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Modern Operating Systems by Herbert Bos and Andrew...
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Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 696
SEC. 9.9
biological viruses reproduce.
The virus can also do other things in addition to
reproducing itself. Worms are like viruses but are self replicating.
That difference
will not concern us for the moment, so we will use the term ‘‘virus’’ to cover both.
We will look at worms in Sec. 9.9.3.
How Viruses Work
Let us now see what kinds of viruses there are and how they work. The virus
writer, let us call him Virgil, probably works in assembler (or maybe C) to get a
small, efficient product. After he has written his virus, he inserts it into a program
on his own machine.
That infected program is then distributed, perhaps by posting
it to a free software collection on the Internet. The program could be an exciting
new game, a pirated version of some commercial software, or anything else likely
to be considered desirable. People then begin to download the infected program.
Once installed on the victim’s machine, the virus lies dormant until the infect-
ed program is executed. Once started, it usually begins by infecting other programs
on the machine and then executing its
In many cases, the payload may
do nothing until a certain date has passed to make sure that the virus is widespread
before people begin noticing it. The date chosen might even send a political mes-
sage (e.g., if it triggers on the 100th or 500th anniversary of some grave insult to
the author’s ethnic group).
In the discussion below, we will examine seven kinds of viruses based on what
is infected. These are companion, executable program, memory, boot sector, device
driver, macro, and source code viruses.
No doubt new types will appear in the fu-
Companion Viruses
companion virus
does not actually infect a program, but gets to run when
the program is supposed to run.
They are really old, going back to the days when
MS-DOS ruled the earth but they still exist. The concept is easiest to explain with
an example. In MS-DOS when a user types
MS-DOS first looks for a program named
If it cannot find one, it looks
for a program named
In Windows, when the user clicks on Start and then
Run (or presses the Windows key and then ‘‘R’’), the same thing happens. Now-
adays, most programs are
files are very rare.
Suppose that Virgil knows that many people run
from an MS-DOS
prompt or from Run on Windows. He can then simply release a virus called
, which will get executed when anyone tries to run
(unless he ac-
tually types the full name:
). When
has finished its work, it then
just executes
and the user is none the wiser.

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