The second method of executing a TSR involves signaling it through some hardware event, such as a predetermined sequence of keystrokes. This type of TSR is called active because it must continually search for its start-up signal. The advantage of active TSRs lies in their accessibility. They can take control from any running application, execute, and return, all on demand.
An active TSR, however, must not seize processor control blindly. It must contain additional code that determines the proper moment at which to execute. The extra code consists of one or more routines called “interrupt handlers,” described in the following section.