typedef struct _IRP
{
     SHORT Type;
     WORD Size;
     PMDL MdlAddress;
     ULONG Flags;
     ULONG AssociatedIrp;
     LIST_ENTRY ThreadListEntry;
     IO_STATUS_BLOCK IoStatus;
     CHAR RequestorMode;
     UCHAR PendingReturned;
     CHAR StackCount;
     CHAR CurrentLocation;
     UCHAR Cancel;
     UCHAR CancelIrql;
     CHAR ApcEnvironment;
     UCHAR AllocationFlags;
     PIO_STATUS_BLOCK UserIosb;
     PKEVENT UserEvent;
     UINT64 Overlay;
     PVOID CancelRoutine;
     PVOID UserBuffer;
     ULONG Tail;
} IRP, *PIRP;

Windows Vista Kernel Structures