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Quantum DLT 7000 Tape System 5-3
5.1.1 SCSI Pointers
SCSI architecture provides a set of three pointers (called saved pointers) for
each I/O process. The three pointers are: Command, Status, and Data. When an
I/O process becomes active, the three saved pointers are copied to the initiator
as current pointers. There is only one set of current pointers in the initiator at
one time. The current pointers point to the next command, data, or status byte
to be transferred between the initiator's memory and the drive. The saved and
current pointers reside in the initiator.
The saved command pointer always points to the start of the Command
Descriptor Block for the I/O process. The saved status pointer always points to
the start of the status area of the I/O process. The saved data pointer always
points to the start of the data area until the drive sends a SAVE DATA
POINTER message for the I/O process back to the initiator.
In response to the SAVE DATA POINTER message, the initiator stores the
value of the current data pointer into the saved data pointer for that I/O
process. The drive can restore the current pointer from the saved pointer value
for the active I/O process by sending a RESTORE POINTERS message to the
initiator. The initiator then copies the set of saved pointers into the set of
current pointers. Whenever a drive disconnects from the SCSI Bus, only the set
of saved pointers is retained in the initiator. The set of current pointers is
restored from the set of saved pointers when the I/O process is reconnected.