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1.4
Command Queuing Function
The IDD is equipped with a command queuing function. Through queuing of commands, the
IDD can receive multiple commands in advance and execute them.
There are two methods used in the queuing function, tagged and untagged. In tagged queuing,
it is possible for the IDD to receive multiple commands from each INIT. In untagged queuing,
it is possible for the IDD to receive a single command from the INIT.
Both cueing methods are possible for the IDD, but an INIT can use only one queuing method
or the other at a time. However, if a different INIT selects a different method, the IDD
controls both methods of command queuing.
1.4.1
Untagged Queuing
In untagged queuing, the IDD can receive a command from an INIT while it is executing
processing of a command from another INIT. The IDD can receive one command at a time
from each INIT. It is the role of the INIT to confirm that only one command is issued every
time.
When the IDD receives a new command from an INIT, if it is processing another command
from a different INIT, or if it is currently executing its initial self-diagnosis, that command is
queued in the command queue. In this case, the IDD executes disconnect processing and
command queuing processing is completed.
After the IDD finishes executing the command it is currently processing, if there is a command
in the queue, it fetches that command and executes it. If there are multiple commands in the
queue, they are fetched and executed in the order in which they were received.
When a command is in the queued state, if a RESET condition occurs, and the IDD receives a
BUS DEVICE RESET message from any INIT, it clears all the commands in the queue. At
this time, the IDD generates a unit attention condition for all the INITs.
When an ABORT message is sent from an INIT that has issued a command which is in the
queue, if the correct LUN (0) is specified, only the command issued by that INIT is cleared
and the other commands in the queue are not effected. An ABORT message which does not
specify a LUN, or one which specifies an illegal LUN (1 to 7), does not effect the commands
in the queue.
Untagged queuing exception processing (events and operations executed by the IDD) is shown
below.
• If the TEST UNIT READY, REQUEST SENSE or INQUIRY command is received.
When one of these commands is received, if there is no link instruction in that command, the
IDD executes that command immediately without queuing the command or executing
disconnect processing. At this time, there is no effect on the commands from other INITs
which are currently being executed, or on the commands in the queue.
If these commands contain link instructions, they are queued.
C141-E039-01EN
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