The SCSI Port driver communicates with its miniport driver by means of a series of pointers to miniport driver callback routines in its dispatch table and driver object. The miniport driver calls ScsiPortInitialize from its DriverEntry routine in order to initialize SCSI Port's dispatch table and driver object with these callback pointers. One such callback pointer is the entry point for the miniport driver's start I/O routine that is used to process I/O requests. The port driver assigns this pointer to the DriverStartIo member of the driver object.
Whenever SCSI Port receives an I/O request from a higher-level driver, it queues the request in an internal queue. For more information about the SCSI Port's internal queues, see SCSI Port Driver's Queue Management.
Once the target device is ready to receive the next I/O request, SCSI Port calls IoStartPacket, which in turn calls the miniport driver start I/O callback routine that is stored in DriverObject->DriverStartIo. For information about the operation and required characteristics of the miniport driver's start I/O routine, see SCSI Miniport Driver's HwScsiStartIo Routine.
SCSI Port raises the IRQL of the processor before calling the miniport driver's start I/O routine, in order to mask out interrupts and to guarantee that the start I/O routine has synchronized access to critical operating system and driver structures.
While the flow of I/O request packets between a storage class driver and the SCSI Port driver is asynchronous, the flow of I/O request packets between the SCSI Port driver and the target device is synchronous. SCSI Port uses an internal queuing system that makes it possible for class drivers to send new I/O requests to SCSI Port before previous I/O requests have completed. However, SCSI Port does not send the next I/O request to the target device until it receives notification from the miniport driver that the miniport driver is ready to receive the next I/O request. The miniport driver notifies SCSI Port by making a call to the ScsiPortNotification library routine.
The Storport Driver offers a more flexible I/O model, in particular with regard to the masking of interrupts. For information about the differences between the Storport I/O model and the SCSI Port I/O model, see Storport I/O Model.