INTRODUCTION TO COMPUTER NOTES - drmuchelule.com€¦ · Web viewAbacusThe history of computer begins with the birth of abacus which is believed to be the first computer. It is

INTRODUCTION TO COMPUTER NOTES

TABLE OF CONTENTS

History of Computers.................................................................................................................4

What is Computer?..................................................................................................................4

Benefits of Using a Computer:...............................................................................................5

Input-Process-Output Model..................................................................................................6

Characteristics of Computer...................................................................................................7

Advantages of Using Computer.............................................................................................7

Disadvantages of Using Computer.........................................................................................7

Booting...................................................................................................................................7

Computer Bus and Data Transmission.................................................................................16

System Software...................................................................................................................17

Language Processor..........................................................................................................18

Device Drivers..................................................................................................................18

Application Software............................................................................................................21

Utility Software....................................................................................................................21

Basics of Computers - System S/W.........................................................................................22

Operating System.................................................................................................................22

Assembler.............................................................................................................................23

Interpreter.............................................................................................................................23

Compiler...............................................................................................................................24

Basics of Computers - Functions of OS...................................................................................25

Processor management.........................................................................................................25

Preemptive Scheduling.........................................................................................................26

Non-preemptive Scheduling.................................................................................................26

Memory Management..........................................................................................................26

Contiguous Storage Allocation.............................................................................................27

Non-contiguous Storage Allocation.....................................................................................27

File Management..................................................................................................................27

Device Management.............................................................................................................27

Basics of Computers - Types of OS.........................................................................................28

GUI OS.................................................................................................................................28

Writen and compiled by Dr. Muchelule Yusuf Wanjala, contact +254701952124 Page 1

Time Sharing OS..................................................................................................................28

Real Time OS.......................................................................................................................28

Distributed OS....................................................................................................................28

Popular Operating Systems...............................................................................................28

Instruction Set.................................................................................................................29

Microprocessor Components................................................................................................29

CPU...................................................................................................................................30

Bus....................................................................................................................................30

Computer Networks: Performance...................................................................................32

Computer Networks: Reliability.......................................................................................33

Computer Networks: Security..........................................................................................33

Properties of a Good Network...........................................................................................33

Uses of Computer Networks..................................................................................................33

Computer Networks: Business Applications....................................................................33

Computer Networks: Home Applications............................................................................34

Computer Networks: Mobile Users......................................................................................34

Line Configuration in Computer Networks.............................................................................34

MultiPoint Connection.........................................................................................................35

Transmission Modes in Computer Networks......................................................................35

SIMPLEX Mode.................................................................................................................36

HALF DUPLEX Mode.......................................................................................................36

FULL DUPLEX Mode.........................................................................................................36

Types of Communication Networks......................................................................................37

Local Area Network (LAN).................................................................................................37

Characteristics of LAN.....................................................................................................38

Applications of LAN........................................................................................................38

Advantages of LAN..........................................................................................................38

Disadvantages of LAN......................................................................................................39

Metropolitan Area Network (MAN)....................................................................................39

Characteristics of MAN....................................................................................................39

Advantages of MAN.........................................................................................................39

Disadvantages of MAN....................................................................................................40

Wide Area Network (WAN)................................................................................................40


Characteristics of WAN....................................................................................................40

Advantages of WAN.........................................................................................................40

Disadvantages of WAN....................................................................................................40

Types of Network Topology.....................................................................................................41

BUS Topology......................................................................................................................41

RING Topology....................................................................................................................42

STAR Topology...................................................................................................................42

MESH Topology.................................................................................................................43

MESH Topology: Routing..............................................................................................43

MESH Topology: Flooding............................................................................................44

TREE Topology..................................................................................................................44

HYBRID Topology.............................................................................................................45

Types of File Organization...................................................................................................46

Heap (unordered) File Organization.....................................................................................46

Pros of Heap storage.........................................................................................................47

Cons of Heap storage........................................................................................................47

Hash File Organization.........................................................................................................47

Pros of Hash file organization..........................................................................................47

Cons of Hash file organization.........................................................................................47

Secondary indexes................................................................................................................48

Indexes Sequential Access Method (ISAM)........................................................................48

Most Effective Antivirus......................................................................................................50

Decimal Number System......................................................................................................51

Binary Number System........................................................................................................52

Octal Number System...........................................................................................................52

Hexadecimal Number System..............................................................................................53

Number System Relationship...............................................................................................53

ASCII....................................................................................................................................54


History of Computers

The first counting device was used by the primitive people. They used sticks, stones and bones as counting tools. As human mind and technology improved with time more computing devices were developed. Some of the popular computing devices starting with the first to recent ones. AbacusThe history of computer begins with the birth of abacus which is believed to be the first computer. It is said that Chinese invented Abacus around 4,000 years ago. It was a wooden rack which has metal rods with beads mounted on them. The beads were moved by the abacus operator according to some rules to perform arithmetic calculations. Abacus is still used in some countries like China, Russia and Japan.

What is Computer?

A computer is a programmable electronic device that accepts raw data as input and processes it with a set of instructions (a program) to produce the result as output. It renders output just after performing mathematical and logical operations and can save the output for future use. It can process numerical as well as non-numerical calculations. The term "computer" is derived from the Latin word "computare" which means to calculate. A computer is designed to execute applications and provides a variety of solutions through integrated hardware and software components. It works with the help of programs and represents the decimal numbers through a string of binary digits. It also has a memory that stores the data, programs, and result of processing. The components of a computer such as machinery that includes wires, transistors, circuits, hard disk are called hardware. Whereas, the programs and data are called software.

It is believed that the Analytical Engine was the first computer which was invented by Charles Babbage in 1837. It used punch cards as read-only memory. Charles Babbage is also known as the father of the computer.

The basic parts without which a computer cannot work are as follows:

o Processor: It executes instructions from software and hardware.o Memory: It is the primary memory for data transfer between the CPU and storage.o Motherboard: It is the part that connects all other parts or components of a

computer.o Storage Device: It permanently stores the data, e.g., hard drive.o Input Device: It allows you to communicate with the computer or to input data, e.g.,

a keyboard.o Output Device: It enables you to see the output, e.g., monitor.

Benefits of Using a Computer:


o Increases your productivity: A computer increases your productivity. For example, after having a basic understanding of a word processor, you can create, edit, store, and print the documents easily and quickly.

o Connects to the Internet: It connects you to the internet that allows you to send emails, browse content, gain information, use social media platforms, and more. By connecting to the internet, you can also connect to your long-distance friends and family members.

o Storage: A computer allows you to store a large amount of information, e.g., you can store your projects, ebooks, documents, movies, pictures, songs, and more.

o Organized Data and Information: It not only allows you to store data but also enables you to organize your data. For example, you can create different folders to store different data and information and thus can search for information easily and quickly.

o Improves your abilities: It helps write good English if you are not good at spelling and grammar. Similarly, if you are not good at math, and don't have a great memory, you can use a computer to perform calculations and store the results.

o Assist the physically challenged: It can be used to help the physically challenged, e.g., Stephen Hawking, who was not able to speak used computer to speak. It also can be used to help blind people by installing special software to read what is on the screen.

o Keeps you entertained: You can use the computer to listen to songs, watch movies, play games and more.

The computer has become a part of our life. There are plenty of things that we do in a day are dependent on a computer. Some of the common examples are as follows:

1. ATM: While withdrawing cash from an ATM, you are using a computer that enables the ATM to take instructions and dispense cash accordingly.

2. Digital currency: A computer keeps a record of your transactions and balance in your account and the money deposited in your account in a bank is stored as a digital record or digital currency.

3. Trading: Stock markets use computers for day to day trading. There are many advanced algorithms based on computers that handle trading without involving humans.

4. Smartphone: The smartphone that we use throughout the day for calling, texting, browsing is itself a computer.

5. VoIP: All voice over IP communication (VoIP) is handled and done by computers.

Input-Process-Output Model

Computer input is called data and the output obtained after processing it, based on user’s instructions is called information. Raw facts and figures which can be processed using arithmetic and logical operations to obtain information are called data.


The processes that can be applied to data are of two types −

Arithmetic operations − Examples include calculations like addition, subtraction, differentials, square root, etc.

Logical operations − Examples include comparison operations like greater than, less than, equal to, opposite, etc.

The corresponding figure for an actual computer looks something like this −

The basic parts of a computer are as follows −

Input Unit − Devices like keyboard and mouse that are used to input data and instructions to the computer are called input unit.

Output Unit − Devices like printer and visual display unit that are used to provide information to the user in desired format are called output unit.

Control Unit − As the name suggests, this unit controls all the functions of the computer. All devices or parts of computer interact through the control unit.

Arithmetic Logic Unit − This is the brain of the computer where all arithmetic operations and logical operations take place.

Memory − All input data, instructions and data interim to the processes are stored in the memory. Memory is of two types – primary memory and secondary memory. Primary memory resides within the CPU whereas secondary memory is external to it.

Control unit, arithmetic logic unit and memory are together called the central processing unit or CPU. Computer devices like keyboard, mouse, printer, etc. that we can see and touch are the hardware components of a computer. The set of instructions or programs that make the computer function using these hardware parts are called software. We cannot see or touch software. Both hardware and software are necessary for working of a computer.

Characteristics of Computer

To understand why computers are such an important part of our lives, let us look at some of its characteristics −

Speed − Typically, a computer can carry out 3-4 million instructions per second.


Accuracy − Computers exhibit a very high degree of accuracy. Errors that may occur are usually due to inaccurate data, wrong instructions or bug in chips – all human errors.

Reliability − Computers can carry out same type of work repeatedly without throwing up errors due to tiredness or boredom, which are very common among humans.

Versatility − Computers can carry out a wide range of work from data entry and ticket booking to complex mathematical calculations and continuous astronomical observations. If you can input the necessary data with correct instructions, computer will do the processing.

Storage Capacity − Computers can store a very large amount of data at a fraction of cost of traditional storage of files. Also, data is safe from normal wear and tear associated with paper.

Advantages of Using Computer

Now that we know the characteristics of computers, we can see the advantages that computers offer−

Computers can do the same task repetitively with same accuracy.

Computers do not get tired or bored.

Computers can take up routine tasks while releasing human resource for more intelligent functions.

Disadvantages of Using Computer

Despite so many advantages, computers have some disadvantages of their own −

Computers have no intelligence; they follow the instructions blindly without considering the outcome.

Regular electric supply is necessary to make computers work, which could prove difficult everywhere especially in developing nations.

Booting

Starting a computer or a computer-embedded device is called booting. Booting takes place in two steps −

Switching on power supply Loading operating system into computer’s main memory Keeping all applications in a state of readiness in case needed by the user

The first program or set of instructions that run when the computer is switched on is called BIOS or Basic Input Output System. BIOS is a firmware, i.e. a piece of software permanently programmed into the hardware.


If a system is already running but needs to be restarted, it is called rebooting. Rebooting may be required if a software or hardware has been installed or system is unusually slow.

There are two types of booting −

Cold Booting − When the system is started by switching on the power supply it is called cold booting. The next step in cold booting is loading of BIOS.

Warm Booting − When the system is already running and needs to be restarted or rebooted, it is called warm booting. Warm booting is faster than cold booting because BIOS is not reloaded.

1.2 Generation of Computers

1.2.1 Computer generations

First generation computers (1946-1956): They made use of vacuum tubes to store and process information. The tubes consumed a lot of power and generated a lot of heat (overheating). They were huge in size and occupy a room. They used magnetic tape. Storage capacity was very low i.e. 2kb and speed of processing was also very low. First machine in this category was ENIAC (electronic discrete variable automatic computer) and later came UNIVAC (universal automatic computers).these computers were mostly computational machines. Their input /output capabilities were usually limited to the keyboard and or punched card input and printer and or punched cart output. Second generation computers (1957-1967): These computers used transistors after invention of transistors. The transistor is smaller cheaper and produced less heat than vacuum tubes and consumed less power. The cost of computers decreased and the speed increased. The second generation saw the introduction of more complex ALU and CPU, the use of high. level languages and provision of system software with the computer. Data access time was measured in micro-seconds. Removable disk storage units were developed for use on these machines. The speed of these machines was described in microseconds (1/1000, 000 of a second). These computers had programming languages whose vocabularies are close to the human natural language, English language.

Third generation computers (1965-1980)

Introduced the use of very small electronic circuit called integrated circuits (IC) by combining several transistors together between 3 transistors to make 1 IC. With IC you can house thousands of transistors in one IC. This change further decreased the size, heat output and the maintenance complexity of the computers while increasing its speed. The small circuitry that resulted improved the processing speed i.e. 10 times the past. The speed of these machines was described in nanoseconds (1/1,000,000,000 of a second). They have higher main memory capacity, reliable and increased processing power (have the capability of holding more than one set of instructions and operate on them) than the second generation computers.

Fourth generation computers (1980s)


Use large scale integration circuits which housed hundreds of transistors and very large IC which are between 200,000 to 400, 000 in one IC. Memory used includes magnetic disc and optical disc. Memory size expanded up to several MB and speed was 10 times faster. This generation marked the origin of mini computers in use today.

Fifth generation computers (1990-current)

The design of these computers was based on VLSI (very large scale integration) technology, the micro chip technology that gave rise to the smaller computers known as the micro computers in use today. These computers are used in networking .examples of micro computers are IBM PCs BBC micro etc. the micro computers are usually described as PCs or stand alone or desktop computers because they were designed primarily to serve single person at a time. The fifth generation is still a state of the art technology that relies on predictions and further technological refinements.

Summary

Research shows that the trend in computer technology revolution is that there is; o Continual decrease in computer sizeo Improved speed and power processing

o Decrease in computers and the related facilities costo Number of components in computer per circuit (IC) greatly increased over 500,000

physical elements e.g. transistors, capacities, diodes etc per chip(IC).

1.3 Characteristics/features of Computers

1. Speed – a computer is a very fast machine. It can perform in a very fewseconds the amount of work that a human being can do in a year if he/she worked day and night doing nothing else.

2. Accuracy – the computer accuracy is consistently high.

3. Diligence – computers are free from monotony, tiredness and lack ofconcentration etc. It can therefore work for hours without creating an error. For example if 10 million calculations are to be done, a computer will do the tenth million calculations with exactly the same speed and accuracy as the first one.

4. Versatility – a computer performs various tasks with ease. I.e. it cansearch for a letter, the next moment prepare an electricity bill, and write a report next then do an arithmetic calculation all with ease.

5. Power of remembering – a computer can store and recall any informationdue to its secondary storage capability.


6. No intelligence Quotient (IQ) – a computer cannot make its owndecisions and has to be instructed on what to do.

7. No feelings – computers are devoid of emotions. They have no feelings orinstincts and none possesses the equivalent of a human heart and soul.

1.4 Basic Computer Concepts

Definition of a computer

A computer is an electronic device capable of executing instructions, developed based on algorithms stored in its memory, to process data fed to it and produce the required results faster than human beings.

Its an electronic device that converts raw data into information. Its an electronic device that receive raw, stores , process it and outputs it as

information.

"one that computes; specifically : a programmable electronic device that can store, retrieve, and process data"

Its an electronic device that receive data and process it as a meaningfull information.

1.5 Types of Computers Computers are classified into four i. functionality & data presentation ii. Size iii. Purpose Types of Computers according to functionality and data representation.1. Analog Computers: Analog computers are used to process continuous data. Analog computers represent variables by physical quantities. Thus any computer which solve problem by translating physical conditions such as flow, temperature, pressure, angular position or voltage into related mechanical or electrical related circuits as an analog for the physical phenomenon being investigated in general it is a computer which uses an analog quantity and produces analog values as output. 2. Digital Computers: Digital computer represents physical quantities with the help of digits or numbers. These numbers are used to perform Arithmetic calculations and also make logical decision to reach a conclusion, depending on, the data they receive from the user.3. Hybrid Computers: Various specifically designed computers are with both digital and analog characteristics combining the advantages of analog and digital computers when working as a system. Hybrid computers are being used extensively in process control system where it is necessary to have a close representation with the physical world. The hybrid system provides the good precision that can be attained with analog computers and the greater control that is possible with digital computers, plus the ability to accept the input data in either form.

Classification of Computer according to size


Super computers: They are very large in size and use multiple processors and superior technology. Super computers are biggest in size, the most expensive in price than any other is classified and known as super computer. It can process trillions of instructions in seconds. This computer is not used as a PC in a home neither by a student in a college. Governments specially use this type of computer for their different calculations and heavy jobs. Different industries also use this huge computer for designing their products.Mainframe computers: A mainframe is another giant computer after the super computer and can also process millions of instruction per second and capable of accessing billions of data .They are physically very large in size with very high capacity of main memory. This computer is commonly used in big hospitals, air line reservations companies, and many other huge companies prefer mainframe because of its capability of retrieving data on a huge basis. They can be linked to smaller computers and handle hundreds of users they are also used in space exploitation. Minicomputers: They are smaller than the main frame but bigger than minicomputers. They support concurrent users. They can be used as servers in companies. They are slower and less costly compared to mainframe computers but more powerful, reliable and expensive than micro computers.

Micro computers: They are of advanced technology i.e. the micro era based on large scale integration that confines several physical components per small elements thumb size IC, hence the size reduced. It is the smallest of the tStrategyee computers. They are usually called personal computers since they are designed to be used by individuals. The micro chip technology has enabled reduction of size of computers. Microcomputers can be a desktop, laptop, notebooks, or even palmtop

Classification of Computers according to purpose

General-Purpose computers — those built for a great variety of processing jobs. Simply by using a general purpose computer and different software, various tasks can be accomplished, including writing and editing (word processing), manipulating facts in a data base, tracking manufacturing inventory, making scientific calculations, or even controlling organization’s security system, electricity consumption, and building temperature. General purpose computers are designed to perform a wide variety of functions and operations. You will probably use this type of computer reading this article and I am using a general purpose computer typing this article in some software (MS Word). A general purpose computer is able to perform a wide variety of operations because it can store and execute different programs in its internal storage.

Special-Purpose Computers are designed to be task specific and most of the times their job is to solve one particular problem. They are also known as dedicated computers, because they are dedicated to perfom a single task over and over again. Such a computer system would be useful in playing graphic intensive Video Games, traffic lights control system, navigational system in an aircraft, weather forecasting, satellite launch / tracking, oil exploration, and in


automotive industries, keeping time in a digital watch, or Robot helicopter. While a special purpose computer may have many of the same features found in a general purpose computer, its applicability to a particular problem is a function of its design rather than to a stored program.

SOFTWARE AND HARDWARE

A computer has to main components software and hardware.

Computer hardware refers to the physical components of a computer such as themonitor, Keyboard, Mouse, system unit etc shown in the diagram below.Computer software

A set of programs associated with the operation of a computer. The two components (hardware, software). Hardware units (Devices) of a computer can be categorized into five units; input, output CPU & Secondary storage.

Input Devices

An input device lets you communicate with a computer. They are used to enter information and issue commands to the computer. Commands tell the computer to do something, like save the file. A keyboard, mouse, scanner, digital camera, touch pads and joystick are examples of input devices.

o Keyboard Used to type data into the computer. It has special keys forgiving the computer commands called command or function keys

o Pointing Devices Pointing devices move some object on the screen andcan do some action Mouse is a common pointing device

o Scanner allows you to scan documents, pictures, or graphics and viewthem on the computer. You can also use software to edit the items you scan. Used to put printed pictures and text into a computer. It Converts an image into dots that the computer can understand .To scan text, optical character recognition (OCR) software is needed

o Digital Camera Used to take electronic pictures of an object. The picturestaken by a digital camera can be used directly by a computer

o Microphone Used to put sound into a computer. Need sound recordingsoftware

o Video Capture Card Usually place inside the computer's case. Use to putvideo into a computer. Need a video source, either a video camera or video recorder


o Voice input device-A computer I/O device in which vocal commands

omay be entered into a computer system.Optical character recognition (OCR) is computer software designed totranslate images of handwritten or typewritten text (usually captured by ascanner) into machine-editable text, or to translate pictures of charactersinto a standard encoding scheme representing them (e.g. ASCII orUnicode).

o Optical Mark Reader (OMR) A special scanning device that can readcarefully placed pencil marks on specially designed documents. OMR is frequently used in forms, questionnaires, and answer-sheets

Output device

An output device displays information on a screen, creates printed copies or generates sound. A monitor, printer, and speakers are examples of output devices.

o Monitors and Displays Shows the processed information on a screen. Amonitor uses a Picture Tube like a television with the image displayed on the front of the tube, which is called the screen.

o Printers produce a hard copy. The information is printed on paper andcan be used when the device is off. It is also called a printout. There different types of printers;

¾ Dot-matrix printers (impact printer)

• Uses metal pins to strike an inked ribbon to make dots on a piece of paper.

• Lowest print quality of all of the printers.

• Very low in cost per page to use.

¾ Ink jet printers (non-impact printer)

• Use drops of magnetic ink to produce dots on a page to produce text or images.

• The print quality is almost the same as a laser printer's.

• The ink is very expensive

• The ink is water soluble and will run if the paper gets wet

• Highest cost per page of all the printers

• For producing color documents, it has the highest quality at a


reasonable price. ¾ Laser printers (non-impact printer)

• A laser or LEDs make dots on a light sensitive drum • Toner (very tiny particles of plastic) stick to the drum where the

dots where made • For black and white printouts, very low cost per page

Printout is permanent

• Color laser printers are still fairly expensive

o SpeakersUsed to output soundo LCD Projectors Similar to monitors but projects an image on to a screen.They are

mainly used for presentations.

Central Processing Unit (CPU)/Processor:

It is the main part of a computer system like the brain of a human being. It interprets the instructions in the program and executes one by one. The CPU of a microcomputer is called a microprocessor. Central Processing Unit is implemented in a single piece of silicon device known as a computer chip.

The processor and main memory of a PC are commonly held on a single board called a mother board. The processor has the following functions:

• It controls the transmission of data from input devices to memory; • It processes the data held in main memory; • It controls the transmission of information from main memory to output devices.

The processor contains the control unit and the arithmetic/logic unit(ALU).The control unit coordinates and controls all the operations carried out by the computer. The control unit operates by repeating tStrategyee operations which are:

• Fetch – cause the next instruction to be fetched from memory;

• Decode – translate the program instruction into commands that thecomputer can process

• Execute – cause the instruction to be executedThe arithmetic/logic unit(ALU) plays two roles.

• Arithmetic operations – these operations are addition, subtraction,multiplication and division..

• Logical operations – it compares two data items to determine whether thefirst one is smaller than, equal to or greater than the second item.

Main Memory: The cycle (input - processing - output) would not be possible without a holding place for the instructions and data that the processors (CPU) can easily reach. This


holding place is known as memory also called main storage and is internal to the computer consisting of RAM and possibly ROM.

Random Access Memory (RAM)

• Is the basic kind of internal memory that holds data and instructions while the computer is in use. It can be read from and written to.

• It is called random access because the processor or computer can access any location in memory in any order as contrasted with sequential access devices which must be accessed in order.

• RAM is volatile; losing the stored information in an event of power loss, and quite expensive.

There are two basic types of RAM.

Static RAM does not need to be refreshed, which makes it faster; but it is alsomore expensive than dynamic RAM. Dynamic RAM needs to be refreshed thousands of times per second. Both types of RAM are volatile, meaning that they lose their contents when the power is turned off.

ROM (Read only memory)

• Is also random access but only for reads, once data has been written onto a ROM chip, it cannot be removed and can only be read.

It refers to special memory used to store programs that boot the computer and perform diagnostics. Most personal computers have a small amount of ROM (a few thousand bytes). Retains its contents even when the computer is turned off and is therefore referred to as being nonvolatile. Secondary storage: These are devices which are used to store huge information for future use. This is mostly hard drives and removable media such as floppy disks, optical media (CD ROM) etc.

Floppy Disk: Floppy disks allow information to be transported easily from onecomputer to another they have limited storage capacity, generally 1.44 MB. Saving and retrieving information from a floppy disk is slower than on a hard drive. They are more susceptible to physical damage and viruses than the hard drive. The size of a hard drive is usually expressed in terms of megabytes and gigabytes.

Compact Disk Read Only Memory (CD ROM): CD ROMs are read only storagemedium. Typically, a CD ROM holds up to 650 MB of information. While information retrieval is faster than from a floppy disk, it is still not as fast as from the hard drive.


Compact Disk-Writable (CD- R): A CD-R is highly effective for storing a largeamount of data. Can hold up to 700MB of information. A CD-R is a one time recordable compact disc.

Compact Disk-Re-Writable (CD-RW):

A CD-RW allows you to read, write, erase and write again. Writing takes place in a single pass of the focused laser beam. This is sometimes referred to as direct overwriting and can be repeated several thousand times per disc.

Computer Bus and Data Transmission

The bus is a medium used to transfer data and controls from one part to other part of computers. Data is normally transferred between main storage and the processor along a device called a bus which is effectively a means of sending multiple bits of data in parallel. There are mainly tStrategyee types of bus: (a) Data bus (b) Address bus (c) Control bus,a) Data bus: A bus which carries a word or a data to or from memory is known as data bus.(ii) The data width is 8 in 8085, 16 in 8086 and 32 in 80386.(iii) Data lines are bidirectional.(iv) One part of the data bus runs between RAM and the microprocessor.(v) Another part of the data bus runs between RAM and various storage devices.b) Address bus: The address bus carries memory address.(ii) The address lines can be 16 in 8085, 20 in 8086 and 32 in 80386.(iii) It is unidirectional, the bits flow in only one direction.c) Control bus: The control bus carries the control signals between the units of computers. The signals like READ/WRITE, START/HALT etc are carried by a control bus.

Software is a Program commercially prepared and tested in software by one or agroup of programmers and system analyst to perform a specified task. Software is simply set of instructions that cause a computer to perform one or more tasks.

Classification of software

Software

System software Application software


Operating Service General /ready Special/tailorSystem programs made made

applications applications

Classification of software

Utilities Development CommunicationPrograms programs

Software can be broadly classified into system software and application software

System software

System Software

Software required to run the hardware parts of the computer and other application software are called system software. System software acts as interface between hardware and user applications. An interface is needed because hardware devices or machines and humans speak in different languages.

Machines understand only binary language i.e. 0 (absence of electric signal) and 1 (presence of electric signal) while humans speak in English, French, German, Tamil, Hindi and many other languages. English is the pre-dominant language of interacting with computers. Software is required to convert all human instructions into machine understandable instructions. And this is exactly what system software does. It Consists of programs that control operations of the computer and enable user to make efficient use of computers. They coordinate computer activities and optimize use of computers. They are used to control the computer and develop and run application programs examples of jobs done by the system software are management of computer resources, defragmentation etc.

Based on its function, system software is of four types −

Operating System Language Processor Device Drivers

Language Processor

As discussed earlier, an important function of system software is to convert all user instructions into machine understandable language. When we talk of human machine interactions, languages are of three types −


Machine-level language − This language is nothing but a string of 0s and 1s that the machines can understand. It is completely machine dependent.

Assembly-level language − This language introduces a layer of abstraction by defining mnemonics. Mnemonics are English like words or symbols used to denote a long string of 0s and 1s. For example, the word “READ” can be defined to mean that computer has to retrieve data from the memory. The complete instruction will also tell the memory address. Assembly level language is machine dependent.

High level language − This language uses English like statements and is completely independent of machines. Programs written using high level languages are easy to create, read and understand.

Program written in high level programming languages like Java, C++, etc. is called source code. Set of instructions in machine readable form is called object code or machine code. System software that converts source code to object code is called language processor.

Device Drivers

System software that controls and monitors functioning of a specific device on computer is called device driver. Each device like printer, scanner, microphone, speaker, etc. that needs to be attached externally to the system has a specific driver associated with it. When you attach a new device, you need to install its driver so that the OS knows how it needs to be managed.

(i) Operating system – is a complex program and most important program that runs on a computer and which controls the operation of a computer. It perform basic tasks, such as recognizing input from the keyboard, sending output to the display screen, keeping track of files and directories on the disk, and controlling peripheral devices such as disk drives and printers. In general the operating system supervises and directs all the software components and the hardware components. Sophisticated operating system could handle multi-processors, many users and tasks simultaneously. Examples of computers operating systems are UNIX, Microsoft windows 95/98, Windows NT, Windows 2000, Windows XP, Windows Vista and Linux.

(ii)Service programs are programs designed for general support of the processes of a computer; "a computer system provides utility programs to perform the tasks needed by most users". The service programs can further be divided into;

o Utilities Performs a variety of tasks that maintain or enhance the computer’s operating system Utility programs are generally fairly small. Each type has a specific job to do. Below are some descriptions of utilities.

� Anti-virus applications protect your computer from the damage that can be caused by viruses and similar programs

� Compression utilities make files smaller for storage (or sendingover the Internet) and then return them to normal size.

� Data recovery utilities attempt to restore data and files that havebeen


damaged or accidentally deleted. � Disk defragmenters reorganize the data stored on disks so that it ismore

efficiently arranged. � Firewalls prevent outsiders from accessing your computer over anetwork such

as the Internet. o Development programs are used in the creation of new software. Theycomprise of

sets of software tools to allow programs to be written and tested. Knowledge of appropriate programming language is assumed. Tools used here are

� Text editors that allows one to enter and modify programsstatements Assembler- allows one to code in machine programs language .i.e.processor specific

� Compilers-makes it possible for programmer to convert sourcecode to object code which can be stored and saved on different computers.

� Interpreters-used to convert source programs statement bystatement as it executes the program without being compiled first.

� Libraries- commonly used parts or portions of a program whichcan be called or included in the programmer’s code without having to recode that portion.

� Diagnostic utilities-used to detect bugs in the logic of programduring program development

o Communication programs- refer to programs that make it possible totransmit data.

Application software

Are programs for user to do their jobs e.g. typing, recording keeping, production of financial statements, drawing, and statistics.

o General/ready made software is developed to perform a variety of tasks,usually determined by use. Such software can be customized by user to achieve specific goals e.g. ms office which is a suit of programs performing a variety of tasks e.g. word processing for producing documents, database for storing, retrieving and manipulating data and various calculations on spreadsheets. General purpose programs are discussed below;

Word processing applications. Writing tasks previously done ontypewriters with considerable effort can now be easily completed with word-processing software. Documents can be easily edited and formatted. Revisions can be made by deleting (cutting), inserting, moving (cutting and pasting), and copying data. Documents can be stored (saved) and opened again for revisions and/or printing. Many styles and sizes of fonts are available to make the document attractive. Example: MS Word, Word Pad etc.


Spreadsheet applications. spreadsheet software permitsperformance of an almost endless variety of quantitative tasks such as budgeting, keeping track of inventory, preparing financial reports, or manipulating numbers in any fashion, such as averaging each of ten departmental monthly sales over a six-month period. A spreadsheet contains cells, the intersection of rows and columns. Example: Excel, Lotus1-2-3 etc.

Database software: A database contains a list of information itemsthat are similar in format and/or nature. An example is a phone book that lists a name, address, and phone number for each entry. Once stored in a database, information can be retrieved in several ways, using reports and queries. For example, all the names listed for a given area code could be printed out and used for a commercial mailing to that area. Examples of database software is Ms Access, Dbase, Oracle etc.

Presentation software: for making slide shows. Allows users tocreate visual presentation A speaker may use presentation software to organize a slide show for an audience. Text, graphics, sound, and movies can easily be included in the presentation. An added feature is that the slide show may be enhanced by inclusion of handouts with two to six slides printed on a page.

Desktop publishing software: This software permits the user toprepare documents by using both word-processing devices and graphics. Desktop publishing software uses word-processing software, with all its ease of entering and revising data, and supplements it with sophisticated visual features that stem from graphics software. For example, one can enhance a printed message with virtually any kind of illustration, such as drawings, paintings, and photographs. . Examples of Desktop publishing software is PageMaker, Corel Draw, and Ms Publisher

Multimedia applications for creating video and music. Allowsusers to create image, audio, video etc. Example: Real Player, Media Player etc. Activity management programs like calendars and address books

o Tailor made/special purpose software Tailor-made computer systemrefers to computer application developed by in-house IT personnel or outside software house according to specific user requirements in a firm. They are developed for given purpose e.g. Payroll system, stock control system etc.

As you know, the hardware devices need user instructions to function. A set of instructions that achieve a single outcome are called program or procedure. Many programs functioning together to do a task make a software.

For example, a word-processing software enables the user to create, edit and save documents. A web browser enables the user to view and share web pages and multimedia files. There are two categories of software −


System Software Application Software Utility Software

Let us discuss them in detail.

Application Software

A software that performs a single task and nothing else is called application software. Application software are very specialized in their function and approach to solving a problem. So a spreadsheet software can only do operations with numbers and nothing else. A hospital management software will manage hospital activities and nothing else. Here are some commonly used application software −

Word processing Spreadsheet Presentation Database management Multimedia tools

Utility Software

Application software that assist system software in doing their work is called utility software. Thus utility software is actually a cross between system software and application software. Examples of utility software include −

Antivirus software Disk management tools File management tools Compression tools Backup tools

Basics of Computers - System S/W


As you know, system software acts as an interface for the underlying hardware system. Here we will discuss some important system software in detail.

Operating System

Operating system (OS) is the lifeline of computer. You connect all the basic devices like CPU, monitor, keyboard and mouse; plug in the power supply and switch it on thinking you have everything in place. System software that is responsible for functioning of all hardware parts and their interoperability to carry out tasks successfully is called operating system (OS). OS is the first software to be loaded into computer memory when the computer is switched on and this is called booting. OS manages a computer’s basic functions like storing data in memory, retrieving files from storage devices, scheduling tasks based on priority, etc.But the computer will not start or come to life unless it has an operating system installed in it because OS −

Keeps all hardware parts in a state of readiness to follow user instructions Co-ordinates between different devices Schedules multiple tasks as per priority Allocates resource to each task Enables computer to access network Enables users to access and use application software

Besides initial booting, these are some of the functions of an operating system −

Managing computer resources like hardware, software, shared resources, etc. Allocating resources Prevent error during software use Control improper use of computer

One of the earliest operating systems was MS-DOS, developed by Microsoft for IBM PC. It was a Command Line Interface (CLI) OS that revolutionized the PC market. DOS was difficult to use because of its interface. The users needed to remember instructions to do their tasks. To make computers more accessible and user-friendly, Microsoft developed Graphical User Interface (GUI) based OS called Windows, which transformed the way people used computers. There are three types of language interpreters−

Assembler − Converts assembly level program into machine level program.


Interpreter − Converts high level programs into machine level program line by line.

Compiler − Converts high level programs into machine level programs at one go rather than line by line.

Assembler

Assembler is a system software that converts assembly level programs to machine level code.

These are the advantages provided by assembly level programming −

Increases efficiency of the programmer as remembering mnemonics is easier Productivity increases as number of errors decreases and hence debugging time Programmer has access to hardware resources and hence has flexibility in writing

programs customized to the specific computer

Interpreter

The major advantage of assembly level language was its ability to optimize memory usage and hardware utilization. However, with technological advancements computers had more memory and better hardware components. So ease of writing programs became more important than optimizing memory and other hardware resources.

In addition, a need was felt to take programming out of a handful of trained scientists and computer programmers, so that computers could be used in more areas. This led to development of high level languages that were easy to understand due to resemblance of commands to English language.

The system software used to translate high level language source code into machine level language object code line by line is called an interpreter. An interpreter takes each line of code and converts it into machine code and stores it into the object file.

The advantage of using an interpreter is that they are very easy to write and they do not require a large memory space. However, there is a major disadvantage in using interpreters, i.e., interpreted programs take a long time in executing. To overcome this disadvantage, especially for large programs, compilers were developed.

Compiler

System software that store the complete program, scan it, translate the complete program into object code and then creates an executable code is called a compiler. On the face of it compilers compare unfavorably with interpreters because they −


are more complex than interpreters need more memory space take more time in compiling source code

However, compiled programs execute very fast on computers. The following image shows the step-by-step process of how a source code is transformed into an executable code −

These are the steps in compiling source code into executable code −

Pre-processing − In this stage pre-processor instructions, typically used by languages like C and C++ are interpreted, i.e. converted to assembly level language.

Lexical analysis − Here all instructions are converted to lexical units like constants, variables, arithmetic symbols, etc.

Parsing − Here all instructions are checked to see if they conform to grammar rules of the language. If there are errors, compiler will ask you to fix them before you can proceed.

Compiling − At this stage the source code is converted into object code.

Linking − If there are any links to external files or libraries, addresses of their executable will be added to the program. Also, if the code needs to be rearranged for actual execution, they will be rearranged. The final output is the executable code that is ready to be executed.

Basics of Computers - Functions of OS

As you know, operating system is responsible for functioning of the computer system. To do that it carries out these three broad categories of activities −

Essential functions − Ensures optimum and effective utilization of resources

Monitoring functions − Monitors and collects information related to system performance

Service functions − Provides services to users

Let us look at some of the most important functions associated with these activities.


Processor management

Managing a computer’s CPU to ensure its optimum utilization is called processor management. Managing processor basically involves allocating processor time to the tasks that need to be completed. This is called job scheduling. Jobs must be scheduled in such a way that −

There is maximum utilization of CPU Turnaround time, i.e. time required to complete each job, is minimum Waiting time is minimum Each job gets the fastest possible response time Maximum throughput is achieved, where throughput is the average time taken to

complete each task

There are two methods of job scheduling done by operating systems −

Preemptive scheduling Non-Preemptive scheduling

Preemptive Scheduling

In this type of scheduling, next job to be done by the processor can be scheduled before the current job completes. If a job of higher priority comes up, the processor can be forced to release the current job and take up the next job. There are two scheduling techniques that use pre-emptive scheduling −

Round robin scheduling − A small unit of time called time slice is defined and each program gets only one time slice at a time. If it is not completed during that time, it must join the job queue at the end and wait till all programs have got one time slice. The advantage here is that all programs get equal opportunity. The downside is that if a program completes execution before the time slice is over, CPU is idle for the rest of the duration.

Response ratio scheduling − A job with shorter response time gets higher priority. So a larger program may have to wait even if it was requested earlier than the shorter program. This improves throughput of the CPU.


Non-preemptive Scheduling

In this type of scheduling, job scheduling decisions are taken only after the current job completes. A job is never interrupted to give precedence to higher priority jobs. Scheduling techniques that use non-preemptive scheduling are −

First come first serve scheduling − This is the simplest technique where the first program to throw up a request is completed first.

Shortest job next scheduling − Here the job that needs least amount of time for execution is scheduled next.

Deadline scheduling − The job with the earliest deadline is scheduled for execution next.

Memory Management

Process of regulating computer memory and using optimization techniques to enhance overall system performance is called memory management. Memory space is very important in modern computing environment, so memory management is an important role of operating systems.

As you know, computers have two types of memory – primary and secondary. Primary memory is fast but expensive and secondary memory is cheap but slower. OS has to strike a balance between the two to ensure that system performance is not hurt due to very less primary memory or system costs do not shoot up due to too much primary memory.

Input and output data, user instructions and data interim to program execution need to be stored, accessed and retrieved efficiently for high system performance. Once a program request is accepted, OS allocates it primary and secondary storage areas as per requirement. Once execution is completed, the memory space allocated to it is freed. OS uses many storage management techniques to keep a track of all storage spaces that are allocated or free.

Contiguous Storage Allocation

This is the simplest storage space allocation technique where contiguous memory locations are assigned to each program. OS has to estimate the amount of memory required for the complete process before allocation.

Non-contiguous Storage Allocation

As the name suggests, program and associated data need not be stored in contiguous locations. The program is divided into smaller components and each component is stored in a separate location. A table keeps a record of where each component of the program is stored. When the processor needs to access any component, OS provides access using this allocation table.

In a real-life scenario primary memory space might not be sufficient to store the whole program. In that case, OS takes the help of Virtual Storage technique, where program is physically stored in secondary memory but appears to be stored in primary memory. This


introduces a miniscule time lag in accessing the program components. There are two approaches to virtual storages −

Program paging − A program is broken down into fixed size page and stored in the secondary memory. The pages are given logical address or virtual address from 0 to n. A page table maps the logical addresses to the physical addresses, which is used to retrieve the pages when required.

Program segmentation − A program is broken down into logical units called segments, assigned logical address from 0 to n and stored in secondary memory. A segment table is used to load segments from secondary memory to primary memory.

Operating systems typically use a combination of page and program segmentation to optimize memory usage. A large program segment may be broken into pages or more than one small segments may be stored as a single page.

File Management

Data and information is stored on computers in form of files. Managing file system to enable users to keep their data safely and correctly is an important function of operating systems. Managing file systems by OS is called file management. File management is required to provide tools for these file related activities −

Creating new files for storing data Updating Sharing Securing data through passwords and encryption Recovery in case of system failure

Device Management

The process of implementation, operation and maintenance of a device by operating system is called device management. Operating system uses a utility software called device driver as interface to the device.

When many processes access the devices or request access to the devices, the OS manages the devices in a way that efficiently shares the devices among all processes. Processes access devices through system call interface, a programming interface provided by the OS.

Basics of Computers - Types of OS

As computers and computing technologies have evolved over the years, so have their usage across many fields. To meet growing requirements more and more customized software have flooded the market. As every software needs operating system to function, operating systems have also evolved over the years to meet growing demand on their techniques and capabilities. Here we discuss some common types of OS based on their working techniques and some popularly used OS as well.

GUI OS


GUI is the acronym for Graphical User Interface. An operating system that presents an interface comprising graphics and icons is called a GUI OS. GUI OS is very easy to navigate and use as users need not remember commands to be given to accomplish each task. Examples of GUI OS includes Windows, macOS, Ubuntu, etc.

Time Sharing OS

Operating systems that schedule tasks for efficient processor use are called time sharing OS. Time sharing, or multitasking, is used by operating systems when multiple users located at different terminals need processor time to complete their tasks. Many scheduling techniques like round robin scheduling and shortest job next scheduling are used by time sharing OS.

Real Time OS

An operating system that guarantees to process live events or data and deliver the results within a stipulated span of time is called a real time OS. It may be single tasking or multitasking.

Distributed OS

An operating system that manages many computers but presents an interface of single computer to the user is called distributed OS. Such type of OS is required when computational requirements cannot be met by a single computer and more systems have to be used. User interaction is restricted to a single system; it’s the OS that distributed work to multiple systems and then presents the consolidated output as if one computer has worked on the problem at hand.

Popular Operating Systems

Initially computers had no operating systems. Every program needed full hardware specifications to run correctly as processor, memory and device management had to be done by the programs themselves. However, as sophisticated hardware and more complex application programs developed, operating systems became essential. As personal computers became popular among individuals and small businesses, demand for standard operating system grew. Let us look at some of the currently popular operating systems −

Windows − Windows is a GUI operating system first developed by Microsoft in 1985. The latest version of Windows is Windows 10. Windows is used by almost 88% of PCs and laptops globally.

Linux − Linux is an open source operating system mostly used by mainframes an supercomputers. Being open source means that its code is available for free and anyone can develop a new OS based on it.

BOSS − Bharat Operating System Solutions is an Indian distribution of Linux based on Debian, an OS. It is localized to enable use of local Indian languages. BOSS consists of −

o Linux kernel


o Office application suite BharteeyaOOo Web browsero Email service Thunderbirdo Chat application Pidgimo File sharing applicationso Multimedia applications

Instruction Set

A command given to a 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 types of operations

Data transfer Arithmetic operations Logical operations Control flow Input/output and machine control

Microprocessor Components

Compared to the first microprocessors, today’s processors are very small but still they have these basic parts right from the first model −

CPU Bus Memory

CPU

CPU is fabricated as a very large scale integrated circuit (VLSI) and has these parts −

Instruction register − It holds the instruction to be executed.

Decoder − It decodes (converts to machine level language) the instruction and sends to the ALU (Arithmetic Logic Unit).

ALU − It has necessary circuits to perform arithmetic, logical, memory, register and program sequencing operations.

Register − It holds intermediate results obtained during program processing. Registers are used for holding such results rather than RAM because accessing registers is almost 10 times faster than accessing RAM.

Bus

Connection lines used to connect the internal parts of the microprocessor chip is called bus. There are three types of buses in a microprocessor −


Data Bus − Lines that carry data to and from memory are called data bus. It is a bidirectional bus with width equal to word length of the microprocessor.

Address Bus − It is a unidirectional responsible for carrying address of a memory location or I/O port from CPU to memory or I/O port.

Control Bus − Lines that carry control signals like clock signals, interrupt signal or ready signal are called control bus. They are bidirectional. Signal that denotes that a device is ready for processing is called ready signal. Signal that indicates to a device to interrupt its process is called an interrupt signal.

Basic Memory

Memory is required in computers to store data and instructions. Memory is physically organized as a large number of cells that are capable of storing one bit each. Logically they are organized as groups of bits called words that are assigned an address. Data and instructions are accessed through these memory address. The speed with which these memory addresses can be accessed determines the cost of the memory. Faster the memory speed, higher the price.

Computer memory can be said to be organized in a hierarchical way where memory with the fastest access speeds and highest costs lies at the top whereas those with lowest speeds and hence lowest costs lie at the bottom. Based on this criteria memory is of two types – primary and secondary. Here we will look at primary memory in detail.

The main features of primary memory, which distinguish it from secondary memory are −

It is accessed directly by the processor It is the fastest memory available Each word is stored as well as It is volatile, i.e. its contents are lost once power is switched off

As primary memory is expensive, technologies are developed to optimize its use. These are broad types of primary memory available.

Microprocessor has two types of memory

RAM − Random Access Memory is volatile memory that gets erased when power is switched off. All data and instructions are stored in RAM.


ROM − Read Only Memory is non-volatile memory whose data remains intact even after power is switched off. Microprocessor can read from it any time it wants but cannot write to it. It is preprogrammed with most essential data like booting sequence by the manufacturer.

RAM: RAM stands for Random Access Memory. The processor accesses all memory

addresses directly, irrespective of word length, making storage and retrieval fast. RAM is the

fastest memory available and hence most expensive. These two factors imply that RAM is

available in very small quantities of up to 1GB. RAM is volatile but my be of any of these

two types

DRAM (Dynamic RAM): Each memory cell in a DRAM is made of one transistor and one

capacitor, which store one bit of data. However, this cell starts losing its charge and hence

data stored in less than thousandth of a second. So it needs to be refreshed thousand times a

second, which takes up processor time. However, due to small size of each cell, one DRAM

can have large number of cells. Primary memory of most of the personal computers is made

of DRAM.

SRAM (SRAM): Each cell in SRAM is made of a flip flop that stores one bit. It retains its bit

till the power supply is on and doesn’t need to be refreshed like DRAM. It also has shorter

read-write cycles as compared to DRAM. SRAM is used in specialized applications.

ROM: ROM stands for Read Only Memory. As the name suggests, ROM can only be read

by the processor. New data cannot be written into ROM. Data to be stored into ROM is

written during the manufacturing phase itself. They contain data that does not need to be

altered, like booting sequence of a computer or algorithmic tables for mathematical

applications. ROM is slower and hence cheaper than RAM. It retains its data even when

power is switched off, i.e. it is non-volatile. ROM cannot be altered the way RAM can be but

technologies are available to program these types of ROMs −

PROM (Programmable ROM): PROM can be programmed using a special hardware device

called PROM programmer or PROM burner.


EPROM (Erasable Programmable ROM): EPROM can be erased and then programmed using

special electrical signals or UV rays. EPROMs that can be erased using UV rays are called

UVEPROM and those that can be erased using electrical signals are called EEPROM.

However, handling electric signals is easier and safer than UV rays.

NETWORK

It’s a connection of two or more devices to share resources.

Computer network is a telecommunication channel using which we can share data with other coomputers or devices, connected to the same network. It is also called Data Network. The best example of computer network is Internet. Computer network does not mean a system with one Control Unit connected to multiple other systems as its slave. That is Distributed system, not Computer Network. A network must be able to meet certain criterias, these are mentioned below:

1. Performance2. Reliability3. Scalability

Computer Networks: Performance

It can be measured in the following ways:

Transit time : It is the time taken to travel a message from one device to another. Response time : It is defined as the time elapsed between enquiry and response.

Other ways to measure performance are :

1. Efficiency of software2. Number of users3. Capability of connected hardware

Computer Networks: Reliability

It decides the frequency at which network failure take place. More the failures are, less is the network's reliability.

Computer Networks: Security

It refers to the protection of data from any unauthorised user or access. While travelling through network, data passes many layers of network, and data can be traced if attempted. Hence security is also a very important characteristic for Networks.

Properties of a Good Network


1. Interpersonal Communication: We can communicate with each other efficiently and easily. Example: emails, chat rooms, video conferencing etc, all of these are possible because of computer networks.

2. Resources can be shared: We can share physical resources by making them available on a network such as printers, scanners etc.

3. Sharing files, data: Authorised users are allowed to share the files on the network.

Uses of Computer Networks

Had it not been of high importance, nobody would have bothered connecting computers over a network. Let's start exploring the uses of Computer Networks with some traditional usecases at companies and for individuals and then move on to the recent developments in the area of mobile users and home networking.

Computer Networks: Business Applications

Following are some business applications of computer networks:

1. Resource Sharing: The goal is to make all programs, equipments(like printers etc), and especially data, available to anyone on the network without regard to the physical location of the resource and the user.

2. Server-Client model: One can imagine a company's information system as consisting of one or more databases and some employees who need to access it remotely. In this model, the data is stored on powerful computers called Servers. Often these are centrally housed and maintained by a system administrator. In contrast, the employees have simple machines, called Clients, on their desks, using which they access remote data.

3. Communication Medium: A computer network can provide a powerful communication medium among employees. Virtually every company that has two or more computers now has e-mail (electronic mail), which employees generally use for a great deal of daily communication

4. e-commerce: A goal that is starting to become more important in businesses is doing business with consumers over the Internet. Airlines, bookstores and music vendors have discovered that many customers like the convenience of shopping from home. This sector is expected to grow quickly in the future.

Computer Networks: Home Applications

Some of the most important uses of the Internet for home users are as follows:

Access to remote information Person-to-person communication Interactive entertainment Electronic commerce

Computer Networks: Mobile Users


Mobile computers, such as notebook computers and Mobile phones, is one of the fastest-growing segment of the entire computer industry. Although wireless networking and mobile computing are often related, they are not identical, as the below figure shows.

Line Configuration in Computer Networks

A Network is nothing but a connection made through connection links between two or more devices. Devices can be a computer, printer or any other device that is capable to send and receive data. There are two ways to connect the devices :

1. Point-to-Point connection2. Multipoint connection

Point-To-Point Connection

It is a protocol which is used as a communication link between two devices. It is simple to establish. The most common example for Point-to-Point connection (PPP) is a computer connected by telephone line. We can connect the two devices by means of a pair of wires or using a microwave or satellite link. Example: Point-to-Point connection between remote control and Television for changing the channels.

MultiPoint Connection


It is also called Multidrop configuration. In this connection two or more devices share a single link. There are two kinds of Multipoint Connections :

If the links are used simultaneously between many devices, then it is spatially shared line configuration.

If user takes turns while using the link, then it is time shared (temporal) line configuration.

Transmission Modes in Computer Networks

Transmission mode refers to the mechanism of transferring of data between two devices connected over a network. It is also called Communication Mode. These modes direct the direction of flow of information. There are three types of transmission modes. They are:

1. Simplex Mode2. Half duplex Mode3. Full duplex Mode

SIMPLEX Mode

In this type of transmission mode, data can be sent only in one direction i.e. communication is unidirectional. We cannot send a message back to the sender. Unidirectional communication is done in Simplex Systems where we just need to send a command/signal, and do not expect any response back. Examples of simplex Mode are loudspeakers, television broadcasting, television and remote, keyboard and monitor etc.


HALF DUPLEX Mode

Half-duplex data transmission means that data can be transmitted in both directions on a signal carrier, but not at the same time.

For example, on a local area network using a technology that has half-duplex transmission, one workstation can send data on the line and then immediately receive data on the line from the same direction in which data was just transmitted. Hence half-duplex transmission implies a bidirectional line (one that can carry data in both directions) but data can be sent in only one direction at a time. Example of half duplex is a walkie- talkie in which message is sent one at a time but messages are sent in both the directions.

FULL DUPLEX Mode

In full duplex system we can send data in both the directions as it is bidirectional at the same time in other words, data can be sent in both directions simultaneously.

Example of Full Duplex is a Telephone Network in which there is communication between two persons by a telephone line, using which both can talk and listen at the same time.

In full duplex system there can be two lines one for sending the data and the other for receiving data.

Types of Communication Networks

Communication Networks can be of following 5 types:

1. Local Area Network (LAN)


2. Metropolitan Area Network (MAN)3. Wide Area Network (WAN)4. Wireless5. Inter Network (Internet)

Local Area Network (LAN)

It is also called LAN and designed for small physical areas such as an office, group of buildings or a factory. LANs are used widely as it is easy to design and to troubleshoot. Personal computers and workstations are connected to each other through LANs. We can use different types of topologies through LAN, these are Star, Ring, Bus, Tree etc. LAN can be a simple network like connecting two computers, to share files and network among each other while it can also be as complex as interconnecting an entire building. LAN networks are also widely used to share resources like printers, shared hard-drive etc.

Characteristics of LAN

LAN's are private networks, not subject to tariffs or other regulatory controls. LAN's operate at relatively high speed when compared to the typical WAN. There are different types of Media Access Control methods in a LAN, the prominent

ones are Ethernet, Token ring. It connects computers in a single building, block or campus, i.e. they work in a

restricted geographical area.

Applications of LAN

One of the computer in a network can become a server serving all the remaining computers called clients. Software can be stored on the server and it can be used by the remaining clients.


Connecting Locally all the workstations in a building to let them communicate with each other locally without any internet access.

Sharing common resources like printers etc are some common applications of LAN.

Advantages of LAN

Resource Sharing: Computer resources like printers, modems, DVD-ROM drives and hard disks can be shared with the help of local area networks. This reduces cost and hardware purchases.

Software Applications Sharing: It is cheaper to use same software over network instead of purchasing separate licensed software for each client a network.

Easy and Cheap Communication: Data and messages can easily be transferred over networked computers.

Centralized Data: The data of all network users can be saved on hard disk of the server computer. This will help users to use any workstation in a network to access their data. Because data is not stored on workstations locally.

Data Security: Since, data is stored on server computer centrally, it will be easy to manage data at only one place and the data will be more secure too.

Internet Sharing: Local Area Network provides the facility to share a single internet connection among all the LAN users. In Net Cafes, single internet connection sharing system keeps the internet expenses cheaper.

Disadvantages of LAN

High Setup Cost: Although the LAN will save cost over time due to shared computer resources, but the initial setup costs of installing Local Area Networks is high.

Privacy Violations: The LAN administrator has the rights to check personal data files of each and every LAN user. Moreover he can check the internet history and computer use history of the LAN user.

Data Security Threat: Unauthorised users can access important data of an organization if centralized data repository is not secured properly by the LAN administrator.

LAN Maintenance Job: Local Area Network requires a LAN Administrator because, there are problems of software installations or hardware failures or cable disturbances in Local Area Network. A LAN Administrator is needed at this full time job.

Covers Limited Area: Local Area Network covers a small area like one office, one building or a group of nearby buildings.

Metropolitan Area Network (MAN)

It was developed in 1980s.It is basically a bigger version of LAN. It is also called MAN and uses the similar technology as LAN. It is designed to extend over the entire city. It can be means to connecting a number of LANs into a larger network or it can be a single cable. It is mainly hold and operated by single private company or a public company.


Characteristics of MAN

It generally covers towns and cities (50 km) Communication medium used for MAN are optical fibers, cables etc. Data rates adequate for distributed computing applications.

Advantages of MAN

Extremely efficient and provide fast communication via high-speed carriers, such as fibre optic cables.

It provides a good back bone for large network and provides greater access to WANs. The dual bus used in MAN helps the transmission of data in both directions

simultaneously. A MAN usually encompasses several blocks of a city or an entire city.

Disadvantages of MAN

More cable required for a MAN connection from one place to another. It is difficult to make the system secure from hackers and industrial espionage(spying)

graphical regions.

Wide Area Network (WAN)

It is also called WAN. WAN can be private or it can be public leased network. It is used for the network that covers large distance such as cover states of a country. It is not easy to design and maintain. Communication medium used by WAN are PSTN or Satellite links. WAN operates on low data rates.


Characteristics of WAN

It generally covers large distances(states, countries, continents). Communication medium used are satellite, public telephone networks which are

connected by routers.

Advantages of WAN

Covers a large geographical area so long distance business can connect on the one network.

Shares software and resources with connecting workstations. Messages can be sent very quickly to anyone else on the network. These messages

can have picture, sounds or data included with them(called attachments). Expensive things(such as printers or phone lines to the internet) can be shared by all

the computers on the network without having to buy a different peripheral for each computer.

Everyone on the network can use the same data. This avoids problems where some users may have older information than others.

Disadvantages of WAN

Need a good firewall to restrict outsiders from entering and disrupting the network. Setting up a network can be an expensive, slow and complicated. The bigger the

network the more expensive it is. Once set up, maintaining a network is a full-time job which requires network

supervisors and technicians to be employed. Security is a real issue when many different people have the ability to use information

from other computers. Protection against hackers and viruses adds more complexity and expense.

Types of Network Topology

Network Topology is the schematic description of a network arrangement, connecting various nodes(sender and receiver) through lines of connection.

BUS Topology

Bus topology is a network type in which every computer and network device is connected to single cable. When it has exactly two endpoints, then it is called Linear Bus topology.


Features of Bus Topology

1. It transmits data only in one direction.2. Every device is connected to a single cable

Advantages of Bus Topology

1. It is cost effective.2. Cable required is least compared to other network topology.3. Used in small networks.4. It is easy to understand.5. Easy to expand joining two cables together.

Disadvantages of Bus Topology

1. Cables fails then whole network fails.2. If network traffic is heavy or nodes are more the performance of the network

decreases.3. Cable has a limited length.4. It is slower than the ring topology.

RING Topology

It is called ring topology because it forms a ring as each computer is connected to another computer, with the last one connected to the first. Exactly two neighbours for each device.

Features of Ring Topology


1. A number of repeaters are used for Ring topology with large number of nodes, because if someone wants to send some data to the last node in the ring topology with 100 nodes, then the data will have to pass through 99 nodes to reach the 100th node. Hence to prevent data loss repeaters are used in the network.

2. The transmission is unidirectional, but it can be made bidirectional by having 2 connections between each Network Node, it is called Dual Ring Topology.

3. In Dual Ring Topology, two ring networks are formed, and data flow is in opposite direction in them. Also, if one ring fails, the second ring can act as a backup, to keep the network up.

4. Data is transferred in a sequential manner that is bit by bit. Data transmitted, has to pass through each node of the network, till the destination node.

Advantages of Ring Topology

1. Transmitting network is not affected by high traffic or by adding more nodes, as only the nodes having tokens can transmit data.

2. Cheap to install and expand

Disadvantages of Ring Topology

1. Troubleshooting is difficult in ring topology.2. Adding or deleting the computers disturbs the network activity.3. Failure of one computer disturbs the whole network.

STAR Topology

In this type of topology all the computers are connected to a single hub through a cable. This hub is the central node and all others nodes are connected to the central node.

Features of Star Topology

1. Every node has its own dedicated connection to the hub.2. Hub acts as a repeater for data flow.3. Can be used with twisted pair, Optical Fibre or coaxial cable.

Advantages of Star Topology

1. Fast performance with few nodes and low network traffic.2. Hub can be upgraded easily.


3. Easy to troubleshoot.4. Easy to setup and modify.5. Only that node is affected which has failed, rest of the nodes can work smoothly.

Disadvantages of Star Topology

1. Cost of installation is high.2. Expensive to use.3. If the hub fails then the whole network is stopped because all the nodes depend on the

hub.4. Performance is based on the hub that is it depends on its capacity

MESH Topology

It is a point-to-point connection to other nodes or devices. All the network nodes are connected to each other. Mesh has n(n-1)/2 physical channels to link n devices. There are two techniques to transmit data over the Mesh topology, they are :

1. Routing2. Flooding

MESH Topology: Routing

In routing, the nodes have a routing logic, as per the network requirements. Like routing logic to direct the data to reach the destination using the shortest distance. Or, routing logic which has information about the broken links, and it avoids those node etc. We can even have routing logic, to re-configure the failed nodes.

MESH Topology: Flooding

In flooding, the same data is transmitted to all the network nodes, hence no routing logic is required. The network is robust, and the its very unlikely to lose the data. But it leads to unwanted load over the network.

Types of Mesh Topology


1. Partial Mesh Topology : In this topology some of the systems are connected in the same fashion as mesh topology but some devices are only connected to two or three devices.

2. Full Mesh Topology : Each and every nodes or devices are connected to each other.

Features of Mesh Topology

1. Fully connected.2. Robust.3. Not flexible.

Advantages of Mesh Topology

1. Each connection can carry its own data load.2. It is robust.3. Fault is diagnosed easily.4. Provides security and privacy.

Disadvantages of Mesh Topology

1. Installation and configuration is difficult.2. Cabling cost is more.3. Bulk wiring is required.

TREE Topology

It has a root node and all other nodes are connected to it forming a hierarchy. It is also called hierarchical topology. It should at least have three levels to the hierarchy.

Features of Tree Topology

1. Ideal if workstations are located in groups.2. Used in Wide Area Network.

Advantages of Tree Topology

1. Extension of bus and star topologies.


2. Expansion of nodes is possible and easy.3. Easily managed and maintained.4. Error detection is easily done.

Disadvantages of Tree Topology

1. Heavily cabled.2. Costly.3. If more nodes are added maintenance is difficult.4. Central hub fails, network fails.

HYBRID Topology

It is two different types of topologies which is a mixture of two or more topologies. For example if in an office in one department ring topology is used and in another star topology is used, connecting these topologies will result in Hybrid Topology (ring topology and star topology).

Features of Hybrid Topology

1. It is a combination of two or topologies2. Inherits the advantages and disadvantages of the topologies included

Advantages of Hybrid Topology

1. Reliable as Error detecting and trouble shooting is easy.2. Effective.3. Scalable as size can be increased easily.4. Flexible.

Disadvantages of Hybrid Topology

1. Complex in design.2. Costly.


Computer File organization

It is used to determine an efficient file organization for each base relation. For example, if we want to retrieve student records in alphabetical order of name, sorting the file by student name is a good file organization. However, if we want to retrieve all students whose marks is in a certain range, a file ordered by student name would not be a good file organization. Some file organizations are efficient for bulk loading data into the database but inefficient for retrieve and other activities.The objective of this selection is to choose an optimal file organization for each relation.

Types of File Organization

In order to make effective selection of file organizations and indexes, here we present the details different types of file Organization. These are:

Heap (unordered) File Organization

An unordered file, sometimes called a heap file, is the simplest type of file organization.Records are placed in file in the same order as they are inserted. A new record is inserted in the last page of the file; if there is insufficient space in the last page, a new page is added to the file. This makes insertion very efficient. However, as a heap file has no particular ordering with respect to field values, a linear search must be performed to access a record. A linear search involves reading pages from the file until the required is found. This makes retrievals from heap files that have more than a few pages relatively slow, unless the retrieval involves a large proportion of the records in the file.

To delete a record, the required page first has to be retrieved, the record marked as deleted, and the page written back to disk. The space with deleted records is not reused. Consequently, performance progressively deteriorates as deletion occurs. This means that heap files have to be periodically reorganized by the Database Administrator (DBA) to reclaim the unused space of deleted records.

Heap files are one of the best organizations for bulk loading data into a table, as records are inserted at the end of the sequence; there is no overhead of calculating what page the record should go on.

Pros of Heap storage

Heap is a good storage structure in the following situations:

When data is being bulk-loaded into the relation.

The relation is only a few pages long. In this case, the time to locate any tuple is Short, even if the entire relation has been searched serially.

When every tuple in the relation has to be retrieved (in any order) every time the relation is accessed. For example, retrieve the name of all the students.

Cons of Heap storage

Heap files are inappropriate when only selected tuples of a relation are to be accessed.

Hash File Organization

In a hash file, records are not stored sequentially in a file instead a hash function is used to calculate the address of the page in which the record is to be stored.


http://ecomputernotes.com/fundamental/what-is-a-database/what-is-dba

http://ecomputernotes.com/fundamental/what-is-a-database/advantages-and-disadvantages-of-dbms

The field on which hash function is calculated is called as Hash field and if that field acts as the key of the relation then it is called as Hash key. Records are randomly distributed in the file so it is also called as Random or Direct files. Commonly some arithmetic function is applied to the hash field so that records will be evenly distributed throughout the file.

Pros of Hash file organization

Hash is a good storage structure in the following situations:

When tuples are retrieve based on an exact match on the hash field value, particularly if the access order is random. For example, if the STUDENT relation is hashed on Name then retrieval of the tuple with Name equal to "Rahat Bhatia" is efficient.

Cons of Hash file organization

Hash is not a good storage structure in the following situations:

When tuples are retrieved based on a range of values for the hash field. For example, retrieve all students whose name begins with the "R".

When tuples are retrieved based on a range of values for the hash field. For example, if STUDENT relation has hash filed Roll Number and the query is to retrieve all students with roll numbers in the range of 3000-5000.

When tuples re retrieved based on a field other than the hash field. For example, if the STUDENT relation is hashed on Roll Number, then hashing cannot be used to search for a tuple based on the Class attribute.

When tuples are retrieved based on only part of the hash field. For example, if the STUDENT relation is hashed on Roll Number and Class, then hashing cannot be used to search for a tuple based on the class attribute alone.

When the hash field frequently updated. When a hash field updated, the DBMS must deleted the entire tuple and possible relocate it to a new address (if the has function results in a new address). Thus, frequent updating of the hash field impacts performance.

Secondary indexes

Secondary indexes provide a mechanism for specifying a...'1additional key for a base relation that can be used to retrieve data more efficiently. For example, the STUDENT relation may be hashed on the Name the primary index. However, there may be frequent access to this relation based on the Roll Number attribute. In this case, we may decide to add Roll Number as a secondary index.

There is an overhead involved in the maintenance and use of secondary indexes that has to be balanced against the performance improvement gained when retrieving data. This overhead includes:

• adding an index record to every secondary index whenever a tuple is inserted into the relation;

• updating a secondary index when the corresponding tuple in the relation is updated;

• The increase in disk space needed to store the secondary index;

• Possible performance degradation during query optimization, as the query optimizer may consider all secondary indexes before selecting an optimal execution strategy.

Indexes Sequential Access Method (ISAM)


http://ecomputernotes.com/fundamental/what-is-a-database/advantages-and-disadvantages-of-dbms

In an ISAM system, data is organized into records which are composed of fixed length fields. Records are stored sequentially. A secondary set of hash tables known as indexes contain "pointers" into the tables, allowing individual records to be retrieved without having to search the entire data set.

It is a data structure that allows the DBMS to locate particular records in a file more quickly and thereby speed response to user queries. An index in a database is similar to an index in a book. It is an auxiliary structure associated with a file that can be referred to when searching for an item of information, just like searching the index of a book, in which we look up a keyword to get a list of one or more pages the keyword appears on. An index obviates the need to scan sequentially through the file each time we want to find the item. In the case of database indexes, the required item will be one or more records in a file. As in the book index analogy, the index is ordered, and each index entry contains the item required and one or more locations (record identifiers) where the item can be found.

While indexes are not strictly necessary to use the DBMS, they can have a significant impact on performance. As with the book index, we could find the desired keyword by looking through the entire book, but this would be tedious and time-consuming. Having an index at the back of the book on alphabetical order to keYW0fd allows us to go directly to the page or pages we want.

An index structure is associated with a particular search key and contains record consisting of the key value and the address of the logical record in the file contains records consisting of the key value of the address of the logical record in the file containing the key value. The file containing the logical records is called the data file and the file containing the index records is called the index file. The value in the index file are ordered according to the indexing field, which is usually based on a single attribute.

A sorted data file with a primary index is called an indexed sequential file. This structure is a compromise between a purely sequential file and a purely random file, in that records can be processed sequentially or individually accessed using a search key value that accesses the record via the index. An indexed sequential file is a more versatile structure, which normally has.

• a primary storage area;

• a separate index or indexes;

• an overflow area.

When to use

ISAM is a more versatile storage structure than hash and it proved better when retrievals are based on exact key match, pattern matching, range of values, and part key specification.

When not to use

However, the ISAM index is static, created when the file is created. Thus, the performance of an ISAM file deteriorates as the relation is updated. Updates also cause an ISAM file to lose the access key sequence, so that retrievals in order of the access key will become slower. These two problems are overcome by the B+-tree file organization. However, unlike B+- tree, concurrent access to the index can be easily managed because the index is static.

Computer viruses


http://ecomputernotes.com/fundamental/information-technology/what-do-you-mean-by-data-and-information

COMPUTER VIRUS

A computer virus is a type of computer program that can replicate itself by making copies of it by making copies of itself without the user's knowledge or consent.

Computer virus spread from following ways

i. Opening the infected email attachment.

ii. By downloading infected programs from the internet.

iii. By using infected floppy disk, pen drives and CDs.

iv. By transferring an infected program over a network and executing it.

Type of computer virus

i) Boot Sector Virus

Boot Sector virus is regarded as one of the most hostile types of virus, a boot sector virus infects the boot record of a hard or a floppy disk. The virus moves the boot sector data to a different part of a disk. This virus allows the actual boot sector data to be read as though a normal start up were occurring.

ii) File Infecting Virus

This type of virus infects program files on a disk. When infected program is launched, the virus code is also launched. After the virus code has been launched, it will normal load and execute the program that has been infected.

iii) Multipartite Virus

This virus is the combination of parasitic and boot sector virus and due to this, it is able to infect both the files or boot sector of a disk. They infect program files and when it is executed, these viruses affect the boot record.

iv) Macro Virus

These viruses infect the Microsoft Office programs such as Excel, Word, Access and PowerPoint. When we open the Microsoft program then the virus is loaded and it affects the normal template or document. Since it is attached in the document, if the infected document is opened on other computer, it affects the computer too.

v) Script Virus

Script Virus is written in script programming languages, such as VBScript and JavaScript. It can infect a computer through various scripting environments, such as Windows Help, Windows installation files and Windows registry files.

The symptoms of computer viruses

i. Programs take long time to load.

ii. Renaming all the files with different names.

iii. Shows unusual error messages on the screen frequently.

iv. Corrupt or deletes system data and programs.\


Anti-virus software

Anti-virus software is a computer program designed to detect and remove viruses from the computer system.

Most Effective Antivirus

Following are the most popular and effective antivirus from which you can choose one for your personal computer −

McAfee Antivirus Plus Symantec Norton Antivirus Avast Pro Antivirus Bitdefender Antivirus Plus Kaspersky Anti-Virus Avira Antivirus Webroot Secure Anywhere Antivirus Emsisoft Anti-Malware Quick Heal Antivirus ESET NOD32 Antivirus

Avoid the virus attacks

i. Scanning the email attachment before executing them.

ii. Use of a good anti-virus program to scan for viruses.

iii. Scanning the disk frequently for viruses.

iv. Not installing the pirated software, especially computer games.

v. Patching up the operating system.

vi. Patching up the client software.

vii. Putting highly secured Passwords and Use of Firewalls.

Computer Numbering systems

The technique to represent and work with numbers is called number system. Decimal number system is the most common number system. Other popular number systems include binary number system, octal number system, hexadecimal number system, etc.

Decimal Number System

Decimal number system is a base 10 number system having 10 digits from 0 to 9. This means that any numerical quantity can be represented using these 10 digits. Decimal number


system is also a positional value system. This means that the value of digits will depend on its position. Let us take an example to understand this.

Say we have three numbers – 734, 971 and 207. The value of 7 in all three numbers is different−

In 734, value of 7 is 7 hundreds or 700 or 7 × 100 or 7 × 102

In 971, value of 7 is 7 tens or 70 or 7 × 10 or 7 × 101

In 207, value 0f 7 is 7 units or 7 or 7 × 1 or 7 × 100

The weightage of each position can be represented as follows −

In digital systems, instructions are given through electric signals; variation is done by varying the voltage of the signal. Having 10 different voltages to implement decimal number system in digital equipment is difficult. So, many number systems that are easier to implement digitally have been developed. Let’s look at them in detail.

Binary Number System

The easiest way to vary instructions through electric signals is two-state system – on and off. On is represented as 1 and off as 0, though 0 is not actually no signal but signal at a lower voltage. The number system having just these two digits – 0 and 1 – is called binary number system.

Each binary digit is also called a bit. Binary number system is also positional value system, where each digit has a value expressed in powers of 2, as displayed here.

In any binary number, the rightmost digit is called least significant bit (LSB) and leftmost digit is called most significant bit (MSB).

And decimal equivalent of this number is sum of product of each digit with its positional value.

110102 = 1×24 + 1×23 + 0×22 + 1×21 + 0×20

= 16 + 8 + 0 + 2 + 0

= 2610


Computer memory is measured in terms of how many bits it can store. Here is a chart for memory capacity conversion.

1 byte (B) = 8 bits 1 Kilobytes (KB) = 1024 bytes 1 Megabyte (MB) = 1024 KB 1 Gigabyte (GB) = 1024 MB 1 Terabyte (TB) = 1024 GB 1 Exabyte (EB) = 1024 PB 1 Zettabyte = 1024 EB 1 Yottabyte (YB) = 1024 ZB

Octal Number System

Octal number system has eight digits – 0, 1, 2, 3, 4, 5, 6 and 7. Octal number system is also a positional value system with where each digit has its value expressed in powers of 8, as shown here −

Decimal equivalent of any octal number is sum of product of each digit with its positional value.

7268 = 7×82 + 2×81 + 6×80

= 448 + 16 + 6

= 47010

Hexadecimal Number System

Octal number system has 16 symbols – 0 to 9 and A to F where A is equal to 10, B is equal to 11 and so on till F. Hexadecimal number system is also a positional value system with where each digit has its value expressed in powers of 16, as shown here −

Decimal equivalent of any hexadecimal number is sum of product of each digit with its positional value.

27FB16 = 2×163 + 7×162 + 15×161 + 10×160

= 8192 + 1792 + 240 +10

= 1023410

Number System Relationship


The following table depicts the relationship between decimal, binary, octal and hexadecimal number systems.

HEXADECIMAL DECIMAL OCTAL BINARY

0 0 0 0000

1 1 1 0001

2 2 2 0010

3 3 3 0011

4 4 4 0100

5 5 5 0101

6 6 6 0110

7 7 7 0111

8 8 10 1000

9 9 11 1001

A 10 12 1010

B 11 13 1011

C 12 14 1100

D 13 15 1101

E 14 16 1110


F 15 17 1111

ASCII

Besides numerical data, computer must be able to handle alphabets, punctuation marks, mathematical operators, special symbols, etc. that form the complete character set of English language. The complete set of characters or symbols are called alphanumeric codes. The complete alphanumeric code typically includes −

26 upper case letters 26 lower case letters 10 digits 7 punctuation marks 20 to 40 special characters

Now a computer understands only numeric values, whatever the number system used. So all characters must have a numeric equivalent called the alphanumeric code. The most widely used alphanumeric code is American Standard Code for Information Interchange (ASCII). ASCII is a 7-bit code that has 128 (27) possible codes.

