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What is a computer

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    What is a computer What is a computer Document Transcript

    • Introduction to computersAre you new to computers? Do you wonder what they do and why you would want to use one? Welcome—youre in the right place. This article gives an overview of computers: what they are, the different types, andwhat you can do with them.Generations of ComputersFirst Generation (1940-1956) Vacuum TubesThe first computers used vacuum tubes for circuitry and magnetic drums for memory, and were oftenenormous, taking up entire rooms. They were very expensive to operate and in addition to using a great dealof electricity, generated a lot of heat, which was often the cause of malfunctions.First generation computers relied on machine language, the lowest-level programming language understoodby computers, to perform operations, and they could only solve one problem at a time. Input was based onpunched cards and paper tape, and output was displayed on printouts.The UNIVAC and ENIAC computers are examples of first-generation computing devices. The UNIVACwas the first commercial computer delivered to a business client, the U.S. Census Bureau in 1951.Second Generation (1956-1963) TransistorsTransistors replaced vacuum tubes and ushered in the second generation of computers. The transistor wasinvented in 1947 but did not see widespread use in computers until the late 1950s. The transistor was farsuperior to the vacuum tube, allowing computers to become smaller, faster, cheaper, more energy-efficientand more reliable than their first-generation predecessors. Though the transistor still generated a great dealof heat that subjected the computer to damage, it was a vast improvement over the vacuum tube. Second-generation computers still relied on punched cards for input and printouts for output.Second-generation computers moved from cryptic binary machine language to symbolic, or assembly,languages, which allowed programmers to specify instructions in words. High-level programming languageswere also being developed at this time, such as early versions of COBOL and FORTRAN. These were alsothe first computers that stored their instructions in their memory, which moved from a magnetic drum tomagnetic core technology.
    • The first computers of this generation were developed for the atomic energy industry.Third Generation (1964-1971) Integrated CircuitsThe development of the integrated circuit was the hallmark of the third generation of computers. Transistorswere miniaturized and placed on silicon chips, called semiconductors, which drastically increased the speedand efficiency of computers.Instead of punched cards and printouts, users interacted with third generation computers through keyboardsand monitors and interfaced with an operating system, which allowed the device to run many differentapplications at one time with a central program that monitored the memory. Computers for the first timebecame accessible to a mass audience because they were smaller and cheaper than their predecessors.Fourth Generation (1971-Present) MicroprocessorsThe microprocessor brought the fourth generation of computers, as thousands of integrated circuits werebuilt onto a single silicon chip. What in the first generation filled an entire room could now fit in the palm ofthe hand. The Intel 4004 chip, developed in 1971, located all the components of the computer—from thecentral processing unit and memory to input/output controls—on a single chip.In 1981 IBM introduced its first computer for the home user, and in 1984 Apple introduced the Macintosh.Microprocessors also moved out of the realm of desktop computers and into many areas of life as more andmore everyday products began to use microprocessors.
    • As these small computers became more powerful, they could be linked together to form networks, whicheventually led to the development of the Internet. Fourth generation computers also saw the development ofGUIs, the mouse and handheld devices.Fifth Generation (Present and Beyond) Artificial IntelligenceFifth generation computing devices, based on artificial intelligence, are still in development, though there aresome applications, such as voice recognition, that are being used today. The use of parallel processing andsuperconductors is helping to make artificial intelligence a reality. Quantum computation and molecular andnanotechnology will radically change the face of computers in years to come. The goal of fifth-generationcomputing is to develop devices that respond to natural language input and are capable of learning and self-organization.
    • Definition ComputersI, Computer: DefinitionA computer is a machine that can be programmed to manipulate symbols. Its principal characteristics are: It responds to a specific set of instructions in a well-defined manner. It can execute a prerecorded list of instructions (a program). It can quickly store and retrieve large amounts of data.Therefore computers can perform complex and repetitive procedures quickly, precisely and reliably. Moderncomputers are electronic and digital. The actual machinery (wires, transistors, and circuits) is calledhardware; the instructions and data are called software. All general-purpose computers require the followinghardware components: Central processing unit (CPU): The heart of the computer, this is the component that actually executes instructions organized in programs ("software") which tell the computer what to do. Memory (fast, expensive, short-term memory): Enables a computer to store, at least temporarily, data, programs, and intermediate results. Mass storage device (slower, cheaper, long-term memory): Allows a computer to permanently retain large amounts of data and programs between jobs. Common mass storage devices include disk drives and tape drives. Input device: Usually a keyboard and mouse, the input device is the conduit through which data and instructions enter a computer. Output device: A display screen, printer, or other device that lets you see what the computer has accomplished.In addition to these components, many others make it possible for the basic components to work togetherefficiently. For example, every computer requires a bus that transmits data from one part of the computer toanother.II, Computer sizes and power Personal Computers Workstations Minicomputer Mainframes Supercomputers s Least powerful Most powerfulComputers can be generally classified by size and power as follows, though there is considerable overlap: Personal computer: A small, single-user computer based on a microprocessor. Workstation: A powerful, single-user computer. A workstation is like a personal computer, but it has a more powerful microprocessor and, in general, a higher-quality monitor. Minicomputer: A multi-user computer capable of supporting up to hundreds of users simultaneously. Mainframe: A powerful multi-user computer capable of supporting many hundreds or thousands of users simultaneously. Supercomputer: An extremely fast computer that can perform hundreds of millions of instructions per second
    • Supercomputer and MainframeSupercomputer is a broad term for one of the fastest computers currently available. Supercomputers are veryexpensive and are employed for specialized applications that require immense amounts of mathematicalcalculations (number crunching). For example, weather forecasting requires a supercomputer. Other uses ofsupercomputers scientific simulations, (animated) graphics, fluid dynamic calculations, nuclear energyresearch, electronic design, and analysis of geological data (e.g. in petrochemical prospecting). Perhaps thebest known supercomputer manufacturer is Cray Research.Mainframe was a term originally referring to the cabinet containing the central processor unit or "mainframe" of a room-filling Stone Age batch machine. After the emergence of smaller "minicomputer" designsin the early 1970s, the traditional big iron machines were described as "mainframe computers" andeventually just as mainframes. Nowadays a Mainframe is a very large and expensive computer capable ofsupporting hundreds, or even thousands, of users simultaneously. The chief difference between asupercomputer and a mainframe is that a supercomputer channels all its power into executing a fewprograms as fast as possible, whereas a mainframe uses its power to execute many programs concurrently. Insome ways, mainframes are more powerful than supercomputers because they support more simultaneousprograms. But supercomputers can execute a single program faster than a mainframe. The distinctionbetween small mainframes and minicomputers is vague, depending really on how the manufacturer wants tomarket its machines.MinicomputerIt is a midsize computer. In the past decade, the distinction between large minicomputers and smallmainframes has blurred, however, as has the distinction between small minicomputers and workstations. Butin general, a minicomputer is a multiprocessing system capable of supporting from up to 200 userssimultaneously.WorkstationIt is a type of computer used for engineering applications (CAD/CAM), desktop publishing, softwaredevelopment, and other types of applications that require a moderate amount of computing power andrelatively high quality graphics capabilities. Workstations generally come with a large, high-resolutiongraphics screen, at large amount of RAM, built-in network support, and a graphical user interface. Mostworkstations also have a mass storage device such as a disk drive, but a special type of workstation, called adiskless workstation, comes without a disk drive. The most common operating systems for workstations areUNIX and Windows NT. Like personal computers, most workstations are single-user computers. However,workstations are typically linked together to form a local-area network, although they can also be used asstand-alone systems.N.B.: In networking, workstation refers to any computer connected to a local-area network. It could be aworkstation or a personal computer.
    • Personal computer:It can be defined as a small, relatively inexpensive computer designed for an individual user. In price,personal computers range anywhere from a few hundred pounds to over five thousand pounds. All are basedon the microprocessor technology that enables manufacturers to put an entire CPU on one chip. Businessesuse personal computers for word processing, accounting, desktop publishing, and for running spreadsheetand database management applications. At home, the most popular use for personal computers is for playinggames and recently for surfing the Internet.Personal computers first appeared in the late 1970s. One of the first and most popular personal computerswas the Apple II, introduced in 1977 by Apple Computer. During the late 1970s and early 1980s, newmodels and competing operating systems seemed to appear daily. Then, in 1981, IBM entered the fray withits first personal computer, known as the IBM PC. The IBM PC quickly became the personal computer ofchoice, and most other personal computer manufacturers fell by the wayside. P.C. is short for personalcomputer or IBM PC. One of the few companies to survive IBMs onslaught was Apple Computer, whichremains a major player in the personal computer marketplace. Other companies adjusted to IBMsdominance by building IBM clones, computers that were internally almost the same as the IBM PC, but thatcost less. Because IBM clones used the same microprocessors as IBM PCs, they were capable of running thesame software. Over the years, IBM has lost much of its influence in directing the evolution of PCs.Therefore after the release of the first PC by IBM the term PC increasingly came to mean IBM or IBM-compatible personal computers, to the exclusion of other types of personal computers, such as Macintoshes.In recent years, the term PC has become more and more difficult to pin down. In general, though, it appliesto any personal computer based on an Intel microprocessor, or on an Intel-compatible microprocessor. Fornearly every other component, including the operating system, there are several options, all of which fallunder the rubric of PCToday, the world of personal computers is basically divided between Apple Macintoshes and PCs. Theprincipal characteristics of personal computers are that they are single-user systems and are based onmicroprocessors. However, although personal computers are designed as single-user systems, it is commonto link them together to form a network. In terms of power, there is great variety. At the high end, thedistinction between personal computers and workstations has faded. High-end models of the Macintosh andPC offer the same computing power and graphics capability as low-end workstations by Sun Microsystems,Hewlett-Packard, and DEC.III, Personal Computer TypesActual personal computers can be generally classified by size and chassis / case. The chassis or case is themetal frame that serves as the structural support for electronic components. Every computer system requiresat least one chassis to house the circuit boards and wiring. The chassis also contains slots for expansionboards. If you want to insert more boards than there are slots, you will need an expansion chassis, whichprovides additional slots. There are two basic flavors of chassis designs–desktop models and tower models–but there are many variations on these two basic types. Then come the portable computers that are computerssmall enough to carry. Portable computers include notebook and subnotebook computers, hand-heldcomputers, palmtops, and PDAs.Tower modelThe term refers to a computer in which the power supply, motherboard, and mass storage devices are stackedon top of each other in a cabinet. This is in contrast to desktop models, in which these components arehoused in a more compact box. The main advantage of tower models is that there are fewer spaceconstraints, which makes installation of additional storage devices easier.
    • Desktop modelA computer designed to fit comfortably on top of a desk, typically with the monitor sitting on topof the computer. Desktop model computers are broad and low, whereas tower model computersare narrow and tall. Because of their shape, desktop model computers are generally limited tothree internal mass storage devices. Desktop models designed to be very small are sometimesreferred to as slimline models.Notebook computerAn extremely lightweight personal computer. Notebook computers typically weigh less than 6pounds and are small enough to fit easily in a briefcase. Aside from size, the principal differencebetween a notebook computer and a personal computer is the display screen. Notebookcomputers use a variety of techniques, known as flat-panel technologies, to produce alightweight and non-bulky display screen. The quality of notebook display screens variesconsiderably. In terms of computing power, modern notebook computers are nearly equivalent topersonal computers. They have the same CPUs, memory capacity, and disk drives. However, allthis power in a small package is expensive. Notebook computers cost about twice as much asequivalent regular-sized computers. Notebook computers come with battery packs that enableyou to run them without plugging them in. However, the batteries need to be recharged everyfew hours.Laptop computerA small, portable computer -- small enough that it can sit on your lap. Nowadays, laptopcomputers are more frequently called notebook computers.Subnotebook computerA portable computer that is slightly lighter and smaller than a full-sized notebook computer.Typically, subnotebook computers have a smaller keyboard and screen, but are otherwiseequivalent to notebook computers.Hand-held computerA portable computer that is small enough to be held in one’s hand. Although extremelyconvenient to carry, handheld computers have not replaced notebook computers because of theirsmall keyboards and screens. The most popular hand-held computers are those that arespecifically designed to provide PIM (personal information manager) functions, such as acalendar and address book. Some manufacturers are trying to solve the small keyboard problemby replacing the keyboard with an electronic pen. However, these pen-based devices rely onhandwriting recognition technologies, which are still in their infancy. Hand-held computers arealso called PDAs, palmtops and pocket computers.PalmtopA small computer that literally fits in your palm. Compared to full-size computers, palmtops areseverely limited, but they are practical for certain functions such as phone books and calendars.Palmtops that use a pen rather than a keyboard for input are often called hand-held computers orPDAs. Because of their small size, most palmtop computers do not include disk drives.However, many contain PCMCIA slots in which you can insert disk drives, modems, memory,and other devices. Palmtops are also called PDAs, hand-held computers and pocket computers.
    • PDAShort for personal digital assistant, a handheld device that combines computing, telephone/fax,and networking features. A typical PDA can function as a cellular phone, fax sender, andpersonal organizer. Unlike portable computers, most PDAs are pen-based, using a stylus ratherthan a keyboard for input. This means that they also incorporate handwriting recognitionfeatures. Some PDAs can also react to voice input by using voice recognition technologies. Thefield of PDA was pioneered by Apple Computer, which introduced the Newton MessagePad in1993. Shortly thereafter, several other manufacturers offered similar products. To date, PDAshave had only modest success in the marketplace, due to their high price tags and limitedapplications. However, many experts believe that PDAs will eventually become commongadgets.PDAs are also called palmtops, hand-held computers and pocket computers.What is SoftwareSoftware refers to the instructions, or programs, that tell the hardware what to do. A wordprocessing program that you can use to write letters on your computer is a type of software. Theoperating system (OS) is software that manages your computer and the devices connected to it.Two well-known operating systems are Windows and Macintosh operating system. Yourcomputer uses the Windows operating system.Computers SimplifiedFor beginning computer users, the computer aisles at an electronics store can be quite a mystery,not to mention overwhelming. However, computers really arent that mysterious. All types ofcomputers consist of two basic parts: Hardware is any part of your computer that has a physical structure, such as the computer monitor or keyboard. Software is any set of instructions that tells the hardware what to do. It is what guides the hardware and tells it how to accomplish each task. Some examples of software are web browsers, games, and word processors such as Microsoft Word.A motherboard (hardware)
    • Anything you buy for your computer can be classified as either hardware or software. Once youlearn more about these items, computers are actually very straightforward.The first electronic computer, the Electronic Numerical Integrator and Computer (ENIAC), wasdeveloped in 1946. It took up 1,800 square feet and weighed 30 tons.What are the Different Types of Computers?When most people hear the word "computer" they think of a personal computer such as adesktop or laptop computer. However, computers come in many shapes and sizes, and theyperform many different functions in our daily lives. When you withdraw cash from an ATM,scan groceries at the store, or use a calculator, youre using a type of computer.Desktop ComputersMany people use desktop computers at work, home, school, or the library. They can be small,medium, or large in style, and usually sit on a desk. Once you add a monitor, mouse, and akeyboard, you have what is typically known as a desktop computer.Most desktop computers are easy to upgrade and expand, or add new parts. Another benefit ofdesktop computers is the cost. If you compare a desktop and a laptop with the same features,you will most likely find that the desktop computer is priced lower.Some desktop computers have a built-in monitor to save space. These are often called all-in-onedesktop computers.Laptop ComputersThe second type of computer that you may be familiar with is a laptop computer, or laptops asthey are often referred to. Laptops are battery or AC-powered personal computers that are moreportable than desktop computers, allowing you to use them almost anywhere.Since a laptop is smaller than a desktop, its more difficult to access the internal components.That means you may not be able to upgrade them as much as a desktop. However, its usuallypossible to add more RAM or a bigger hard drive.Other Types of ComputersToday, there lots of everyday devices that are basically specialized computers, even though wedont always think of them as computers. Here are a few common examples: Tablet Computers: These use a touch-sensitive screen for typing and navigation. Since they dont require a keyboard or mouse, tablet computers are even more portable than laptops. The iPad is an example of a tablet computer. Mobile Phones: Many mobile phones can do a lot of things a computer can do, such as browsing the internet or playing games. These phones are often called smartphones.
    • Game Consoles: A game console is a specialized kind of computer that is used for playing video games. Although they are not as fully-featured as a desktop computer, many newer consoles, such as the Nintendo Wii, allow you to do non-gaming tasks like browsing the internet.What is Hardware?Your PC (Personal Computer) is a system, consisting of many components. Some of thosecomponents, like Windows XP, and all your other programs, are software. The stuff you canactually see and touch, and would likely break if you threw it out a fifth-story window, ishardware.Not everybody has exactly the same hardware. But those of you who have a desktop system, likethe example shown in Figure 1, probably have most of the components shown in that samefigure. Those of you with notebook computers probably have most of the same components.Only in your case the components are all integrated into a single book-sized portable unit.The system unit is the actual computer; everything else is called a peripheral device. Yourcomputers system unit probably has at least one floppy disk drive, and one CD or DVD drive,into which you can insert floppy disks and CDs. Theres another disk drive, called the hard diskinside the system unit, as shown in Figure 2. You cant remove that disk, or even see it. But itsthere. And everything thats currently "in your computer" is actually stored on that hard disk.(We know this because there is no place else inside the computer where you can storeinformation!).The floppy drive and CD drive are often referred to as drives with removable media orremovable drives for short, because you can remove whatever disk is currently in the drive, and
    • replace it with another. Your computers hard disk can store as much information as tens ofthousands of floppy disks, so dont worry about running out of space on your hard disk any timesoon. As a rule, you want to store everything you create or download on your hard disk. Use thefloppy disks and CDs to send copies of files through the mail, or to make backup copies ofimportant items.Random Access Memory (RAM)Theres too much "stuff" on your computers hard disk to use it all at the same time. During theaverage session sitting at the computer, youll probably use only a small amount of all thatsavailable. The stuff youre working with at any given moment is stored in random accessmemory (often abbreviated RAM, and often called simply "memory"). The advantage usingRAM to store whatever youre working on at the moment is that RAM is very fast. Much fasterthan any disk. For you, "fast" translates to less time waiting and more time being productive.So if RAM is so fast, why not put everything in it? Why have a hard disk at all? The answer tothat lies in the fact that RAM is volatile. As soon as the computer is shut off, whetherintentionally or by an accidental power outage, every thing in RAM disappears, just as quickly asa light bulb goes out when the plug is pulled. So you dont want to rely on RAM to holdeverything. A disk, on the other hand, holds its information whether the power is on or off.The Hard DiskAll of the information thats "in your computer", so to speak, is stored on your computers harddisk. You never see that actual hard disk because its sealed inside a special housing and needs tostay that way. Unlike RAM, which is volatile, the hard disk can hold information forever -- withor without electricity. Most modern hard disks have tens of billions of bytes of storage space onthem. Which, in English, means that you can create, save, and download files for months oryears without using up all the storage space it provides.In the unlikely event that you do manage to fill up your hard disk, Windows will start showing alittle message on the screen that reads "You are running low on disk space" well in advance ofany problems. In fact, if that message appears, it wont until youre down to about 800 MB offree space. And 800 MB of empty space is equal to about 600 blank floppy disks. Thats stillplenty of room!The MouseObviously you know how to use your mouse, since you must have used it to get here. But letstake a look at the facts and buzzwords anyway. Your mouse probably has at least two buttons onit. The button on the left is called the primary mouse button, the button on the right is called thesecondary mouse button or just the right mouse button. Ill just refer to them as the left and rightmouse buttons. Many mice have a small wheel between the two mouse buttons, as illustrated inFigure 3.
    • The idea is to rest your hand comfortably on the mouse, withyour index finger touching (but not pressing on) the left mousebutton. Then, as you move the mouse, the mouse pointer (thelittle arrow on the screen) moves in the same direction. Whenmoving the mouse, try to keep the buttons aimed toward themonitor -- dont "twist" the mouse as that just makes it all theharder to control the position of the mouse pointer.If you find yourself reaching too far to get the mouse pointer where you want it to be on thescreen, just pick up the mouse, move it to where its comfortable to hold it, and place it backdown on the mousepad or desk. The buzzwords that describe how you use the mouse are asfollows: Point: To point to an item means to move the mouse pointer so that its touching the item. Click: Point to the item, then tap (press and release) the left mouse button. Double-click: Point to the item, and tap the left mouse button twice in rapid succession - click-click as fast as you can. Right-click: Point to the item, then tap the mouse button on the right. Drag: Point to an item, then hold down the left mouse button as you move the mouse. To drop the item, release the left mouse button. Right-drag: Point to an item, then hold down the right mouse button as you move the mouse. To drop the item, release the right mouse button.The KeyboardLike the mouse, the keyboard is a means of interacting with your computer. You really only needto use the keyboard when youre typing text. Most of the keys on the keyboard are laid out likethe keys on a typewriter. But there are some special keys like Esc (Escape), Ctrl (Control), andAlt (Alternate). There are also some keys across the top of the keyboard labeled F1, F2, F3, andso forth. Those are called the function keys, and the exact role they play depends on whichprogram you happen to be using at the moment.Most keyboards also have a numeric keypad with the keys laid out like the keys on a typicaladding machine. If youre accustomed to using an adding machine, you might want to use thenumeric keypad, rather than the numbers across the top of the keyboard, to type numbers. Itdoesnt really matter which keys you use. The numeric keypad is just there as a convenience topeople who are accustomed to adding machines.
    • Most keyboards also contain a set of navigation keys. You can use the navigation keys to movearound around through text on the screen. The navigation keys wont move the mouse pointer.Only the mouse moves the mouse pointer.On smaller keyboards where space is limited, such as on a notebook computer, the navigationkeys and numeric keypad might be one in the same. There will be a Num Lock key on thekeypad. When the Num Lock key is "on", the numeric keypad keys type numbers. When theNum Lock key is "off", the navigation keys come into play. The Num Lock key acts as a toggle.Which is to say, when you tap it, it switches to the opposite state. For example, if Num Lock ison, tapping that key turns it off. If Num Lock is off, tapping that key turns Num Lock on.ComputerA programmable machine. The two principal characteristics of a computer are: It responds to a specific set of instructions in a well-defined manner. It can execute a prerecorded list of instructions (a program).Modern Computers:Modern computers are electronic and digital. The actual machinery -- wires, transistors, andcircuits -- is called hardware; the instructions and data are called software.All general-purpose computers require the following hardware components:
    • memory : Enables a computer to store, at least temporarily, data and programs. mass storage device : Allows a computer to permanently retain large amounts of data. Common mass storage devices include disk drives and tape drives. input device : Usually a keyboard and mouse, the input device is the conduit through which data and instructions enter a computer. output device : A display screen, printer, or other device that lets you see what the computer has accomplished. central processing unit (CPU): The heart of the computer, this is the component that actually executes instructions.In addition to these components, many others make it possible for the basic components to worktogether efficiently. For example, every computer requires a bus that transmits data from one partof the computer to another.Computer Classification:Computers can be generally classified by size and power as follows, though there is considerableoverlap: personal computer : A small, single-user computer based on a microprocessor. In addition to the microprocessor, a personal computer has a keyboard for entering data, a monitor for displaying information, and a storage device for saving data. workstation : A powerful, single-user computer. A workstation is like a personal computer, but it has a more powerful microprocessor and a higher-quality monitor. minicomputer : A multi-user computer capable of supporting from 10 to hundreds of users simultaneously. mainframe : A powerful multi-user computer capable of supporting many hundreds or thousands of users simultaneously. supercomputer : An extremely fast computer that can perform hundreds of millions of instructions per second.Operating SystemAn operating system (OS) is a set of programs that manage computer hardware resources and providecommon services for application software. The operating system is the most important type of systemsoftware in a computer system. A user cannot run an application program on the computer without anoperating system, unless the application program is self booting.
    • Time-sharing operating systems schedule tasks for efficient use of the system and may alsoinclude accounting for cost allocation of processor time, mass storage, printing, and otherresources.For hardware functions such as input and output and memory allocation, the operating systemacts as an intermediary between application programs and the computer hardware,[1][2] althoughthe application code is usually executed directly by the hardware and will frequently call the OSor be interrupted by it. Operating systems are found on almost any device that contains acomputer—from cellular phones and video game consoles to supercomputers and web servers.Types Of Operating SystemReal-time A real-time operating system is a multitasking operating system that aims at executing real-time applications. Real-time operating systems often use specialized scheduling algorithms so that they can achieve a deterministic nature of behavior. The main objective of real-time operating systems is their quick and predictable response to events. They have an event-driven or time-sharing design and often aspects of both. An event- driven system switches between tasks based on their priorities or external events while time-sharing operating systems switch tasks based on clock interrupts.Multi-user vs. Single-user A multi-user operating system allows multiple users to access a computer system concurrently. Time-sharing system can be classified as multi-user systems as they enable a multiple user access to a computer through the sharing of time. Single-user operating systems, as opposed to a multi-user operating system, are usable by a single user at a time. Being able to have multiple accounts on a Windows operating system does not make it a multi-user system. Rather, only the network administrator is the real user. But for a Unix-like operating system, it is possible for two users to login at a time and this capability of the OS makes it a multi-user operating system.Multi-tasking vs. Single-tasking When only a single program is allowed to run at a time, the system is grouped under a single-tasking system. However, when the operating system allows the execution of multiple tasks at one time, it is classified as a multi-tasking operating system. Multi- tasking can be of two types: pre-emptive or co-operative. In pre-emptive multitasking, the operating system slices the CPU time and dedicates one slot to each of the programs. Unix-like operating systems such as Solaris and Linux support pre-emptive multitasking, as does AmigaOS. Cooperative multitasking is achieved by relying on each process to give time to the other processes in a defined manner. MS Windows prior to Windows 2000 and Mac OS prior to OS X used to support cooperative multitasking.Distributed A distributed operating system manages a group of independent computers and makes them appear to be a single computer. The development of networked computers that could be linked and communicate with each other gave rise to distributed computing. Distributed computations are carried out on more than one machine. When computers in a group work in cooperation, they make a distributed system.Embedded Embedded operating systems are designed to be used in embedded computer systems. They are designed to operate on small machines like PDAs with less autonomy. They are
    • able to operate with a limited number of resources. They are very compact and extremely efficient by design. Windows CE and Minix 3 are some examples of embedded operating systems.Summary Of Operating SystemEarly computers were built to perform a series of single tasks, like a calculator. Operatingsystems did not exist in their modern and more complex forms until the early 1960s. [4] Someoperating system features were developed in the 1950s, such as resident monitor programs thatcould automatically run different application programs in succession to speed up processing.Hardware features were added that enabled use of runtime libraries, interrupts, and parallelprocessing. When personal computers by companies such as Apple Inc., Atari, IBM and Amigabecame popular in the 1980s, vendors added operating system features that had previouslybecome widely used on mainframe and mini computers. Later, many features such as graphicaluser interface were developed specifically for personal computer operating systems.An operating system consists of many parts. One of the most important components is the kernel,which controls low-level processes that the average user usually cannot see: it controls howmemory is read and written, the order in which processes are executed, how information isreceived and sent by devices like the monitor, keyboard and mouse, and decides how to interpretinformation received from networks. The user interface is a component that interacts with thecomputer user directly, allowing them to control and use programs. The user interface may begraphical with icons and a desktop, or textual, with a command line. Application programminginterfaces provide services and code libraries that let applications developers write modular codereusing well defined programming sequences in user space libraries or in the operating systemitself. Which features are considered part of the operating system is defined differently in variousoperating systems. For example, Microsoft Windows considers its user interface to be part of theoperating system, while many versions of Linux do not.History Of Operating SystemIn the 1940s, the earliest electronic digital systems had no operating systems. Electronic systemsof this time were so primitive compared to those of today that instructions were often enteredinto the system one bit at a time on rows of mechanical switches or by jumper wires on plugboards. These were special-purpose systems that, for example, generated ballistics tables for themilitary or controlled the printing of payroll checks from data on punched paper cards. Afterprogrammable general purpose computers were invented, machine languages (consisting ofstrings of the binary digits 0 and 1 on punched paper tape) were introduced that speed up theprogramming process (Stern, 1981).
    • OS/360 was used on most IBM mainframe computers beginning in 1966, including thecomputers that helped NASA put a man on the moon.In the early 1950s, a computer could execute only one program at a time. Each user had sole useof the computer for a limited period of time and would arrive at a scheduled time with programand data on punched paper cards and/or punched tape. The program would be loaded into themachine, and the machine would be set to work until the program completed or crashed.Programs could generally be debugged via a front panel using toggle switches and panel lights. Itis said that Alan Turing was a master of this on the early Manchester Mark 1 machine, and hewas already deriving the primitive conception of an operating system from the principles of theUniversal Turing machine.[citation needed]Later machines came with libraries of software, which would be linked to a users program toassist in operations such as input and output and generating computer code from human-readablesymbolic code. This was the genesis of the modern-day operating system. However, machinesstill ran a single job at a time. At Cambridge University in England the job queue was at one timea washing line from which tapes were hung with different colored clothes-pegs to indicate job-priority.MainframesHistory of IBM mainframe operating systemsThrough the 1950s, many major features were pioneered in the field of operating systems,including batch processing, input/output interrupt, buffering, multitasking, spooling, runtimelibraries, link-loading, and programs for sorting records in files. These features were included ornot included in application software at the option of application programmers, rather than in aseparate operating system used by all applications. In 1959 the SHARE Operating System wasreleased as an integrated utility for the IBM 704, and later in the 709 and 7090 mainframes,although it was quickly supplanted by IBSYS/IBJOB on the 709, 7090 and 7094.During the 1960s, IBMs OS/360 introduced the concept of a single OS spanning an entireproduct line, which was crucial for the success of the System/360 machines. IBMs currentmainframe operating systems are distant descendants of this original system and applicationswritten for OS/360 can still be run on modern machines.[citation needed]OS/360 also pioneered the concept that the operating system keeps track of all of the systemresources that are used, including program and data space allocation in main memory and filespace in secondary storage, and file locking during update. When the process is terminated forany reason, all of these resources are re-claimed by the operating system.The alternative CP-67 system for the S/360-67 started a whole line of IBM operating systemsfocused on the concept of virtual machines. Other operating systems used on IBM S/360 seriesmainframes included systems developed by IBM: COS/360 (Compatabililty Operating System),DOS/360 (Disk Operating System), TSS/360 (Time Sharing System), TOS/360 (Tape OperatingSystem), BOS/360 (Basic Operating System), and ACP (Airline Control Program), as well as afew non-IBM systems: MTS (Michigan Terminal System), MUSIC (Multi-User System forInteractive Computing), and ORVYL (Stanford Timesharing System).
    • Control Data Corporation developed the SCOPE operating system in the 1960s, for batchprocessing. In cooperation with the University of Minnesota, the Kronos and later the NOSoperating systems were developed during the 1970s, which supported simultaneous batch andtimesharing use. Like many commercial timesharing systems, its interface was an extension ofthe Dartmouth BASIC operating systems, one of the pioneering efforts in timesharing andprogramming languages. In the late 1970s, Control Data and the University of Illinois developedthe PLATO operating system, which used plasma panel displays and long-distance time sharingnetworks. Plato was remarkably innovative for its time, featuring real-time chat, and multi-usergraphical games. Burroughs Corporation introduced the B5000 in 1961 with the MCP, (MasterControl Program) operating system. The B5000 was a stack machine designed to exclusivelysupport high-level languages with no machine language or assembler, and indeed the MCP wasthe first OS to be written exclusively in a high-level language – ESPOL, a dialect of ALGOL.MCP also introduced many other ground-breaking innovations, such as being the firstcommercial implementation of virtual memory. During development of the AS400, IBM madean approach to Burroughs to licence MCP to run on the AS400 hardware. This proposal wasdeclined by Burroughs management to protect its existing hardware production. MCP is still inuse today in the Unisys ClearPath/MCP line of computers.UNIVAC, the first commercial computer manufacturer, produced a series of EXEC operatingsystems. Like all early main-frame systems, this was a batch-oriented system that managedmagnetic drums, disks, card readers and line printers. In the 1970s, UNIVAC produced the Real-Time Basic (RTB) system to support large-scale time sharing, also patterned after the DartmouthBC system.General Electric and MIT developed General Electric Comprehensive Operating Supervisor(GECOS), which introduced the concept of ringed security privilege levels. After acquisition byHoneywell it was renamed to General Comprehensive Operating System (GCOS).Digital Equipment Corporation developed many operating systems for its various computer lines,including TOPS-10 and TOPS-20 time sharing systems for the 36-bit PDP-10 class systems.Prior to the widespread use of UNIX, TOPS-10 was a particularly popular system in universities,and in the early ARPANET community.In the late 1960s through the late 1970s, several hardware capabilities evolved that allowedsimilar or ported software to run on more than one system. Early systems had utilizedmicroprogramming to implement features on their systems in order to permit differentunderlying architecture to appear to be the same as others in a series. In fact most 360s after the360/40 (except the 360/165 and 360/168) were microprogrammed implementations. But soonother means of achieving application compatibility were proven to be more significant.The enormous investment in software for these systems made since 1960s caused most of theoriginal computer manufacturers to continue to develop compatible operating systems along withthe hardware. The notable supported mainframe operating systems include:MicrocomputersThe first microcomputers did not have the capacity or need for the elaborate operating systemsthat had been developed for mainframes and minis; minimalistic operating systems were
    • developed, often loaded from ROM and known as monitors. One notable early disk-basedoperating system was CP/M, which was supported on many early microcomputers and wasclosely imitated by Microsofts MS-DOS, which became wildly popular as the operating systemchosen for the IBM PC (IBMs version of it was called IBM DOS or PC DOS). In the 80s, AppleComputer Inc. (now Apple Inc.) abandoned its popular Apple II series of microcomputers tointroduce the Apple Macintosh computer with an innovative Graphical User Interface (GUI) tothe Mac OS.ComponentsThe components of an operating system all exist in order to make the different parts of acomputer work together. All software—from financial databases to film editors—needs to gothrough the operating system in order to use any of the hardware, whether it be as simple as amouse or keyboard or complex as an Internet connection.InterruptsInterrupts are central to operating systems, as they provide an efficient way for the operatingsystem to interact with and react to its environment. The alternative — having the operatingsystem "watch" the various sources of input for events (polling) that require action — can befound in older systems with very small stacks (50 or 60 bytes) but are unusual in modern systemswith large stacks. Interrupt-based programming is directly supported by most modern CPUs.Interrupts provide a computer with a way of automatically saving local register contexts, andrunning specific code in response to events. Even very basic computers support hardwareinterrupts, and allow the programmer to specify code which may be run when that event takesplace.When an interrupt is received, the computers hardware automatically suspends whateverprogram is currently running, saves its status, and runs computer code previously associated withthe interrupt; this is analogous to placing a bookmark in a book in response to a phone call. Inmodern operating systems, interrupts are handled by the operating systems kernel. Interruptsmay come from either the computers hardware or from the running program.ModesModern CPUs support multiple modes of operation. CPUs with this capability use at least twomodes: protected mode and supervisor mode. The supervisor mode is used by the operatingsystems kernel for low level tasks that need unrestricted access to hardware, such as controllinghow memory is written and erased, and communication with devices like graphics cards.Protected mode, in contrast, is used for almost everything else. Applications operate withinprotected mode, and can only use hardware by communicating with the kernel, which controlseverything in supervisor mode. CPUs might have other modes similar to protected mode as well,such as the virtual modes in order to emulate older processor types, such as 16-bit processors ona 32-bit one, or 32-bit processors on a 64-bit one.When a computer first starts up, it is automatically running in supervisor mode. The first fewprograms to run on the computer, being the BIOS or EFI, bootloader, and the operating systemhave unlimited access to hardware - and this is required because, by definition, initializing a
    • protected environment can only be done outside of one. However, when the operating systempasses control to another program, it can place the CPU into protected mode.In protected mode, programs may have access to a more limited set of the CPUs instructions. Auser program may leave protected mode only by triggering an interrupt, causing control to bepassed back to the kernel. In this way the operating system can maintain exclusive control overthings like access to hardware and memory.The term "protected mode resource" generally refers to one or more CPU registers, whichcontain information that the running program isnt allowed to alter. Attempts to alter theseresources generally causes a switch to supervisor mode, where the operating system can dealwith the illegal operation the program was attempting (for example, by killing the program).Memory managementAmong other things, a multiprogramming operating system kernel must be responsible formanaging all system memory which is currently in use by programs. This ensures that a programdoes not interfere with memory already in use by another program. Since programs time share,each program must have independent access to memory.Cooperative memory management, used by many early operating systems, assumes that allprograms make voluntary use of the kernels memory manager, and do not exceed their allocatedmemory. This system of memory management is almost never seen any more, since programsoften contain bugs which can cause them to exceed their allocated memory. If a program fails, itmay cause memory used by one or more other programs to be affected or overwritten. Maliciousprograms or viruses may purposefully alter another programs memory, or may affect theoperation of the operating system itself. With cooperative memory management, it takes onlyone misbehaved program to crash the system.Memory protection enables the kernel to limit a process access to the computers memory.Various methods of memory protection exist, including memory segmentation and paging. Allmethods require some level of hardware support (such as the 80286 MMU), which doesnt existin all computers.In both segmentation and paging, certain protected mode registers specify to the CPU whatmemory address it should allow a running program to access. Attempts to access other addresseswill trigger an interrupt which will cause the CPU to re-enter supervisor mode, placing the kernelin charge. This is called a segmentation violation or Seg-V for short, and since it is both difficultto assign a meaningful result to such an operation, and because it is usually a sign of amisbehaving program, the kernel will generally resort to terminating the offending program, andwill report the error.Windows 3.1-Me had some level of memory protection, but programs could easily circumventthe need to use it. A general protection fault would be produced, indicating a segmentationviolation had occurred; however, the system would often crash anyway.
    • Virtual MemoryThe use of virtual memory addressing (such as paging or segmentation) means that the kernelcan choose what memory each program may use at any given time, allowing the operatingsystem to use the same memory locations for multiple tasks.If a program tries to access memory that isnt in its current range of accessible memory, butnonetheless has been allocated to it, the kernel will be interrupted in the same way as it would ifthe program were to exceed its allocated memory. (See section on memory management.) UnderUNIX this kind of interrupt is referred to as a page fault.When the kernel detects a page fault it will generally adjust the virtual memory range of theprogram which triggered it, granting it access to the memory requested. This gives the kerneldiscretionary power over where a particular applications memory is stored, or even whether ornot it has actually been allocated yet.In modern operating systems, memory which is accessed less frequently can be temporarilystored on disk or other media to make that space available for use by other programs. This iscalled swapping, as an area of memory can be used by multiple programs, and what that memoryarea contains can be swapped or exchanged on demand."Virtual memory" provides the programmer or the user with the perception that there is a muchlarger amount of RAM in the computer than is really thereDiagramMultitaskingMultitasking refers to the running of multiple independent computer programs on the samecomputer; giving the appearance that it is performing the tasks at the same time. Since most
    • computers can do at most one or two things at one time, this is generally done via time-sharing,which means that each program uses a share of the computers time to execute.An operating system kernel contains a piece of software called a scheduler which determineshow much time each program will spend executing, and in which order execution control shouldbe passed to programs. Control is passed to a process by the kernel, which allows the programaccess to the CPU and memory. Later, control is returned to the kernel through some mechanism,so that another program may be allowed to use the CPU. This so-called passing of controlbetween the kernel and applications is called a context switch.An early model which governed the allocation of time to programs was called cooperativemultitasking. In this model, when control is passed to a program by the kernel, it may execute foras long as it wants before explicitly returning control to the kernel. This means that a maliciousor malfunctioning program may not only prevent any other programs from using the CPU, but itcan hang the entire system if it enters an infinite loop.Modern operating systems extend the concepts of application preemption to device drivers andkernel code, so that the operating system has preemptive control over internal run-times as well.The philosophy governing preemptive multitasking is that of ensuring that all programs are givenregular time on the CPU. This implies that all programs must be limited in how much time theyare allowed to spend on the CPU without being interrupted. To accomplish this, modernoperating system kernels make use of a timed interrupt. A protected mode timer is set by thekernel which triggers a return to supervisor mode after the specified time has elapsed. (See abovesections on Interrupts and Dual Mode Operation.)Device driversA device driver is a specific type of computer software developed to allow interaction with hardwaredevices. Typically this constitutes an interface for communicating with the device, through the specificcomputer bus or communications subsystem that the hardware is connected to, providing commands toand/or receiving data from the device, and on the other end, the requisite interfaces to the operatingsystem and software applications. It is a specialized hardware-dependent computer program which isalso operating system specific that enables another program, typically an operating system orapplications software package or computer program running under the operating system kernel, tointeract transparently with a hardware device, and usually provides the requisite interrupt handlingnecessary for any necessary asynchronous time-dependent hardware interfacing needs.NetworkingCurrently most operating systems support a variety of networking protocols, hardware, andapplications for using them. This means that computers running dissimilar operating systems canparticipate in a common network for sharing resources such as computing, files, printers, andscanners using either wired or wireless connections. Networks can essentially allow a computersoperating system to access the resources of a remote computer to support the same functions as itcould if those resources were connected directly to the local computer. This includes everythingfrom simple communication, to using networked file systems or even sharing another computersgraphics or sound hardware. Some network services allow the resources of a computer to be
    • accessed transparently, such as SSH which allows networked users direct access to a computerscommand line interface.SecurityA computer being secure depends on a number of technologies working properly. A modernoperating system provides access to a number of resources, which are available to softwarerunning on the system, and to external devices like networks via the kernel.The operating system must be capable of distinguishing between requests which should beallowed to be processed, and others which should not be processed. While some systems maysimply distinguish between "privileged" and "non-privileged", systems commonly have a formof requester identity, such as a user name. To establish identity there may be a process ofauthentication. Often a username must be quoted, and each username may have a password.Other methods of authentication, such as magnetic cards or biometric data, might be usedinstead. In some cases, especially connections from the network, resources may be accessed withno authentication at all (such as reading files over a network share). Also covered by the conceptof requester identity is authorization; the particular services and resources accessible by therequester once logged into a system are tied to either the requesters user account or to thevariously configured groups of users to which the requester belongs.In addition to the allow/disallow model of security, a system with a high level of security willalso offer auditing options. These would allow tracking of requests for access to resources (suchas, "who has been reading this file?"). Internal security, or security from an already runningprogram is only possible if all possibly harmful requests must be carried out through interrupts tothe operating system kernel. If programs can directly access hardware and resources, they cannotbe secured.External security involves a request from outside the computer, such as a login at a connectedconsole or some kind of network connection. External requests are often passed through devicedrivers to the operating systems kernel, where they can be passed onto applications, or carriedout directly. Security of operating systems has long been a concern because of highly sensitivedata held on computers, both of a commercial and military nature. The United StatesGovernment Department of Defense (DoD) created the Trusted Computer System EvaluationCriteria (TCSEC) which is a standard that sets basic requirements for assessing the effectivenessof security. This became of vital importance to operating system makers, because the TCSECwas used to evaluate, classify and select computer systems being considered for the processing,storage and retrieval of sensitive or classified information.What is Computer Virus?Computer viruses are small software programs that are designed to spread from one computer toanother and to interfere with computer operation. A virus might corrupt or delete data on yourcomputer, use your e-mail program to spread itself to other computers, or even erase everythingon your hard disk.Computer viruses are often spread by attachments in e-mail messages or instant messagingmessages. That is why it is essential that you never open e-mail attachments unless you knowwho its from and you are expecting it.
    • Viruses can be disguised as attachments of funny images, greeting cards, or audio and videofiles. Computer viruses also spread through downloads on the Internet. They can be hidden inillicit software or other files or programs you might download. Computer VirusesHow Computer Viruses Work?Here is the general way that viruses work: 1. An infected program is run. This is either a program file (in the case of a file-infecting virus) or a boot sector program at boot time. In the case of a Microsoft Word document the virus can be activated as soon as the document that contains it is opened for reading within Microsoft Word. If the "NORMAL.DOT" document template is infected (and this is the most common target of these viruses) then the virus may be activated as soon as Microsoft Word is started up. 2. The infected program has been modified so that instead of the proper code running, the virus code runs instead. This is usually done by the virus modifying the first few instructions to "jump" to where the virus code is stored. The virus code begins to execute. 3. The virus code becomes active and takes control of the PC. There are two ways that a virus will behave when it is run: direct-action viruses will immediately execute, often seeking other programs to infect and/or exhibiting whatever other possibly malicious behavior their author coded into them. Many file-infector viruses are direct-action. In contrast, memory-resident viruses dont do anything immediately; they load themselves into memory and wait for a triggering event that will cause them to "act". Many file infectors and all boot infectors do this (boot infectors have to become memory resident, because at the time they are executed the system is just starting up and there isnt that much "interesting" for them to do immediately.) 4. What exactly the virus does depends on what the virus is written to do. Their primary goals however include replication and spreading, so viruses will generally search for new targets that they can infect. For example, a boot sector virus will attempt to install itself
    • on hard disks or floppy disks that it finds in the system. File infectors may stay in memory and look for programs being run that they can target for infection. 5. "Malevolent" viruses that damage files or wreak havoc in other ways will often act on triggers. There are viruses that will only activate on particular days of the year (such as the infamous "Friday the 13th"), or act randomly, say, deleting a file every 8th time they are run. Some viruses do nothing other than trying to maximize their own infection to as many files and systems as possible.Computer VirusesMost Common Types of Viruses and Other Malicious Programs1. Resident VirusesThis type of virus is a permanent which dwells in the RAM memory. From there it can overcomeand interrupt all of the operations executed by the system: corrupting files and programs that areopened, closed, copied, renamed etc.Examples include: Randex, CMJ, Meve, and MrKlunky.2. Multipartite VirusesMultipartite viruses are distributed through infected media and usually hide in the memory.Gradually, the virus moves to the boot sector of the hard drive and infects executable files on thehard drive and later across the computer system.3. Direct Action VirusesThe main purpose of this virus is to replicate and take action when it is executed. When aspecific condition is met, the virus will go into action and infect files in the directory or folderthat it is in and in directories that are specified in the AUTOEXEC.BAT file PATH. This batchfile is always located in the root directory of the hard disk and carries out certain operationswhen the computer is booted.4. Overwrite VirusesVirus of this kind is characterized by the fact that it deletes the information contained in the filesthat it infects, rendering them partially or totally useless once they have been infected.The only way to clean a file infected by an overwrite virus is to delete the file completely, thuslosing the original content.Examples of this virus include: Way, Trj.Reboot, Trivial.88.D.5. Boot VirusThis type of virus affects the boot sector of a floppy or hard disk. This is a crucial part of a disk,in which information on the disk itself is stored together with a program that makes it possible to
    • boot (start) the computer from the disk.The best way of avoiding boot viruses is to ensure that floppy disks are write-protected and neverstart your computer with an unknown floppy disk in the disk drive.Examples of boot viruses include: Polyboot.B, AntiEXE.6. Macro VirusMacro viruses infect files that are created using certain applications or programs that containmacros. These mini-programs make it possible to automate series of operations so that they areperformed as a single action, thereby saving the user from having to carry them out one by one.Examples of macro viruses: Relax, Melissa.A, Bablas, O97M/Y2K.7. Directory VirusDirectory viruses change the paths that indicate the location of a file. By executing a program(file with the extension .EXE or .COM) which has been infected by a virus, you are unknowinglyrunning the virus program, while the original file and program have been previously moved bythe virus.Once infected it becomes impossible to locate the original files.8. Polymorphic VirusPolymorphic viruses encrypt or encode themselves in a different way (using different algorithmsand encryption keys) every time they infect a system.This makes it impossible for anti-viruses to find them using string or signature searches (becausethey are different in each encryption) and also enables them to create a large number of copies ofthemselves.Examples include: Elkern, Marburg, Satan Bug, and Tuareg.9. File InfectorsThis type of virus infects programs or executable files (files with an .EXE or .COM extension).When one of these programs is run, directly or indirectly, the virus is activated, producing thedamaging effects it is programmed to carry out. The majority of existing viruses belongs to thiscategory, and can be classified depending on the actions that they carry out.10. Encrypted VirusesThis type of viruses consists of encrypted malicious code, decrypted module. The viruses useencrypted code technique which make antivirus software hardly to detect them. The antivirusprogram usually can detect this type of viruses when they try spread by decrypted themselves.
    • 11. Companion VirusesCompanion viruses can be considered file infector viruses like resident or direct action types.They are known as companion viruses because once they get into the system they "accompany"the other files that already exist. In other words, in order to carry out their infection routines,companion viruses can wait in memory until a program is run (resident viruses) or actimmediately by making copies of themselves (direct action viruses).Some examples include: Stator, Asimov.1539, and Terrax.106912. Network VirusNetwork viruses rapidly spread through a Local Network Area (LAN), and sometimesthroughout the internet. Generally, network viruses multiply through shared resources, i.e.,shared drives and folders. When the virus infects a computer, it searches through the network toattack its new potential prey. When the virus finishes infecting that computer, it moves on to thenext and the cycle repeats itself.The most dangerous network viruses are Nimda and SQLSlammer.13. Nonresident VirusesThis type of viruses is similar to Resident Viruses by using replication of module. Besides that,Nonresident Viruses role as finder module which can infect to files when it found one (it willselect one or more files to infect each time the module is executed).14. Stealth VirusesStealth Viruses is some sort of viruses which try to trick anti-virus software by intercepting itsrequests to the operating system. It has ability to hide itself from some antivirus softwareprograms. Therefore, some antivirus program cannot detect them.15. Sparse InfectorsIn order to spread widely, a virus must attempt to avoid detection. To minimize the probability ofits being discovered a virus could use any number of different techniques. It might, for example,only infect every 20th time a file is executed; it might only infect files whose lengths are withinnarrowly defined ranges or whose names begin with letters in a certain range of the alphabet.There are many other possibilities.16. Spacefiller (Cavity) VirusesMany viruses take the easy way out when infecting files; they simply attach themselves to theend of the file and then change the start of the program so that it first points to the virus and thento the actual program code. Many viruses that do this also implement some stealth techniques soyou dont see the increase in file length when the virus is active in memory.
    • A spacefiller (cavity) virus, on the other hand, attempts to be clever. Some program files, for avariety of reasons, have empty space inside of them. This empty space can be used to house viruscode. A spacefiller virus attempts to install itself in this empty space while not damaging theactual program itself. An advantage of this is that the virus then does not increase the length ofthe program and can avoid the need for some stealth techniques. The Lehigh virus was an earlyexample of a spacefiller virus.17. FAT VirusThe file allocation table or FAT is the part of a disk used to connect information and is a vitalpart of the normal functioning of the computer.This type of virus attack can be especially dangerous, by preventing access to certain sections ofthe disk where important files are stored. Damage caused can result in information losses fromindividual files or even entire directories.18. WormsA worm is technically not a virus, but a program very similar to a virus; it has the ability to self-replicate, and can lead to negative effects on your system and most importantly they are detectedand eliminated by antiviruses.Examples of worms include: PSWBugbear.B, Lovgate.F, Trile.C, Sobig.D, Mapson.19. Trojans or Trojan HorsesAnother unsavory breed of malicious code (not a virus as well) are Trojans or Trojan horses,which unlike viruses do not reproduce by infecting other files, nor do they self-replicate likeworms.20. Logic BombsThey are not considered viruses because they do not replicate. They are not even programs intheir own right but rather camouflaged segments of other programs.Their objective is to destroy data on the computer once certain conditions have been met. Logicbombs go undetected until launched, and the results can be destructive.Type of Antivirus1) Microsoft Security EssentialsI would say Microsoft Security Essentials is the most simple and easiest way to protect yourcomputer. The main advantage of this antivirus software is, its completely free and its interfaceis really very easy and user friendly. The only thing you have to remember is that your WindowsOperation system which you are using must be a genuine one. If you have a genuine WindowsOperating system, Microsoft Security Essentials will automatically updates and installs newitems daily. If not you have to try your luck with other antivirus or atleast get a new genuine
    • Windows key.Microsoft Security Essentials can guard your system from various viruses,malicious software’s and spywares2) Panda Cloud AntivirusThis free antivirus is really worth to give it a try.Panda Cloud Antivirus actually is very usefulfor scanning and finding viruses in your computer. Its unique interface is really easy to work andmore consistent than any other. This antivirus comes from the very famous Cloud AntivirusSecurity. You can deactivate many unwanted process operations running behind your computerand make a quick background scan. When installed, the antivirus will first try to connect to therePanda Labs Collective Intelligence servers and protects your computer from viruses andmalicious software’s.3) Avast Free AntivirusAvast is really a great way of protecting your computer from viruses. I would say there NewInternet Security software is pretty impressive. But while coming to the Avast Free Antivirus itsreally gives maximum protection to your computer for home machines.Though it is not quiteimpressive with the Online shopping and bank account safety, Doesn’t really stop hackers toharm your computer automatically. And also quite unsatisfactory with the SPAM messages andmails. But you can find all of this features available in Pro and Internet Security. Avast FreeAntivirus best part is that it will automatically update when there is a new update available.4) AVG Free AntivirusAVG antivirus was really a great software when they launched its service as a free antivirus. Theupdates are really easily to download and very consistent. There was a false information aboutthe software that it will automatically creates a virus application system in ones computer after3months and will cause the whole system to crash. This is just a bad rumor about the software.They took this as a challenge and released a new interface which is very impressive and veryconsistent.Since the software was very famous from past years, it got many users who trust thesoftware. The AVG Free Antivirus is really very very simple to install and use. All you have todo is to download the software and install it. AVG Free Antivirus also a upgraded version whichacts similar to the Free AVG Antivirus Software.5) Avira AntiVir PersonalOur last free antivirus is Avira AntiVir Personal which is really the simplest antivirus in themarket. The simple user interface is easy to understand. Avira AntiVir Personal is very good indetecting root kit threats and other viruses. One of the most important feature of Avira AntiVirPersonal is that it can find the malware very easily. People say that Avira AntiVir Personal is notgood at finding the false and virus emails in our system, which is completely a false imagination.