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


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.


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.


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.


        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.

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

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


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.

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.

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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