Quantum 10K II - Manual (Page 355)

Appendix B. SCSI Bus Signal Timing
B–34 Quantum Atlas 10K II Ultra 160/m SCSI Hard Disk Drives
B.8.16.1 HEAD OF QUEUE TAG Message (21h)
The HEAD OF QUEUE TAG message specifies that the task is to be placed first in
the drive’s task queue. An in-progress task is not pre-empted by the command that
accompanies this message. A later task, also received with a HEAD OF QUEUE
TAG message, would be placed at the head of the drive’s task queue for execution in
a last-in, first-out order.
B.8.16.2 ORDERED QUEUE TAG Message (22h)
The ORDERED QUEUE TAG message specifies that the task be placed in the drive’s
task queue for execution in the order received. All tasks placed in the task queue
before this process is received get executed before this process. Except for tasks
received with a HEAD OF QUEUE TAG message, all later tasks are executed after
this task.
B.8.16.3 SIMPLE Message (20h) (SIMPLE QUEUE TAG)
The SIMPLE message specifies that the task be placed in the drive’s task queue. If
command reordering is enabled, commands queued with a SIMPLE QUEUE TAG
may be executed in an order that optimizes overall drive throughput, though the
execution order will observe the HEAD OF QUEUE TAG and ORDERED QUEUE
TAG message order rules.
B.8.17 RELEASE RECOVERY Message (10h)
The RELEASE RECOVERY message terminates an Auto Contingent Allegiance
condition established by a previous INITIATE RECOVERY message. See Chapter 5
for a description of the drive’s processing done upon Auto Contingent Allegiance
termination. An IDENTIFY Message must immediately precede this message.
The drive rejects the message if there is no Auto Contingent Allegiance condition
active when the message is received or if the host fails to precede it with an
IDENTIFY message.
B.8.18 SAVE DATA POINTER Message (02h)
The SAVE DATA POINTER message is sent from the drive to direct the initiator to
copy the current data pointer to the saved data pointer for the current task. The
message is sent prior to the drive performing a bus disconnect.