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2024年7月26日发(作者:)
武汉纺织大学2011届毕业论文
英文
COMPUTER VIRUSES
What are computer viruses?
According to Fred Cohen’s well-known definition, a computer virus is a
computer program that can infect other computer programs by modifying them in
such a way as to include a (possibly evolved) copy of itself. Note that a program
does not have to perform outright damage (such as deleting or corrupting files) in
order to be called a “virus”. However, Cohen uses the terms within his definition
(e.g. “program” and “modify”) a bit differently from the way most anti-virus
researchers use them, and classifies as viruses some things which most of us would
not consider viruses.
Computer viruses are bits of code that damage or erase information, files, or
software programs in your computer, much like viruses that infect humans,
computer viruses can spread, and your computer can catch a virus when you
download an infected file from the Internet or copy an infected file from a diskette.
Once the viruses is embedded into your computer’s files, it can immediately start
to damage or destroy information, or it can wait for a particular date or event to
trigger its activity.
What are the main types of viruses?
武汉纺织大学2011届毕业论文
Generally, there are two main classes of viruses. The first class consists of the
file Infectors which attach themselves to ordinary program files. These usually
infect arbitrary .COM and/or .EXE programs, though some can infect any program
for which execution is requested, such as .SYS,.OVL,.PRG,&.MNU files.
File infectors can be either direct action or resident. A direct-action virus
selects one or more other programs to infect each other time the program which
contains it is executed ,and thereafter infects other programs when “they” are
executed (as in the case of the Jerusalem) or when certain other conditions are
fulfilled. The Vienna is an example of a direct-action virus. Most other viruses are
resident.
The second class is system or boot-record infectors: those viruses, which infect
executable code, found in certain system areas on a disk that are not ordinary files.
On DOS systems, there are ordinary boot-sector viruses, which infect only the DOS
boot sector on diskettes. Examples include Brain, Stoned, Empire, Azusa, and
Michelangelo. Such viruses are always resident viruses.
Finally, a few viruses are able to infect both (the Tequila virus is one example).
There are often called “multipartite” viruses, though there has been criticism of
this name; another name is “boot-and -file” virus.
File system or cluster viruses (e.g. Dir-II) are those that modify directory table
entries so that the virus is loaded and executed before the desired program is.
Note that the program itself is not physically altered; only the directory entry is.
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