A signal to a processor indicating that an asynchronous event has occurred. The current sequence of instructions is temporarily suspended (interrupted), and a sequence appropriate to the interruption is started in its place. Interrupts can be broadly classified as being associated with one of the following.

1. (a) Events occurring on peripheral devices. A processor having initiated a transfer on a peripheral device on behalf of one process may start some other process. When the transfer terminates, the peripheral device will cause an interrupt. See also interrupt I/O.

2. (b) Voluntary events within processes. A process wishing to use the services of the operating system may use a specific type of interrupt, a supervisor call (SVC), as a means of notifying the supervisor.

3. (c) Involuntary events within processes. A process that attempts an undefined or prohibited action will cause an interrupt that will notify the supervisor.

4. (d) Action by operators. An operator wishing to communicate with the supervisor may cause an interrupt.

5. (e) Timer interrupts. Many systems incorporate a timer that causes interrupts at fixed intervals of time as a means of guaranteeing that the supervisor will be entered periodically.