Ciprico Rimfire 3880, User Manual

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User’s Guide 21020285 D
Chapter 8 Details of Usage
Tagged Queuing Page 8 - 9
2. Choose a tag message.
There are three tag messages specified by the SCSI-2 and SCSI-3 protocol. You
need only set the appropriate TAG OPTS bits in the Flags-2 field of the Pass-
through Parameter Block for the command you are issuing to the SCSI device.
These are the messages defined by the SCSI-2 and SCSI-3 specification, their
meaning, and the TAG OPTS bit assignments for the Flags-2 field of the
RF3880 command:
As you determine the types of tag messages you wish to use, you should be
aware that there are rules of execution that your SCSI-2 or SCSI-3 device must
follow for each tag message or combination of tag message commands. For
example, determination of priority in the case of mixed tag messages, which
executes first — an ORDERED QUEUE TAG or a SIMPLE QUEUE TAG?
This situation and others are detailed in your SCSI-2 or SCSI-3 specification.
3. The RF3880 sends the tag message.
The RF3880 decodes these two bits, assigns a tag number and then manages
the protocol for sending the proper message for the command.
How It Works
This is how the RF3880 processes commands that use tag messages:
1. The tag message is sent after the identify message and before the SCSI
command.
2. The adapter sends the commands in the exact order received from the
host.
3. The queue tag number that is sent with the tag message is assigned by
the adapter in ascending order beginning with 1, maximum of 255.
When 255 is reached, the RF3880 starts over again with 1.
BITS
7 6 5 4 3 2 1 0
0 0 0 0 JSM IAD TAG OPTS
Table 113: Meaning of Flags-2 Field
TAG OPT
Value
Meaning
00
SIMPLE QUEUE TAG MESSAGE: The command may be processed by the
SCSI-2 or SCSI-3 device out of sequence for efficiency. Example: device may
sort commands by logical block address.
01
ORDERED QUEUE TAG MESSAGE: The command must be executed by the
SCSI-2 or SCSI-3 device in the order sent by the adapter (in order with respect
to other commands sent with the Ordered Queue Tag Message.
10
HEAD OF QUEUE TAG MESSAGE: The command is executed by the SCSI-
2 or SCSI-3 device before all other commands previously sent, including other
Head of Queue Tag commands but excluding the command currently in process.
Table 114: Meaning of TAG OPTS Bits