typedef struct _WAIT_CONTEXT_BLOCK
{
     KDEVICE_QUEUE_ENTRY WaitQueueEntry;
     PIO_ALLOCATION_ACTION DeviceRoutine;
     PVOID DeviceContext;
     ULONG NumberOfMapRegisters;
     PVOID DeviceObject;
     PVOID CurrentIrp;
     PKDPC BufferChainingDpc;
} WAIT_CONTEXT_BLOCK, *PWAIT_CONTEXT_BLOCK;

Windows Vista Kernel Structures