Computer Viruses

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A computer virus is a relatively small program that attaches itself to data and program files before it delivers its malicious act. There are many distinct types of viruses and each one has a unique characteristic. Viruses are broken up into to main classes, file infectors and system or boot-record infectors.
A file infector attaches to ordinary programs usually affecting .COM and .EXE files. These two files are subject to infection though some can infect any program from which execution is requested. On the other spectrum, system or boot-record infectors infect executable codes found in certain system areas on a disk, which are not ordinary files.
Not only are there two main classes of viruses, but there are also many different types. A virus can be classified as a boot sector virus, a program virus, a trojan horse virus, a macro virus, a memory resident virus, a stealth virus, or a polymorphic virus.
A boot sector virus replaces the startup code on a disk with a modified version, which is loaded into the memory when the computer is started.
Program viruses attach to legitimate program files. After attachment the program file will then intercept the program’s execution and take control of the computer system.
Trojan horse viruses are also legitimate programs that are advertised as something desirable, but are actually something that is undesirable. It performs an unauthorized destructive act on your computer when executed. They received their name from the mythical Trojan Horse that was delivered to the gates of the city Troy.
Macro viruses are written in macro language of specific computer programs, such as Word or Excel. These viruses infect files when a program document is accessed, or triggered by user actions, such as certain keystrokes or menu choices.
Memory resident viruses hide in memory and are difficult to detect. When the virus is in memory it will infect any uninfected file that is executed.
A stealth virus hides their presence. They can infect a program by adding bytes to the infected file. It then subtracts the directory entry of the infected file by the same number of bytes. This gives the impression that the file’s size has not changed, therefore, antiviral programs can not detect them through these changes.
Polymorphic viruses modify their appearance and scramble their code periodically. This allows them to replicate so that none of its copies are the same.
These viruses also all have different characteristics.
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