Computer viruses can be classified into several different
types. The first and most common type is the virus which infects
any application program. On IBM PC’s and clones running under
PC-DOS or MS-DOS, most programs and data which do not belong
to the operating system itself are stored as files. Each file has a file
nameeight characters long, and an extent which is three characters
long. A typical file might be called “TRUE.TXT”, where “TRUE”
is the name and “TXT” is the extent. The extent normally gives
some information about the nature of a file—in this case
“TRUE.TXT” might be a text file. Programs must always have an
extent of “COM”, “EXE”, or “SYS”. Under DOS, only files with
these extents can be executed by the central processing unit. If the
user tries to execute any other type of file, DOS will generate an
error and reject the attempt to execute the file.
The Basics of the Computer Virus 14
Since a virus’ goal is to get executed by the computer, it
must attach itself to a COM, EXE or SYS file. If it attaches to any
other file, it may corrupt some data, but it won’t normally get
executed, and it won’t reproduce. Since each of these types of
executable files has a different structure, a virus must be designed
to attach itself to a particular type of file. A virus designed to attack
COM files cannot attack EXE files, and vice versa, and neither can
attack SYS files. Of course, one could design a virus that would
attack two or even three kinds of files, but it would require a separate
reproduction method for each file type.
The next major type of virus seeks to attach itself to a
specific file, rather than attacking any file of a given type. Thus, we
might call it an application-specific virus. These viruses make use
of a detailed knowledge of the files they attack to hide better than
would be possible if they were able to infiltrate just any file. For
example, they might hide in a data area inside the program rather
than lengthening the file. However, in order to do that, the virus
must know where the data area is located in the program, and that
differs from program to program.
This second type of virus usually concentrates on the files
associated to DOS, like COMMAND.COM, since they are on
virtually every PC in existence. Regardless of which file such a
virus attacks, though, it must be very, very common, or the virus
will never be able to find another copy of that file to reproduce in,
and so it will not go anywhere. Only with a file like COM-MAND.COM would it be possible to begin leaping from machine
to machine and travel around the world.
The final type of virus is known as a “boot sector virus.”
This virus is a further refinement of the application-specific virus,
which attacks a specific location on a computer’s disk drive, known
as the boot sector. The boot sector is the first thing a computer loads
into memory from disk and executes when it is turned on. By
attacking this area of the disk, the virus can gain control of the
computer immediately, every time it is turned on, before any other
program can execute. In this way, the virus can execute before any
other program or person can detect its existence
types. The first and most common type is the virus which infects
any application program. On IBM PC’s and clones running under
PC-DOS or MS-DOS, most programs and data which do not belong
to the operating system itself are stored as files. Each file has a file
nameeight characters long, and an extent which is three characters
long. A typical file might be called “TRUE.TXT”, where “TRUE”
is the name and “TXT” is the extent. The extent normally gives
some information about the nature of a file—in this case
“TRUE.TXT” might be a text file. Programs must always have an
extent of “COM”, “EXE”, or “SYS”. Under DOS, only files with
these extents can be executed by the central processing unit. If the
user tries to execute any other type of file, DOS will generate an
error and reject the attempt to execute the file.
The Basics of the Computer Virus 14
Since a virus’ goal is to get executed by the computer, it
must attach itself to a COM, EXE or SYS file. If it attaches to any
other file, it may corrupt some data, but it won’t normally get
executed, and it won’t reproduce. Since each of these types of
executable files has a different structure, a virus must be designed
to attach itself to a particular type of file. A virus designed to attack
COM files cannot attack EXE files, and vice versa, and neither can
attack SYS files. Of course, one could design a virus that would
attack two or even three kinds of files, but it would require a separate
reproduction method for each file type.
The next major type of virus seeks to attach itself to a
specific file, rather than attacking any file of a given type. Thus, we
might call it an application-specific virus. These viruses make use
of a detailed knowledge of the files they attack to hide better than
would be possible if they were able to infiltrate just any file. For
example, they might hide in a data area inside the program rather
than lengthening the file. However, in order to do that, the virus
must know where the data area is located in the program, and that
differs from program to program.
This second type of virus usually concentrates on the files
associated to DOS, like COMMAND.COM, since they are on
virtually every PC in existence. Regardless of which file such a
virus attacks, though, it must be very, very common, or the virus
will never be able to find another copy of that file to reproduce in,
and so it will not go anywhere. Only with a file like COM-MAND.COM would it be possible to begin leaping from machine
to machine and travel around the world.
The final type of virus is known as a “boot sector virus.”
This virus is a further refinement of the application-specific virus,
which attacks a specific location on a computer’s disk drive, known
as the boot sector. The boot sector is the first thing a computer loads
into memory from disk and executes when it is turned on. By
attacking this area of the disk, the virus can gain control of the
computer immediately, every time it is turned on, before any other
program can execute. In this way, the virus can execute before any
other program or person can detect its existence
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