Interrupts

A program enters a waiting loop in which it repeatedly tests the device status. During this period, the processor is not performing any useful computation. There are many situations where other tasks can be performed while waiting for an I/O device to become ready. To allow this to happen, we can arrage for the I/O device to alert the processor when it becomes ready. It can do so by sending a hardware signal called an interrupt to the processor. Atleast one of the bus control lines, called an interrupt request line is usually dedicated for most of these kind of purposes. Since the processor is no longer required to continuously check the status of external devices, it can use the waiting period to perform other useful functions. Indeed, by using interrupts such waiting periods can ideally be eliminated.