When you install a TSR, you define a special key sequence (usually a control character) that will invoke the TSR program. You can then press this hot key from within any application to run the TSR program. Many programs can be installed as a TSR, but TSRs reduce the amount of memory available to other programs. In addition, not all TSRs interact well with each other. You may have difficulties, therefore, if you try to keep too many TSRs in main memory at once.