Figure 1. Interrupt Control Flow

DPCs can be targeted so that they will only be queued on a certain processor with the call KeSetTargetProcessorDpc. this function, although undocumented, is prototyped in NTDDK.H, and takes a pointer to the DPC object being targeted and the number of the processor it will be queued upon. KeInsertQueueDpc looks at the Number field, and if it is less than 32, treats the DPC as non-targeted. KeInitializeDpc sets the number field to the number passed as a parameter plus 32. Thus, when KeInitializeDpc sets the number field to 0, it is initializing a DPC object as non-targeted. Another way of describing a non-targeted DPC is to say that its targeted at the current CPU (the one on which the ISR executed), which also describes DPCs that are actually targeted, but happen to be aimed at the processor that the ISR ran on.

Before KeInsertQueueDpc places a dpc on dpc queue it first checks the Lock field. If this field is non-null it indicates that the DPC object already resides on one of the system's DPC queues. When this is the case KeInsertQueueDpc returns immediately with a FALSE result. If the Lock field is null, KeInsertQueueDpc links the dpc object onto the target processor's DPC queue through the DpcListEntry links in the DPC object. It then sets the Lock value to non-null and returns TRUE, informing the caller that the DPC has been freshly queued.

The NT DDK states that a DISPATCH_LEVEL software interrupt is issued. This is also known as a DPC queue-drain interrupt, but it is also used for invocations of the scheduler.