Glossary

MBR: Master Boot Record
The master boot record is, in a sense, a small program that is automatically executed when the computer is booted. It resides in the hard drive's master boot sector which is located at the very beginning of the drive. The main function of the code contained within the MBR is to give the operating system valuable information about how the hard drive is organized. Since the MBR is accessed so early on in the boot process, it is an excellent target for viral infection. A boot sector virus will overwrite the MBR's code with its own code so that it is executed first. The virus will generally copy the actual MBR to another place on the hard drive and give control back to it after the virus gets a chance to execute.
Partition Table
The partition table is a small storehouse of information that tells the operating system where to look for its specific boot code. It is located in the master boot sector and is read by the master boot record at bootup. Thus, if you had both DOS and Linux installed on your hard drive, the partition table would contain the information pointing to the boot code of each of these operating systems. This information is often either moved, or encrypted by boot sector viruses.
CMOS
The CMOS, complimentary Metal Oxide Semiconductor, is a small segment of internal memory which contains vital information about your entire computer: its number of drives, their size, amount of RAM, etc. Without the information contained in the CMOS your computer would be virtually useless. At the present time, only a handful of viruses, most notably exebug, will target the CMOS.
.com file
A .com file is a program that ends with an extension of .com. The vast majority of PC-based viruses are .com programs. There are several reasons for this. The most important reasons are:
1) Since .com programs contain instructions that can be executed by a computer without interpretation they tend to operate faster.
2) .com programs are much more compact than their .exe counterparts so they are easier to hide.
3) In DOS, except for internal commands, .com files will always execute before any other program of the same name with a different extension. For example, if you have three programs called chart.com, chart.exe, and chart.bat in the same directory, typing "chart" will execute chart.com. A special type of virus called a companion virus exploits this situation by searching for a file with an .exe extension and creating a hidden file of the same name with a .com extension containing a virus. Thus, typing a program's name will execute the virus first, (since it has a .com extension), then code contained within the virus will start the actual .exe program.
.exe file
A .exe file is the most common type of program in the PC world. Though they are not as compact as .com programs, they provide a great deal of functionality and flexibility in terms of what they can accomplish. Viruses that can infect .exe files generally have a better chance of surviving because there are more places in an .exe file for a virus to hide. All .exe files begin with a header that tells the program how large it is an how much memory it needs to allocate. After the header there is a blank space, usually about 512 bytes long, that contains nothing but blank characters. This space is a perfect place for a virus to hide itself. Since the virus is simply filling a blank space in the file, the size of the infected file does not change, making the infection much more inconspicuous.
TSR
TSR stands for terminate, but stay resident. A TSR program will remain resident in your computer's memory after it executes. Programs such as memory managers, disk caching software, and device drivers reserve a section of your computer's memory so that they can continue to perform their function for the whole time your system is turned on. Many viruses, (particularly boot sector viruses), will stay resident in memory so they can spread to other disks and programs much faster and more transparently. In addition, once a virus becomes memory-resident it is much harder to detect because it can monitor every action taken by your computer and cover its tracks accordingly.

The opinions expressed in this article are solely my own and do not necessarily reflect the views of my employer, MicroLine, Inc., and further are not intended as endorsements for any of the products mentioned therein.