Should SIP require detection of duplicate tags?
chen at corp.cirrus.com
chen at corp.cirrus.com
Thu Apr 14 09:52:02 PDT 1994
I have the same concern about the target responsible of checking the
duplicated queue. Indeed the possibility of tag duplication is in the
order that Steve pointed out -- 65,280 cases. In order to verify the
tag duplication, there are few ways to check it:
1. Firmware: This verification requires firmware to search for the
65,280 possibilities to find out the duplication. With performance in
mind, the target has to be force to implement greater MIP's, higher
cost CPU. Even then, it is not sure for me to see what are we buying.
2. Silicon: Everything is possible with the cost and die size. My
study shows that a significant increase of die size required for this
implementation (if it is not impossible for 65,280 cases). A Ram
based tag table has to be constructed in order for the hardware (or
firmware) to check it. Outrages to me.
Any other thought?
Cirrus Logic, Inc.
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Subject: Should SIP require detection of duplicate tags?
Author: Steve Finch <TFINCHS%A1.VAX2T.mrouter at vax2t.tus.ssi1.com> at novmailpc
Date: 4/13/94 8:56 AM
Currently, SAM states that the requirement to detect duplicate tags is
determined by the appropriate protocol standard. The reason is that for
some protocols the set of possible tags is enormous.
How bit is enormous? SIP can have 15 initiators and up to 256 tags per
initiator, for a total of 65,280 possible tags. Is this enormous enough?
There are only two conditions that duplicate tags will ever be issued: a
broken host computer, or a qualification program designed explicitly to
test duplicate tag detection by a target.
The real question is: should a target device be REQUIRED to protect the
system from an errant host who is issuing duplicate tags? Yes, this
capability would help detect strange errors like the host picking or
dropping bits in hardware as it assigned tag numbers. But if this is a
concern, maybe we should add a command verification phase to SIP to make
sure that the write he just issued was really to block 1354 and not 1350
with a bit error.
Why burden a target with the requirement to add a capability that is only
needed if the system is broken? No sane host ever issues duplicate tags,
and insane hosts can do a lot worse things than sending duplicate tags.
Since SAM does not require duplicate tag detection, let's not require it in
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