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.