require duplicate queue tag checking

Gerry Houlder (Notes) Gerry_Houlder at
Thu Apr 14 14:39:13 PDT 1994

Don't go overboard on the number of possible queue tags overlaps that must be
checked. Remember that different initiators are allowed to send the same tag,
so there are only 256 possible conflicts (the maximum possible number of tags
|from one initiator). Also the target only has to check against the number of
commands in its queue - if only 64 commands are accepted before "queue full" is
generated, then only 63 other entries must be checked for conflicts. Thus the
real impact on how hard it is to check duplicate tags is the queue depth of the
target, not the number of possible number combinations.
I agree that this requirement adds significant command processing overhead  for
the target, however. I have heard of designs that intend to do the cache hit
determination in a hardware engine (so it takes only a few microseconds). These
designs can take in the command bytes, find a cache hit, and transfer the data
to the initiator before the queue tag has even been checked for duplicity!
Checking for overlap between tagged and untagged commands is not as bad, but
still unconvenient. Untagged commands can be treated as tagged commands with a
"unique" tag value (like 257, so it doesn't overlap any tagged commands) and no
problems will result because of mixing tagged and untagged commands in the
queue. If overlapped duplicate tagged commands can be ignored, overlapped
duplicate untagged commands can too!
Gerry Houlder (Notes)  (Gerry_Houlder at
Seagate Technology   -   920 Disc Drive   -   Scotts Valley, CA 95066 USA
Main Phone 408-438-6550   -   Email Problems postmaster at
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### OGATE Version 6 message trace and attachment information:
### MsgFileName: m:\mgate\outbound\916.MSG
### Org Date:    04-14-94 01:30:12 PM
### From:        Gerry Houlder at SEAGATE
### To:          scsi @ WichitaKS.NCR.COM @ internet
### Subject:     RE: require duplicate queue tag checking
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