An Interrupt Request Level (IRQL) is the priority given to any interrupt request generated by computer hardware. If for instance, a hard disk drive generates an interrupt request signal, and another device, such as a USB printer, generates an interrupt request signal, the two interrupts can not be serviced simultaneously. The IRQL is used to determine which one is serviced first. In this example, a hard disk interrupt requires a higher priority than a printer interrupt.
The criteria used to prioritize interrupt request levels are determined by the operating system, which is programmed to give higher priority to more important devices. For example, if a gamepad and a hard disk generated simultaneous interrupt requests, the operating system would assign a higher priority to the hard disk, because the hard disk contains information necessary for the operating system or its applications to function. If hard disk requests are delayed, many applications or even the operating system will be slowed down.