typedef struct {SHORT
SHORT Type;
UCHAR Number;
UCHAR Importance;
LIST_ENTRY DpcListEntry;
PKDEFERRED_ROUTINE DeferredRoutine;
PVOID DeferredContext;
PVOID SystemArgument1;
PVOID SystemArgument2;
PULONG Lock;
} KDPC, *PKDPC;
The self-explantory fields are DeferredRoutine, DeferredContext, and SystemArgument1 and SystemArgument2. DeferredRoutine and DeferredContext are obtained from the parameters that are passed to KeInitializeDpc. the system arguments are set in the call to KeInsertQueueDpc. However, if the call to KeInsertQueueDpc was made through the IoRequestDpc macro then SystemArgument1 is taken from the IRP parameter and SystemArgument2 corresponds to the Context parameter.
The Type field is also pretty obvious. Each object, whether it is an Executive or Kernel object, is tagged with a type so that functions can ensure that they are handed objects of the type that they expect. The Kernel object type values are simply determined from an enumerated list that contains entries for APCs, Events, Processes, Queues, Semaphores, Timers, as well as DPCs.
The final four fields, Number, Importance, DpcListEntry and Lock are more obscure and are undocumented. I'll describe each of these in the following sections.