1.4Command Queuing Function

The IDD equips 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, the IDD can receive multiple commands from each INIT. In untagged queuing, the IDD can receive a single command from each INIT.

Both queuing methods are possible for the IDD, but an INIT can use only one queuing method at a time. However, if another INIT selects a different method, the IDD controls both methods of command queuing.

1.4.1Untagged queuing

Using 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 after command queuing processing is completed.

After the IDD finishes executing the current processing command, 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, or if the IDD receives a TARGET RESET message from any INIT, it clears all the commands in the queue. At this time, the IDD generates on 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. If the LUN is not identified, an ABORT message 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.

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Fujitsu MAN3184, MAN3367, MAN3735 SERIES DISK DRIVES specifications Command Queuing Function, Untagged queuing
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MAN3735, MAN3184, MAN3367, MAN3735 SERIES DISK DRIVES, MAN3367, MAN3184 specifications

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