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TSR(1) See TRS-80.
(2) (Terminate and Stay Resident) A program loaded into memory (RAM) in order to be immediately available. Terminate and stay resident means "stop running and remain in memory."
The TSR was popular in the days of DOS to quickly pop up a calendar, calculator or other utility with the press of a hotkey. Because DOS had no task switching, and no standards were initially defined for writing TSRs, they often conflicted with each other and with applications. Starting with Windows 3.0, TSRs were no longer needed, because any app could be conveniently task switched.
Task Switching and Multitasking
Although not called TSRs, apps today exhibit the same behavior as TSRs. With task switching, a feature of most operating systems, every time a user switches to another program, the previous one stops running but remains in memory. With true multitasking, that may not be the case because the previous app may continue to process data in the background if required (see batch process). See multitasking.