TSR Viruses 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.