typedef struct _DRIVER_EXTENSION
{
     PDRIVER_OBJECT DriverObject;
     LONG * AddDevice;
     ULONG Count;
     UNICODE_STRING ServiceKeyName;
     PIO_CLIENT_EXTENSION ClientDriverExtension;
     PFS_FILTER_CALLBACKS FsFilterCallbacks;
} DRIVER_EXTENSION, *PDRIVER_EXTENSION;

Windows Vista Kernel Structures