non –impact printers. They print on the paper by using ink- jet, thermal, electrostatic or laser beam technology. They are faster and less noisy than impact printers’ .The main disadvantage of non -impact printer is that they cannot produce multiple copies of text at a time. 

Different types of printer:

1. Dot matrix printer

It is a kind of impact printer which prints character by printing or selected number of dots from a matrix of dots. It has a print head containing or number of pins.

Then it prints by striking the head on the inked ribbon and paper. It does not have fixed character font so it prints any type of font or alphabets.

Advantages:

i. Printing cost is cheaper as it uses the simple technology.

ii. It can print multiple copies of output at a time by using carbon paper. Hence it is very useful for printing bills.

2. Inkjet printer: It is all types of non-impact printers which prints one character at time. It print text and graphic by spraying ink from a nozzle on to the paper. The Ink cartridge contains a column of tiny heaters. They heat the ink near the nozzle to very high temperature for a fraction of second.

    This heating process causes the ink to boil and form vapor bubble. As the vapor bubble expands, it pushes a drop of ink thorough the nozzle and forces it on the paper. It is faster, less noisy and provides higher print quality than impact printers but its printing cost is higher.

3. Laser Printer: It is a non-impact high resolution printer which uses the photocopy principle and prints text and image of a complete page at a time. It uses the laser beam to produce an image of the page on a photosensitive drum.

    The drum coated with negatively charge photoconductive material. When the laser beam is exposed it covers the negative charge of the drum into positive charge. Then the drum is rolled through the toner and only the laser –exposed areas of the drum attract the toner. Then finally, the toner is transferred to the paper through the combination of heat and pressure.

Hardware Interfaces:

    Interfaces is the point where a connection is made between two different parts of a computer system, such as between two hardware devices, between a user and a program or between two application programs. The hardware interface is also called port. Data and information flow in and out through these port is computer system.

a) Parallel Port: it allows the transfer of all the bits of a word simultaneously or in a parallel way. These are multiple lines or wires to connect the peripherals to the port in parallel port.

b)Serial Port: It allows the transfer of data in a serial way. In serial port, only one line/ wire is used to transefer data. It is used for slow speed peripherals as keyboard, mouse, modem etc.

c) USB Port: USB stands for ‘Universal Serial Bus’. It is the universal port and it is also used to connect different peripherals to a computer system. Data and information can flow in and out through the USB port. It is high speed serial bus. It can transfer data at the speed upto 12 MB per second.

d)HDMI: HDMI (High Definition Multimedia Interface) is a specification that combines video and audio into a single digital interface for use with digital versatile disc (DVD) players, digital television (DTV) players, set-top boxes, and other audiovisual devices.

e) Expansion Slots: Slots are the inbuilt interfaces in the motherboard of computer system to connect with different devices such as RAM, internal modem, sound cards etc.