Interrupts - Event driven devices
Interrupts: An option to polling is the interrupt feature. The device sends an interrupt signal to the processor while an event happens. The processor breaks from its usual execution and executes several special interrupt-handling routine or task.
After complete the task, the control returns to the important program. To handle conditions while more than one event happens, various priorities are allocated to tasks hence higher priorities tasks may interrupt tasks of lower priority.
Some events may arise before the program is prepared for them. While more than one event occurs, the relate information is entered in the event queue. A polling loop can be utilized to check the status of the event queue. The event queue can after that pass input data from the polling task to the main program in the accurate order. The main program takes events off the head of the queue and raises the suitable process. The devices require not be checked frequently for occurrence of events. Such devices can interrupt although the processor being unaware of this.
Two types of light pen interrupts may arise. If the user points that pen at an item upon the screen to choose it, as in figure (a) of Light Pen Application, a selection interrupt arises. If the user is positioning along with the pen, as in figure (b) of Light Pen Application, a pattern termed as tracking pattern in displayed along the pen's tracking interrupts and movement arise when the pen observes the tracking pattern.
Modified versions of the light pen may also be utilized to draw lines, read barcodes or perform transformation operations on things on a tablet or on the screens.