Terminate-and-stay-resident program

A terminate-and-stay-resident program (commonly TSR) is a computer program running under DOS that uses a system call to return control to DOS as though it has finished, but remains in computer memory so it can be reactivated later. This technique partially overcame DOS's limitation of executing only one program, or task, at a time. TSRs are used only in DOS, not in Windows.

Some TSRs are utility software that a computer user might call up several times a day, while working in another program, using a hotkey. Borland Sidekick was an early and popular example of this type. Others serve as device drivers for hardware that the operating system does not directly support.