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TSR's were popular in the MS-DOS days as pseudo multi-tasking. They would 'terminate and stay resident'. You could call them up with a hotkey and you could have them accessible from another program.

These were VERY hard to program as MS-DOS was intended as a single tasking program.

Many debuggers ran as TSR's.

I think the most popular TSR was Sidekick from Borland. Sidekick included a calculator, notepad, appointment book, basic terminal program, clip board among other things.

TSR's typically hooked the keyboard interrupt looking for a special sequence that would call them up, and interrupt the underlying program. More sophisticated ones would also hook the timer, and allow the 'foreground' program to continue to execute, a form of multitasking.