IBM DCAS-32160 - Ultrastar 2.1 GB Hard Drive, Технические характеристики

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9.1.17 QUEUE TAG MESSAGES(20h, 21h, 22h)
Byte Value Description
ÄÄÄÄ ÄÄÄÄÄ ÄÄÄÄÄÄÄÄÄÄÄ
0 20H Simple Queue Tag message
21H Head of Queue Tag message
22H Ordered Queue Tag message
1 XXh Queue Tag
Figure 130. Queue Tag Messages
Queue Tag messages are used to specify an identifier, called a Queue Tag, for an I/O process which establish
the I_T_L_Q nexus.
The queue tag filed is an 8-bit unsigned integer assigned by the initiator during an
initial connection. The Queue Tag for every I/O process for each I_T_L nexus must be unique. If the target
receives a Queue Tag that is currently in use for the I_T_L nexus it will respond as "Incorrect Initiator
Response". A Queue Tag becomes available for re-assignment when I/O process ends. The numeric value of
a Queue Tag has no effect on the order of execution.
Whenever an initiator connects to the target, the appropriate Queue Tag message must be sent immediately
following the Identify message and within the same MESSAGE O U T phase to establish the I_T_L_Q nexus
for the I/O process.
Whenever the target reconnects to an initiator to continue a tagged I/O process, the Simple Queue Tag
message is sent immediately following the Identify and within the same MESSAGE IN phase to revive the
I_T_L_Q nexus for the I/O process.
9.1.17.1 SIMPLE QUEUE TAG (20h)
The Simple Queue Tag Message specifies that the current I/O process be placed in the command queue. The
order of execution, with respect to other I/O processes received with Simple Queue Tag Messages, is up to
the discretion of the target. The target will send a Simple Queue Tag Messages after reselection for I/O proc-
esses that were received with either Simple, Ordered, or Head of Queue Tag messages.
9.1.17.2 HEAD OF QUEUE TAG (21h)
Commands with this tag should be inserted into the head of the queue. When a command is being exe-
cuted, this tagged command will be inserted to the head of queue to be executed after the command being
currently executed.
The previous command being executed will not terminated by this tagged command.
This tagged command will wait until the previous command is through. If plural head-of-queue tagged com-
mands are received, those command will be executed in LIFO(Last in First out) order.
9.1.17.3 ORDERED QUEUE TAG (22h)
This tagged command is executed in the order received. All commands received before this command
should be executed before this command, and all commands received after this commands should be exe-
cuted after this command.
9.1.18 IGNORE WIDE RESIDUE (23h)
SCSI MESSAGE SYSTEM
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