Sysinternals Freeware - Information for Windows NT and Windows 2000 - Advanced DPCs

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DPCs are a fundamental building block in NT's interrupt handling architecture. Interrupt servicing typically consists of two driver-provided components: an interrupt service routine (ISR) and a Deferred Procedure Call (DPC). ISR's execute at IRQL's above DISPATCH_LEVEL in the DIRQL range. Because one of NT's design goals is to spend as little time as possible at elevated IRQL, ISR's usually perform minimal interactions with their devices, normally just reading some state and getting the device to stop interrupting. The mechanism by which an ISR can finish processing at a lower IRQL is a DPC. Some time after an ISR requests a DPC, its DPC function will be called by NT at DISPATCH_LEVEL. While there are restrictions on what can be done at DISPATCH_LEVEL (e.g. touching pageable memory or blocking the thread), virtually nothing can be done from within an ISR. And of course, it is always possible to queue a worker thread work-item from a DPC so that interrupt processing can continue at PASSIVE_LEVEL.

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