Dispatch Routines and IRQLs
Most drivers' dispatch routines are called in an arbitrary thread context at IRQL = PASSIVE_LEVEL, with the following exceptions:
Any highest-level driver's dispatch routines are called in the context of the thread that originated the I/O request, which is commonly a user-mode application thread.
In other words, the dispatch routines of file system drivers and other highest-level drivers are called in a nonarbitrary thread context at IRQL = PASSIVE_LEVEL.
The DispatchRead, DispatchWrite, and DispatchDeviceControl routines of lowest-level device drivers, and of intermediate drivers layered above them in the system paging path, can be called at IRQL = APC_LEVEL and in an arbitrary thread context.
The DispatchRead and/or DispatchWrite routines, and any other routine that also processes read and/or write requests in such a lowest-level device or intermediate driver, must be resident at all times. These driver routines can neither be pageable nor be part of a driver's pageable-image section; they must not access any pageable memory. Furthermore, they should not be dependent on any blocking calls (such as KeWaitForSingleObject with a nonzero time-out).
The DispatchPower routine of drivers in the hibernation and/or paging paths can be called at IRQL = DISPATCH_LEVEL. The DispatchPnP routines of such drivers must be prepared to handle PnP IRP_MN_DEVICE_USAGE_NOTIFICATION requests.
The DispatchPower routine of drivers that require inrush power at start-up can be called at IRQL = DISPATCH_LEVEL.
For additional information, see Managing Hardware Priorities.