Computer viruses can be classified into the following types depending upon the target of attack. They are of the following types: boot sector viruses and file infecting viruses.
1. Boot Sector Viruses
Boot sector viruses are designed to infect the boot sector of the disk. The boot sector is the first sector of a disk. Boot sector virus contains an executable program that enables the hardware to load the operating system with certain settings. A boot sector virus either replaces or modifies the executable program in the boot sector of the disk. This prevents the hardware from loading the operating system when the computer is turned on. A boot sector virus is executed every time a computer is switched ON or restarted. The most common method by which computer systems become infected is through the use of infected diskettes or CDs during start up. The examples of boot sector viruses are Disk Killer, Stoned and Michelangelo.
2. File Infecting Viruses
File infecting viruses infect executable program files having an extension.exe,.com, or .dll. These viruses either overwrite or modify the contents of executable files. When these infected files are executed, the virus gets activated and remains present in the main memory even after the execution of the file is over. When another file is executed, this virus attaches itself to the new file and thus another file gets infected. File viruses can spread by using floppy disks, CDs, over the network, or through remote access. The examples of file viruses are Friday the 13th, Enigma, Nemesis and Cascade.
1. Boot Sector Viruses
Boot sector viruses are designed to infect the boot sector of the disk. The boot sector is the first sector of a disk. Boot sector virus contains an executable program that enables the hardware to load the operating system with certain settings. A boot sector virus either replaces or modifies the executable program in the boot sector of the disk. This prevents the hardware from loading the operating system when the computer is turned on. A boot sector virus is executed every time a computer is switched ON or restarted. The most common method by which computer systems become infected is through the use of infected diskettes or CDs during start up. The examples of boot sector viruses are Disk Killer, Stoned and Michelangelo.
2. File Infecting Viruses
File infecting viruses infect executable program files having an extension.exe,.com, or .dll. These viruses either overwrite or modify the contents of executable files. When these infected files are executed, the virus gets activated and remains present in the main memory even after the execution of the file is over. When another file is executed, this virus attaches itself to the new file and thus another file gets infected. File viruses can spread by using floppy disks, CDs, over the network, or through remote access. The examples of file viruses are Friday the 13th, Enigma, Nemesis and Cascade.
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