Terminate-and-stay-resident (TSR) programs or utilities

A TSR, or a TERMINATE AND STAY RESIDENT program, is a program that loads (from Autoexec.bat or the command line) and remains in memory until it is explicitly deactivated or the computer is turned off. Some TSRs are 'pop-up' utilities like calculators or task switchers, that load into memory, remain in the background, and are immediately activated by pressing a special keystroke combination. Other TSRs load into memory and serve as middlemen between programs and the disk (e.g., 'on the fly' disk compression programs like DoubleSpace, Stacker, or SuperStor). For example, when another program tries to write to the disk, the disk compression TSR intercepts the data, compresses it, and writes only the compressed form to the disk. No user action is required for this type of TSR to do its work.