Types of Operating Systems

           Operating system can be broadly classified into two categories based on the type of interface provided to the user.

Command Line Interface

         In these types of operating systems the user can enter commands through the keyboard and each command will perform a specific action. But this type of interface is very difficult for a beginner to use, because the commands have to be remembered.

          MS-DOS is such a Operating System. Dos stands for Disk Operating system. It was developed by Microsoft as an operating system in the 1980s. Windows 3x needed DOS to run, but the later versions of Windows do not.

Graphical User Interface

         These operating systems provide the user the ability to manipulate the computer in a more interactive way. These systems are more user-friendly and easier to use than command driven interface, because the commands need not be learnt.

E.g. Windows, MacOS, Linux

Windows 7

MacOS

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Operating System

         An Operating System or OS is a software program that enables the computer hardware to communicate and operate with the computer software. Without an Operating System, a computer would be useless.

Features Expected of an Operating System

• To Startup and Shutdown the computer. 

• To act as an interface between the user and the hardware.

• To perform house keeping tasks such as file handling, disk/ memory management.

• To be able to allow the user to configure the system. This includes changing the types of printers, adding new hardware (such as sound cards), etc.

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The Internet

   The Internet is a network of autonomous computer networks that spans the globe. It provides an infrastructure for global electronic information exchange. Today the Internet is used by millions of people of facilitate various activities such as business, education and plain communication.

   The Internet grew out of a United States Department of Defense project, the ARPANET (Advanced Research Project Agency Network) started in late 1960s. Its main objective was to provide network links between universities, research organizations and remote computer centers.

            The following figure shows how the Internet is structured. At the bottom level are the organizations and home uses connected to the Internet. Typically users connect to the Internet through local Internet Service Providers (ISPs). The ISPs are in turn connected to the Internet Backbone through regional networks. Typically ISPs lease high-bandwidth lines that connect into the Internet and rent that bandwidth to Internet users.

             Typically a home user (client) connects to the Internet via telephone lines. If the telephone network is analog a modem is needed in between the computer and the telephone line to convert digital signals generated by the comuter to analog signals that are being transmitted by the telephone network.

              When a client registers with an ISP for Internet usage he will get an account. To access the Internet by using this account, the client has to dial the ISP and log on to this account.

             On the Internet information is available mainly through three mediums: the World Wide Web (WWW), Usenet News (News) and Electronic Mail (e-mail). The WWW is built on top of the Internet. It can be viewed as a large notice board with posters and printed notices stuck to it. On the WWW Web servers provide information on the Internet. A user can access the information by using a web browser.

• To Connect to the Internet, your dialer software calls an access number.

• Your modem converts the signals from your computer to signals that travel over wire lines to an Internet service provider (IPS).

• Your IPS provides the connection to the Internet.

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How to Protect a Computer from Computer Viruses

        There are many ways of preventing a computer from virus attack. Some of these precautions are listed below.

• Verbal Warning
• Avoiding the use of unchecked disks
• Avoiding the use of unauthorized or unchecked software

Anti-virus Software

        Anti-virus software contains anti-virus scanners, which look for viruses.

• On-access scanners - Automatically check the hard disk and diskette each time the system starts up.

• On-demand scanners - Executes only on users demand and can be scheduled to run at various times

       Most commercial anti-virus software provides both types of scanners. It will take some time for an anti-virus software designer get to know about a new type of virus and to write software to detect it. Therefore anti-virus software gets out of date very often.

Firewalls

        A computer generally has many ports (think of them as doorways), which allow external programs to access the computer. Hackers use programs to automatically detect vulnerabilities in the computer and gain access to the computer by using this weak access points. Firewall protection secures these ports so malicious programs can sneak into your machine. Firewall software can also be used to check whether files downloaded from the internet follow a defined security protocol and free of viruses.

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Main Types of Virus

Boot Virus

          Boot virus infects the boot sector of the hard disk and is activated every time the computer is switched on. Once infected, the computer may fail to start properly. Usually boot viruses spread from the boot sectors in floppy diskettes to the boot sector of the hard disk. When you start up the system with an infected diskette in the diskette drive, the virus transfers from the diskette to the boot sector of the hard disk. Once infected, the virus may remain in the main memory and infect other diskettes.

File Virus (Parasitic Virus)

         File viruses are virus programs attached to executable files. They become active every time the program is executed.

Macro Virus

         A macro is a collection of application specific instructions coded to automate some manual process such as formatting a word document. Macro Viruses are either complete macros or macro segments embedded as parts of legitimate macros. They may come straight from the software manufacturer's original disks. Once a computer is infected with a macro virus the following action may result.

• Unusual messages
• Unknown macros listed in macros list
• Save data in unintended formats
• Lost data

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Main Phases of Virus Activity

The main phases of virus generally follows are;

• Gains access to the system

• Lies dormant

• Propagates

• Virus is triggered by some event

• Virus Action

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Popular Viruses and Worm Programmes

• 2000

           May : The VBS/Loveletter ('ILOVEYOU') worm appeard. As of 2004 this is the most costly virus to business, causing upwards of 10 billion dollars in damage.

 

• 2001

            January : A worm strikingly similar to the Morris worm, names the Ramen worm infected only Red Hat Linux machines running version 6.2 and 7.

 

             July 13 : The Code Red worm attacking Microsoft Internet Information Services.

 

             October 26 : The Klez worm is first identified. 

 

• 2003

             January 24 : The SQL slammer worm also known as the Sapphire worm, attacked vulnerabilities in Microsoft SQL Server and causes widespread problems on the internet.

 

             August 12 : The Blaster worm, also known as the Lovesan worm, spread rapidly by exploiting Microsoft Windows computers.

 

• 2004

             January : MyDoom emerges, and currently holds the record for the fastest-spreading mass mailer worm.

             December : Santy, the first known "webworm" is launched. It infected around 40000 sites before Google filtered the search query by the worm.

 

• 2005

           August 16 : The Zotob worm and several variations of malware exploiting the vulnerability described in MS05-039 are discovered. The effect was overblown because several United States media outlets were infected.

 

• 2006

              January 20 : The Nyxem work discovered. It spread by mass-mailing. It attempts to certain types, such as Microsoft Office files.

 

               February 16 : Discovery of the first-ever virus for Mac OS X, a low-threat worm known as OSX/Leap-A or OSX/Oompa-A, is announced.

 

• 2007

               January 7 :  A worm generated by hackers of the popular website MySpace was discovered by many users on the site.

 

 

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Components of a Virus

A virus comprises of four general components.

• The replicator (or Engine) : Replicates the virus.

• The safeguard : Prevents the detection and the removal of the virus.

• The trigger monitor : Monitors for a specific condition to deliver payload. These specific conditions may be the arrival of a particular date, time, keyboard stroke, etc.

• The payload : the action it does apart from propagation. This action may be an amusing or malicious action resulting in distraction from the current work or destruction of data.

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Computer Viruses

Some of the main reasons for writing viruses are;

• Hacking.
• To prevent copying software.
• Revenge - Employees may plant "Time Bombs" in programs.
• Fraud - Viruses may allow access to an otherwise secure system.
• Political and terrorist motives.
• Commercial sabotage - Damage the reputation of a competitor.
• Warfare - Incorporate a virus to an enemy's computer.

Viruses can be transmitted from one system to another through various media types such as;

• Floppy disks.
• Tape backups.
• Internet.
• Software.

Some of the symptoms that a computer will show up when infected with a virus are listed below.

• Programs take long time to lead than normal.
• The floppy disk drive or hard drive runs when you are not using it.
• New files keep appearing on the system and you do not know where they come from.
• Strange sound or beeping noises come from the computer or keyboard.
• Strange graphics are displayed on your computer monitor.
• Files have strange names you do not recognize.
• Unable to access the hard drive when booting from the floppy drive.
• Program sizes keep changing.
• Conventional memory is less than it used to be and you cannot explain it.

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Network Protocols

           A network protocol is "an agreement on how to converse". The four most commonly used network protocols are TCP/IP, IPX/SPX, X.25 and SNA. Each protocol performs essentially the same functions, but each protocol is incompatible with other unless there is a special device to translate between them.

TCP/IP Network Addressing

              TCP/IP protocol uses 3 different types of addressing to move data between computers: Physical, Logical and Port addresses. The physical addresses (MAC addresses) are utilized to move data within a single LAN, logical addresses (IP addresses) are used to move data from one LAN to another LAN across the network and the port address is used to transmit data to the intended application.

              MAC addresses burned into hardware. IP address is a 32-bit value. This means that there are fore billion different IP address possibilities. IP addresses may be sent in software and are thus flexible. An IP address contains the address of the device itself as well as the address of the network on which the device is located. Therefore, if a device is moved from one network to a different network, the IP address of the device must be changed. IP addresses are hierarchical addresses like phone numbers and zip codes. They provide a better way to organize computer addresses like social security numbers. 

              The International Network Information Center (InterNIC) assigns to companies blocks of IP addresses based on the size of their networks.

How to Determine Your IP Address?

                                                     Go to the Start menu and select Run... Then type "cmd" in the box and click OK. Then type "ipconfig" in the command prompt and press Enter.

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Backbone Network Components

         A backbone network is a network that connects many networks in a single site. There are many backbone network components.

• Router - A router is a special device that enables the connection of two or more networks so that the computers in different networks can communicate with each other. A router may be a special device, a computer with several network interface cards or a special network module in a computer or other network device. Routers also allow a network to be segmented into smaller logical networks or subnets.

• Bridge - A Bridge is an interface that enables similar networks to communicate.

• Gateway - A Gateway is an interface that enables dissimilar networks to communicate. 

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Data Transmission Types

       In a network data can be transmitted in two different ways, serial transmission and in parallel transmission.

• Serial Data Transmission - Bits are transmitted sequentially, one after the other.

• Parallel Data Transmission - Bits are transmitted through separate channels simultaneously.

     The standard unit of measure used to specify the speed of the data flow through a network is bits per second (bps).

Network Interface Card (NIC)

 A network Interface Card provides the physical connection between the computer and a network cable and enables access to a network. Most modern computers come with NIC cards pre-installed, and ready to be connected to networks.

 Types of Networks

             There are two primary types of networks.

• Local Area Networks (LAN) - LAN is a communication network that serves users within a confined geographical area. Private parties generally own LANs and their effective range is limited.

• Wide Area Network (WAN) - WAN is a communications network that has no geographical limit. A WAN may span hundreds or thousands of miles. In general, a WAN is made up of a number of interconnected LANs. WANs can use either analog (telephone lines) or digital signals or a combination of both. WANs may be privately owned by large organizations or may be public.

• Local Area Networks (LAN) - Local area networks are two types: client server and peer- to-peer. 

            - Client Sever

                                   A Client server LAN consists of devices that provide services called servers, and devices that request services from servers, called clients. Typically servers are the devices that manage shared devices such as printers.

           - Pear-to-Peer Networks 

                                         In a peer-to-peer LAN there is no server, instead all devices communicate directly with each other. Peer-to-Peer networks are also called work groups. In a peer-to-peer network there is no hierarchy among the computers. Typically the number of computers in a peer-to-peer network would be less than 10. All the computers in the network are treated equal and are referred to s peers. Each computer may function as a client and a sever and there is no central administrator to manage the entire network. The users at each computer determine what data on his computer should be shared with the other peers.

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Communication Media

           The communication media is the matter or substance that carries the voice or data. Many different types of transmission media are currently in use. All these different media types can be grouped into two categories: Guided media and Radiated media.

Guided Media

          The guided media are those in which the data flows through physical media.

• Twisted-pair Wire - A twisted-pair wire consists of two insulated copper wires, twisted around each other and covered in another layer of plastic insulation.

• Coaxial Cable - A coaxial cable consists of insulated copper wire wrapped in a metal shield, which is then wrapped in an outer external cover. Often many coaxial cables are  bundled together.

• Fiber-Optical Cable - Consists of hundreds of thin glass wires that transmit pulsation beams of light.

Radiated Media

             And the radiated media are those in which the data is broadcast through the air.

• Radio Transmission - Radio transmission use the same basic principle as standard radio transmission. When using radio transmission each device/computer on the network should equipped with a radio transmitter/receiver on a specific frequency that does not interfere with commercial radio stations. The transmitters are very low power and are designed typically to transmit a signal to a very short distance, typically up to 500 feet.

• Infrared Transmission - Infrared transmission uses low-frequency light wave, below the visible spectrum, to transmit data through the air. Infrared transmitters are seldom uses to transmit data to and from portable or handheld computers.

• Microwave Transmission - Microwave is a high-frequency beam with short wave length. Microwave can be transmitted over a direct line-of sight path between any two points. This transmission medium is typically used for long-distance data/voice transmission.

• Satellite Transmission - In satellite transmission, singles are transmitted to a satellite 500 to 22,000 miles in space. One disadvantage of satellite transmission is the delay that occurs in transmission, which is known as the propagation delay.

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Network Topologies

           Networks can be laid out in a number of different ways. The physical layout of a network is called its topology. The basic network topologies are Star, Ring, Bus. These basic topologies can be combined in a variety of ways to build complex hybrid network topologies.

Star Topology

           In a star network all computers and other communications devices are connected to a central point such as a hub, file sever or a host computer.

Ring Topology

            In a ring network all communications devices are connected in a continuous ring. Messages are passed around the ring until they reach the right destination.

Bus Topology

           In a bus network communications devices are connected to a common channel. There is no central computer and the communications devices transmit message to the other devices.

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Data Communications

           Data Communication is the process of moving information from one point to another by means of some transmission system.

Network

          A network is a collection of entities that exchange information or good. Few examples of networks are railway system, nervous system of animals, telephone system.

Communications Network

          A communications network is a system of interconnected computers, and communication devices that can communicate with one another and share resources. At the most elementary level, a computer network consists  of two computers connected with each other by a cable to allow them to share data. A device connected to a network is called a node. A node may be a device such as a computer, a printer, workstation etc.

Advantages of Networks

         Networks offer several advantages over stand along systems. Some of these advantages listed below:

• Sharing of peripheral devices: Expensive resources can be shared by connecting them to the network.

• Sharing of programs and data: Some programs may be expensive, and it may not be possible to purchase multiple copies. Such programs can be shared a network.

• Better communications: Networks can be used to establish communication such as e-mail.

• Access to databases: Users can access numerous databases by making them available on the network.

Common Network Elements

            The different types of components that you can find in a network are:

• Servers: Computers that provide shared resources to the network users.

• Clients: Computers that access shared network resources provided by the users.

• Communication Media: The media through which data is being transmitted.

• Network Operating System: Software that manages the activities of a network.

• Shared resources: Any service or resource made available for use by the members of the network.

• Modem: Used to connect a computer to another computer over phone lines.

• Hub: A hub can be thought of as a junction box, permitting new computers to be connected to a network as easily as plugging a power cord into an electrical socket. Hubs are commonly available in 4, 8, 16 port sizes, enabling anywhere from 4 to 16 network devices to be plugged into a network. There is no need to connect all the ports of a hub to network devices simultaneously. When no cables are plugged in, the signals bypass the unused ports. Some hubs have an additional interface port that connects to another hub, increasing the size of the network.

• Switch: An enhanced version of the hub. Typically a switch can distribute the communication load so all the computers connected to the switch can communicate effectively.

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Application Software

               Application software is designed to perform useful general-purpose tasks. For example, an application program called Solitaire is a single program that lets you play a card game. Application software can be grouped as customized software or as application packages. Customized software is generally designed for a particular customer whereas application packages are "off the shelf" programmes designed for the general public. Common types of application software are:

• Word Processing
• Spreadsheet
• Database Management System
• Presentation Software
• Desktop Publishing and Graphic Design
• Web Design and Development

Device Drivers

               Virtually every hardware component located inside or connected externally to a computer requires associated software component to enable that hardware to communicate and function with the operating system, other applications and other hardware components of the computer. This software component is referred ti as driver software of that hardware component. Until you install the proper software derive for a hardware component that hardware component remains logically isolated from the rest of the components of the computer.



Booting the Computer

            The process of loading the operating system into the computer's main memory from the hard disk, floppy disk or CD is called booting. A programme known as the boot routine or bootstrap loader is in change of the booting process and this boot routine is stored permanently in the computer's electronic circuitry, typical in ROM. Another series of programmes which start up prior to the booting process are the diagnostic routines. These programs test the main memory, the CPU and other parts of the system to make sure that they are running properly.



System Start up Process

            When you switch on a computer it automatically loads a program called the Basic Input / Output System (BIOS). The BIOS is stored on a special chip on the computer's motherboard. As it runs, it performs a number of essential tests on the computer hardware. These tests are called Power On Self Test (POST).



Software Evaluation

              Software evaluation is the process of checking the software for suitability for the required function. the major steps involved in software evaluation are:

• Identity clearly the problem or application for which the software is required.

• Inspect the manual or instructions supplied with the software.

           - Is the manual clearly written? 

           - Does it explain the main functions of the software? 

           - Does it provide examples?

• Input / Output Interfaces

             - Do the input and output of the software meet your requirements?

• Test the system with your own data sets to see its performance.

• Reliability

              - Is the system consistent? 

              - Does it give consistent answers to similar or the same problems?

• User Friedliness

              - User interface is easy to use. 

              - Error message are clear. 

              - Time periods to wait between successive screens are acceptable.

• Adaptability

              - To what extent the original problem be modified without affecting the functionality of the software.

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System Software

Software

          Software refers to instructions which are used by  the computer to perform various tasks. The word programme is synonymous with software. Software is created with programming languages and related utilities. Software is generally classified into two types: System Software and Application Software.

System Software

          System software is responsible for controlling, integrating and managing the individual hardware components of a computer system so that other software and the users of the system see it as a functional unit without having to be concerned with the low-level details of the computer hardware.
System software can be further classified as:

• Operating Systems.
• Utilities and service programs.

Operating Systems (OS)

              The Operating System manages the resources and the basic operations of the computer. Without an operating system the computer would not work. The operating system (or a part of it) is loaded into the main memory when the computer is switched on and handles many tasks, which we are unaware of when we use the computer. A part of the operating system remains in main memory until the computer is turned off.

             There are several operating systems used with personal computers such as Windows 98, Windows XP, Linux and Mac OS. Different systems are designed for different hardware platforms or for different functions.

            Operating systems may provide different types of user interfaces such as Command Line Interface and Graphical User Interface (GUI). In the command line interface, users communicate with the operating system by typing commands using the keyboard. One of the main disadvantages of the command line interface is that the user has to memorize the commands exactly.

               In the graphical user interface, a user invokes a command by means of graphical objects shown on the screen and the system translates the user action to the operating system commands and executes them on behalf of the user. The graphical user interface was first developed for Apple Macintosh Computers.

              The general-purpose operating systems are typically supplied on CD-ROMs and must be installed before using the computer. Some computers such as hand held computers come with pre-installed operating systems on the computers' ROM. Such operating systems provide only limited capabilities and are virtually impossible to be upgraded.


Utilities and Service Programs 

             Another category of software is utility software, which is a collection of useful programs that enhance the capabilities of the operating system. These software programmes are designed to perform various tasks. Some examples of utility software are Norton Utilities and Download Accelerator.

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Different Types of Printers

            Many different types of printers are in use today. Two primary technologies used for printing are impact printing and non-impact printing. Impact Printers use a print head containing a number of metal pins which strike an inked ribbon placed between the print head and the paper. The non-impact printers are much quieter than impact printers as their printing heads do not strike the paper.

The Most Common Printer Types Are:

• Dot matrix printers,
• Ink-jet printers,
• Laser printers. 

Dot Matrix Printers

              These printers work by firing a matrix of tiny pins (which are located in the print head), through a ribbon similar to that found on a typewriter. As the head moves across the paper the correct pins are fired out to hit an inked ribbon and form the shape of the character required. The greater the number of pins, the higher the quality of the print. Dot matrix printers are impact printers and are used to print multipart stationery. So print several copies of a document at the same time you will need to use a dot matrix printer. Such printers are cheap and have the lowest running cost compared to any other type of printer.

Laser Printers

            These non-impact printers offer high-speed printing and an excellent quality of text and graphics. A laser beam is used to from an image on a rotating, charged metal drum. This then picks up toner from the toner cartridge and transfers it onto paper. Very soon after wards heat and pressure are applied so the toner sticks to the paper. Since they are page printers they are very fast. Colour laser printers are now available and are mainly used for desktop publishing. Although they are expensive, they are likely eventually to come into widespread use.

Inkjet Printers

             Inkjet printers can produce high quality text and graphics. They are quieter than dot matrix printers. The technology involves ink flowing through the appropriate nozzles (usually in an array of 64) where it is then heated and a bubble is formed. This expands to release a tiny droplet of ink onto the paper.

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Different Types of Display Screens

CRT Monitor

                  A cathode-ray tube and associated electronics connected to the video output of a computer. These have higher resolution than TVs. Lager monitors with high resolution are used for specialized application such as desktop publishing and CAD.

LCD (Liquid Crystal Display) Monitor

               LCDs are screens made from two glass/plastic plates with liquid in between. LCDs are commonly used for calculators and laptop computers, as they are far flatter than is possible with cathode ray screens used in TVs and Standard computer monitors.

Main difference of LCDs as compared with CRT monitors are:

• Takes very little space.
• Small energy consumption.
• Sharp pictures.
• Fairly expensive.
• Limited viewing angle.
• Slower response time.

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Different Types of Output Devices

             Output devices translate information processed by the computer into a form that either humans or other machines can understand. There are a variety of output devices, which can be connected to a computer. Some of the common output devices are explained in the following sections.

Monitor

            The most common form of output is the computer screen. It is more correctly called 'monitor' and sometimes referred to as the visual display or VDU.
             The quality of a computer monitor is based on the following properties:

• Resolution: Resolution is the number of pixels, which the screen can display. A 'pixel' is the 'picture element' and refers to the smallest area of the screen that the computer can change.

• The number of colours: it can display (dependent on the computer as well)

• Radiation output. 

More About Monitors..

Printer

                                                               More About Printers..

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Different Types of Input Devices

Formatting

            Hard disks and Diskettes must be formatted before saving data on them. Formatting can be considered as an initialization process that prepares the disk or diskette electronically so that it can store data or programs.


Input Devices

            Input devices enable data to be fed into the computer in a form that the computer can use. Input devices are categorized into two types: keyboard entry devices and direct entry devices. A devices through which data can be entered to the computer by pressing keys is called a keyboard entry device whereas all the other types of input devices are called direct entry devices.


Keyboard

The keyboard is the most commonly used input device and has been used since computers were first introduced. They are intelligent devices and contain their own chips. Each key is switch, which closes when that particular key is pressed. The microprocessor scans the keyboard hundreds of times a second to see if a key has been pressed; if it has, a code that corresponds to that key is sent to the Processing Unit. The CPU then translates this code into the ASCII code (the code that computers use to represent characters on the computer keyboard), which is then used by the computer program.


Mouse

A mouse is another popular input device that forms an essential part of a computer system. Its movements on the desktop are translated into digital information, which in turn is fed to the computer, causing the cusor to move on the screen. Underneath the mouse there is a ball which rotates when the mouse is moved by the user and sensors pick up this movement. A mouse usually has two or three buttons, and these are used to make selections on the screen. The Mouse is classified as a pointing device. Joysticks, Touch Screens and Light pens are a few other examples for pointing devices.


          A newer type of mouse is the optical mouse. An optical mouse uses a light-emitting diode (LED), an optical sensor, and a digital signal processing (DSP) in place of the traditional mouse ball and electro-mechanical transducer. Movement is detected by sensing changes in reflected light, rather than by interpreting the motion of a rolling sphere.


Digital Camera

              Digital Cameras store the images digitally rather than on film. Digital still cameras can be connected to a computer and the pictures taken on them can be transferred while digital motion cameras or web cams can be used to create video and for video conferencing purposes.

Microphone

            A microphone can be used to record sound into a computer. Microphones are also used for voice communication through a computer, for example, for making telephone calls through a computer.

Scanners

            Scanners are input devices normally used to scan text or pictures. The scans are then stored in a computer's memory where they can then be accessed and modified using a desktop publishing package, before being printed. Both black and white, and color scanners are available.

            Scanners often come with OCR software. Optical Character Recognition (OCR) is where the text on a page is scanned and then converted and fed into a word processing package so that it can be modified according to the needs of the user. OCR software may not recognize certain characters, be they handwritten, typed fonts or symbols.


Other Devices 

           A variety of devices exist which allow people to input different kinds of information into a computer. Some such devices are:

• Fingerprint Reader - Used to capture a fingerprint of a person into the computer.

• Magnetic Ink Character Recognition (MICR) - Used to recognize characters printed in special ink. e.g. on cheques.

• Environment Sensors - Movement, Temperature, Humidity, etc., can be monitored through a computer using these devices.

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Different Types of Memory

Floppy Disk

           A floppy disk or a diskette is a round flat removable piece of plastic coated with a Ferric Oxide layer, and encased in a protective plastic cover, called the disk jacket. The Ferric Oxide layer is capable of holding a magnetic field. Data is stored on a floppy disk by means of a disk drive, which alters the magnetic orientation of the particles in the Ferric Oxide layer. Reading data from a diskette means that the data stored as a magnetic pattern on a diskette is converted to electronic signals and transmitted to the memory of the computer. Writing data on a diskette means electronic information is transformed into magnetic patterns and recorded magnetically on the diskette.

                                              Evolution of Diskettes

Year Introduced	Physical Size (in inches)	R/W Capability	Maximum Capacity	Number of sides used
1970	8	R	100 Kilobytes	Single
1973	8	R/W	250 Kilobytes	Double
1976	5.25	R/W	100 Kilobytes	Single
	5.25	R/W	1.2 Megabytes	Double
1980	3.5	R/W	1.44 Megabytes	Double

Zip disks and Jaz disks

          These disks are very similar to floppy disks except that they can be used to store large amounts of data. Zip disk capacities from 100 to 750 MB and Jaz disks come in 1 and 2GB capacities. Both these disks are removable and portable and provide much better performance than floppy disks.



Magneto optical disk

           Magneto optical disk drives use a combination of a laser, to heat the surface of the disk, and a magnetic head, to record data. While these disks are high performance, they are expensive. Magneto optical disk have capacities ranging from 128 MB to several Gigabytes.

Magnetic Tape

          Magnetic tapes are similar to audio tapes and store data in a serial manner (i.e. data is stored one after the other on the tape). These tapes can store large amounts of data but are primarily used for backup because the access speeds are slow.



CD/DVD

          A Compact Disk (CD) is a plastic disk where disk is optically 'written' on its surface. The data written on the disk is read using a CD-ROM drive which uses lasers for this purpose. Usually once the data has been 'written' (or 'burnt') on to the CD, it is not possible to change it, hence Read Only Memory. Some CDs are re-writable. To 'write' to a CD, a drive called 'CD Writer' must be used. The  faster the CD ROM drive can read a CD, the faster the computer will be able to get data from the CD. Digital Versatile Discs (DVD0 use a similar concept to CDs except that are able to use multiple surfaces on a single disc and so can be used to store larger amounts of data. A typical CD can store 650 to 700 MB of data while a DVD can store from 2GB to 8GB of data.

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Components of a Computer System

                  By considering the functions performed by various components, a computer can be represented as a collection of logical components as in Figure 1.3. The main hardware components of a modern computer system are the central processing unit (CPU), the main memory, the secondary storage and the In put-Out put devices.

 

System Unit

                  The system unit, or the cabinet houses most of the essential components of the computer system such as the power supply, the motherboard, the CPU chip, specialized chips, the system clock, RAM, ROM, expansion board and bus lines.


Central Processing Unit (CPU)

The central processing unit performs the majority of calculations and controls the operation of a computer. CPUs are rated by the speed at which they can execute instructions. The speed of a CPU is measured in Megahertz (MHz), and is also known as the clock speed. The higher the value of the speed the faster the computer can run programs. The capacity of a Central Processing Unit is expressed in terms of word size. A word is the maximum number of bits that the CPU can manipulate or store at one time.

The CPU consists of two parts: the control unit (CU) and the arithmetic/logic unit (ALU)

Different manufactures are making CPUs today. Some of these popular CPUs available today are the Intel, AMD, Cyrix and Motorola.


Control Unit (CU)

                  The control unit controls and directs the operation of the entire computer system. Although it does perform any actual processing on the data, the control unit acts as a central nervous system for the other components of the computer. It obtains instructions from the program stored in main memory, interprets the instructions, and issues signals, which cause other units in the system to execute them.


Arithmetic-Logic Unit (ALU)

                  The arithmetic-Logic unit performs arithmetic operations such as addition, subtraction, multiplication and division on data. It also performs logical operations, which involve comparison of data. In microcomputers, the entire CPU is typically fabricated on a single chip.


Motherboard

                  The motherboard is the main circuit board inside the computer. It can be considered as the main communication center through which all the components of the computer transmit data back and forth. All the main parts including the central processing unit of the computer are typically plugged in to the mother board. The mother board also provides sockets, called expansion slots, to plug in special electronic circuitboards to enhance the functionality of the computer. These extra circuitboards are called expansion cards, by using which you can customize a computer to suit your needs. One other major function of the mother board is to supply the necessary power to all its expansion cards.

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Data Representation

             Computers use the two-state system to represent data. Such a two state system is referred to as a binary system. In computers these two states are denoted by the digits 0 and 1 and any such value is called a binary digit or bit. In side the computer these two states are typically implemented by using two different voltages.

             The smallest unit of data that can be stored in a computer is a bit. A bit will have the value '0' or '1' . A large collection of such '0's or '1's is what makes up data in a computer.

Smallest Unit           = 1 bit

8 bits                     = 1 byte (B)

1024 bytes             = 1 kilobyte (KB)

1024 kilobytes        = 1 megabyte (MB)

1024 megabyte       = 1 gigabyte (GB)

1024 gigabyte         = 1 terabyte (TB)
(In computer jargon, One Kilo = 1024)

              The capacity of a particular storage device and the space taken up by a file are always measured using these units.

              Letters, numbers and special characters are represented within a computer by means of a binary coded scheme. Three popular binary coding schemes use eight bits to represent characters whereas Unicode uses 16 bits.

Character	ASCII-8	EBCDIC	Character	ASCII-8	EBCDIC
A B C D E F G H I J K L M	0100 0001 0100 0010 0100 0011 0100 0100 0100 0101 0100 0110 0100 0111 0100 1000 0100 1001 0100 1010 0100 1011 0100 1100 0100 1101	1100 0001 1100 0010 1100 0011 1100 0100 1100 0101 1100 0110 1100 0111 1100 1000 1100 1001 1101 0001 1101 0010 1101 0011 1101 0100	N O P Q R S T U V W X Y Z	0100 1110 0100 1111 0101 0000 0101 0001 0101 0010 0101 0011 0101 0100 0101 0101 0101 0110 0101 0111 0101 1000 0101 1001 0101 1010	1101 0101 1101 0110 1101 0111 1101 1000 1101 1001 1110 0010 1110 0011 1110 0100 1110 0101 1110 0110 1110 0111 1110 1000 1110 1001
0 1 2 3 4 !	0011 0000 0011 0001 0011 0010 0011 0011 0011 0100 0010 0001	1111 0000 1111 0001 1111 0010 1111 0011 1111 0100 0101 1010	5 6 7 8 9 ;	0011 0101 0011 0110 0011 0111 0011 1000 0011 1001 0011 1011	1111 0101 1111 0110 1111 0111 1111 1000 1111 1001 0101 1110

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