Developing Drivers with the Windows® Driver Foundation

Interrupt Request Levels

An IRQL defines the hardware priority at which a processor operates at any given time. When a processor is running at a given IRQL, interrupts at that IRQL and lower are masked off on the processor. A thread running at a low IRQL can be interrupted to run code at a higher IRQL, but a thread running at a higher IRQL cannot be interrupted to run code at an equal or lower IRQL. For example, a processor that is running at IRQL DISPATCH_LEVEL can be interrupted only by a request at an IRQL greater than DISPATCH_LEVEL.

The number of IRQLs and their specific values are processor dependent. The x64 and Intel Itanium architectures have 16 IRQLs, and x86-based architectures have 32 IRQLs. The difference is due primarily to the types ...