QAS and non-packetized mode

Bill Galloway BillG at
Tue May 30 09:27:53 PDT 2000

* From the T10 Reflector (t10 at, posted by:
* "Bill Galloway" <BillG at>
Messages are one of the hardest things in SCSI to get right. It is very scary
to think that hardware would have to parse and understand messages correctly.
I can easily come up with 2^7 legal message sequences that may end a
connection.  The real number is closer to 2^9 legal (but strange) message
sequences. The hardware would have to know and interpret the length of all
messages to know where message boundaries are.  Currently there are reserved
messages without any defined length.  We would have to lock down in hardware
the length of all reserved messages.  If in the future we had a good reason to
make message codes 0x30-0x3f three bytes long we would not be able to.  Two
spi-5 devices could not implement any messages that could not be snooped by
today's devices. This breaks the current model where only the two communicating
devices have to understand the message.

Would this hardware have to deal with all possible message protocol/parity
errors?? I bet someone will create a tester that will check for these.  When
they find a bug you will be turning silicon not reving firmware.

Just the act of snooping is problematic.  We have said in the past that if a
device did not successfully snoop the QAS REQUEST message then no real harm was
done, it just did not participate in that arbitration.  If we require all
devices to parse all messages to find the QAS message then ALL bytes must be
snooped successfully.  If a device misses a single message byte it can
mis-interpret all remaining message bytes.

I understand that limiting QAS to packetized causes problems for tapes but
snooping ALL message bytes is NOT the answer.

Bill Galloway
BREA Technologies, Inc.
P: (281) 530-3063
F: (281) 988-0358
BillG at

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