Resident viruses are viruses that stay active in memory after
they are first run. These viruses usually trap one or more system
functions, usually file access and execution functions. When a
trapped system function is called, virus code gets control first
and a virus can infect a file or a sector which is accessed by a
system. After that the control is passed to a system function
that was called.
NON (non-resident)
Non-memory resident viruses are viruses that search for
infectable files themselves. When such virus is run, it searches
for files of specific name, type or extension on a hard drive and
infects them. Some viruses have a limit on a number of files they
can infect during one operation. This is done to hide a virus
presence in a system as search and infection actions cause a lot
of disk activity and can slow down an infected system
considerably.
OVE
Overwriting viruses are viruses that replace the contents of
other files with their own code. The content of an infected file
is destroyed. A system hit by an overwriting virus quickly
becomes unusable. Overwriting viruses are the most destructive
viruses among all others.
[Description: F-Secure Anti-Virus Research Team; F-Secure Corp.; July 14th, 2003]
