TSR Viruses:  Terminate and Stay Resident

TSR stands for Terminate and Stay Resident.  This term was often used with DOS programs that closed but still ran, unnoticed, inside the computer.  These are also considered memory resident.  In the computer world a program can install themselves in memory, and this part can remain active after the program has ended. This memory resident part is called a resident extension.  Many viruses install themselves as resident extensions. 

Being memory resident provides several advantages for viruses.  This would account for the fact that the majority of viruses stay in the memory.  They will reside there until something external occurs like inserting a disk or executing a program.  By being in the memory, a resident virus can do anything the operating system can do.  Some will modify or damage the computer's software to hide in the memory.  Stealth viruses are always memory resident.

Another advantage to being memory resident is that the user cannot delete the virus while the computer is running.  Deleting these viruses may require doing a cold boot (restarting the computer after it has been off for a short time period) or booting from a clean, write protected boot disk.

Examples of TSR viruses:  all boot viruses, many file viruses, some macro viruses, some network viruses