Tagged Queuing - Fujitsu MAX3073NC - SCSI,80, Dell DC961,Hard Drive Scsi Logical Interface Specifications

Command Processing
a) When there is an error in the CDB, the IDD responds with a CHECK CONDITION status
at the point when that command is fetched from the queue.
b) If the IDD is in the not ready state at the point when the queued command is fetched, it
responds with a CHECK CONDITION status.
c) If a UNIT ATTENTION condition is generated before the queued command is fetched, a
CHECK CONDITION status may be replied.
1.4.2

Tagged queuing

Through the tagged queuing function, the IDD can receive multiple commands from the same INIT or
from different INITs until the command queue is full. The number of commands that it is possible to
receive by the IDD is 128 maximum, without relation to the INIT. When the IDD receives a new
command, if the command queue is full, it responds to the new command with the TASK SET FULL
status.
The IDD manages the command queue, but it is possible for the INIT to add or clear commands from the
queue. When adding a command to the queue, it is possible for the INIT to specify the order in which
commands should be executed or the command that should be executed next to the IDD.
If the disconnect right is not recognized in the IDENTIFY message of a tagged command, the IDD
responds with a BUSY status.
Through the QUEUE TAG message, the INIT can attach a unique tag (ID) to each command. The INIT
can set that command's pointer correctly by the tag sent when the IDD reconnects. One INIT can issue
multiple commands to the IDD only when the respective commands have unique tags.
If an ORDERED message is used, the IDD executes the other commands not included in the ORDERED
message in the order in which they are received. All commands received with a SIMPLE message before
commands are received with an ORDERED message are executed before those commands received with
the ORDERED message. All commands with SIMPLE messages received after commands received with
the ORDERED message are executed after the commands received with the ORDERED message.
Commands received with a HEAD OF QUEUE message are registered at the top of the queue for waiting
execution. The IDD does not interrupt the current command execution and executes them after
completion of current command execution. When commands with the HEAD OF QUEUE message are
received continuously, the IDD first executes the command which was received last.
During the IDD executes or queues a tagged command, the same INIT must issue untagged commands
except when the IDD is in the sense hold state.
The IDD handles a series of linked commands as if it were processing a single command and processes
the series of commands by the tag received with the first command. A command with a HEAD OF
QUEUE received before processing of a series of linked commands is completed is executed by the IDD
after all the linked commands in the series have been executed.
The RESERVE, RELEASE and RESERVE EXTENDED commands should be issued together with an
ORDERED message. If the HEAD OF QUEUE message is used with these commands, previously issued
commands and reserved states may become redundant.
The TEST UNIT READY and INQUIRY commands do not influence the state of the IDD, so they can be
issued together with a HEAD OF QUEUE message.
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