Computer Viruses

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A computer virus is a simple set of computer instructions or code that is written by some unscrupulous person. This code is attached to some part of the normal computer operating system or computer program. Instructions in this code tell the computer to perform some task. This task is often destructive, such as deleting important information or crashing the hard disk. There are other that may be only a slight aggravation. Some viruses have been known to do nothing more than put a large happy face on your computer screen (Berger).

There are different types of viruses that can infect different parts of system. Some viruses are harmless that a computer user might not notice for years. Sometimes viruses might cause random damage to data files and over a long period of time they might destroy files and disk (Hanhisalo). Boot viruses are infected floppy disk that boot records or master boot records in hard disks. They replace copying it elsewhere on the disk overwriting it. Boost viruses load into memory if the computer tries to read it while it is booting. Examples of a boot virus are Form, Disk Killer, Michelangelo, and Stone virus. Another type of virus is a program virus. This virus infect executable program files, such as those writing extensions like .BIN, .COM, .EXE, .OVL, .DRV [driver and .SYS [device driver]. These programs are loaded in memory during execution, taking the virus with them. The virus becomes active in memory, making copies of itself and infecting files on disk. Examples of this type of virus are Sunday and Cascade. Multipartite viruses are hybrid of Boot and Program viruses that infect program files and when the infected program is executed, these viruses infect the boot record. When you boot the computer the next time the virus from the boot record in the memory starts to infect other program files on disk. Examples of these types of viruses are Invader, Flip, and Tequila.

Stealth viruses use certain techniques to avoid detection. It may either redirect the disk head to read another sector instead of the one which reside or they make alter the reading of the infected file's size shorter than the directory listing. For instance, the Whale virus adds 9216 bytes to an infected file; than the virus subtracts the same number of bytes (9216) from the size in the directory. Examples of this type of virus are Frodo, Joshi, and Whale.
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