The operating system employs a class/port driver architecture to communicate with devices on the SCSI bus. The class/port design allows the responsibilities for communication between the operating system and the device to be cleanly divided between two separate driver modules (see Figure 8-1).

Figure 8-1 OpenVMS SCSI Class/Port Interface

The SCSI port driver transmits and receives SCSI commands and data. It knows the details of transmitting data from the local processor's SCSI port hardware across the SCSI bus. Although it understands SCSI bus phases, protocol, and timing, it has no knowledge of which SCSI commands the device supports, what status messages it returns, or the format of the packets in which this information is delivered. Strictly speaking, the port driver is a communications path. When directed by a SCSI class driver, the port driver forwards commands and data from the class driver onto the SCSI bus to the device. On any given OpenVMS system, a single SCSI port driver handles bus-level communications for all SCSI class drivers that may exist on the system.

The SCSI class driver acts as an interface between the user and the SCSI port, translating an I/O function as specified in a user's $QIO request to a SCSI command targeted to a device on the SCSI bus. Although the class driver knows about SCSI command descriptor buffers, status codes, and data, it has no knowledge of underlying bus protocols or hardware, command transmission, bus phases, timing, or messages. A single class driver can run on any given OpenVMS system, in conjunction with the SCSI port driver that supports that system. The operating system supplies a standard SCSI disk class driver and a standard SCSI tape class driver to support its disk and tape SCSI devices.