An interrupt request (IRQ) is an asynchronous signal sent from a device to a processor indicating that in order to process a request, attention is required. A hardware IRQ is induced by a hardware peripheral or device request, whereas a software IRQ is induced by a software instruction. Both result in processor status savings, and revert to serving the IRQ using an interrupt handler routine.
Interrupts can be level-triggered or edge-triggered. Level-triggered interrupts indicate that the line is held by the device at an active level, triggering the interrupt until it is served. Edge-triggered interrupts indicate that the device triggers the line shortly from level 1 to 0 (or vice versa).
IRQ levels are allocated to devices to indicate their identities. For example, in a PC, IRQ0 through IRQ15 denote the 16 levels associated with the mouse, keyboard, serial port, sound card...etc.
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