Introduction to DPCs
Any driver that has an ISR typically also has at least one DpcForIsr or CustomDpc routine to complete processing of interrupt-driven I/O operations. A typical lowest-level driver's DpcForIsr or CustomDpc routine does the following:
Finishes handling an I/O operation that the ISR began processing.
Dequeues the next IRP so that the driver can begin processing it.
Completes the current IRP, if possible.
Sometimes the current IRP cannot be completed because several data transfers are required, or a recoverable error was detected. In these cases, the DpcForIsr or CustomDpc routine typically reprograms the device for either another transfer or a retry of the last operation.
A DpcForIsr or CustomDpc routine is called in an arbitrary DPC context at IRQL DISPATCH_LEVEL. Running at DISPATCH_LEVEL restricts the set of support routines a DpcForIsr or CustomDpc routine can call. See Managing Hardware Priorities for more information.
DPC objects and DPCs can also be used with timers. For more information, see Timer Objects and DPCs.