B3) What are the main types of PC viruses?

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 time
the program which contains it is executed.  A resident virus hides
itself somewhere in memory the first time an infected program 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 category is SYSTEM or BOOT-RECORD INFECTORS: those viruses
which infect executable code found in certain system areas on a disk
which are not ordinary files.   On DOS systems, there are ordinary
boot-sector viruses, which infect only the DOS boot sector, and MBR
viruses which infect the Master Boot Record on fixed disks and 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).  These are often called "MULTI-PARTITE" 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 which 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.  Some consider these
infectors to be a third category of viruses, while others consider
them to be a sub-category of the file infectors.


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