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CUSTOM SCSI PACKETS
2-4
2
This first class is used for command execution and/or message passing
through the MVME147 SCSI firmware while the module is playing the
initiator role. According to SCSI definition, an initiator is a SCSI device
(usually a host system) which requests an operation to be performed by
another SCSI device; a TARGET is a SCSI device which performs an operation
requested by an initiator. Initiator role custom packets request operations to
be performed by other SCSI devices. For the initiator role, certain data
structures are needed by the SCSI firmware. These are:
1. SCRIPT: A "script" is a sequence of SCSI bus phases that the initiator
expects the TARGET to perform when executing a requested command.
For example, a disk read (under SCSI rules) would typically require the
following SCSI bus phases:
MESSAGE-OUT: The IDENTIFY message is sent from the initiator to the
TARGET. This message contains the identification of the desired logical
unit of the selected disk controller that the initiator wishes to read. The
message also indicates whether the initiator is capable of reselection.
COMMAND: The Command Descriptor Block (CDB) is sent during the
command phase to specify the block number to read, the logical unit to
read from, the number of blocks to read, and whether the command is
linked.
DATA-IN: The actual data is transferred from the TARGET to the initiator
during the data-in phase.
STATUS: The disk controller sends the status of the command that was
executed during this phase.
MESSAGE-IN: During this phase, the disk controller sends a message
describing the execution of the command it just executed. The linked
command information would appear in the message sent during this
phase, for example.
BUS DISCONNECT: After a disk controller sends a command complete
message, it disconnects from the SCSI bus by releasing the BSY* signal.
The justification for a script is as follows. On the SCSI bus, the TARGET
is always the SCSI device that dictates the sequence of bus phases that
occurs during a communication with the initiator (this communication is
NOTES:
1. Refer to Chapter 3.
2. Command Table = 384 bytes
RAM work area.