qpc field of CIP in SBP-2

Stephen Finch/SSI1 Stephen_Finch/SSI1.SSI1 at notes-gw.tus.ssi1.com
Thu Jun 26 08:35:51 PDT 1997

* From the SCSI Reflector (scsi at symbios.com), posted by:
* Stephen Finch/SSI1 <Stephen_Finch/SSI1.SSI1 at notes-gw.tus.ssi1.com>
><Stephen_Finch/SSI1.SSI1 at notes-gw.tus.ssi1.com> asks:
>> I think I understand the purpose of qpc.  My question is:
>> Are the quadlets that are logically added to the application-dependent 
>> data block to make it equal to the value obtained by multiplying dbs by 
>> the value of 2 raised to the fn actually sent on in the isoc packet?
>> Okay everyone, does anyone out there understand my question?
>The padding quadlets are transmitted in the isochronous packet which 
>contains the last source-packet-data-block (2^fn)-1 
>There is a helpful note in the SBP-2 rev 2d draft following Figure 65
>illustrating how this source-packet-data-block may be located.
>Rob  <rlash at qntm.com (Rob Lash) >

That's an interesting answer.  The note says which block in the current 
isoc packet is the first block of a source packet, but it doesn't state that 
block previous is really full, or that the ordinal number of the block could be 
multiplied by the dbs value to determine the offset of the first packet.  This
just gives a block number....

I would think that qpc is saying that the last block of a source packet is 
this number of quadlets and NOT have to send these quadlets. 

Your answer states what may be the correct answer, but I don't see it explicitly
stated in SBP-2, either directly (preferred) or even indirectly, ala the note 
referenced.  Is there another document (e.g., 1883) that contains a statement 
on which you base your assumption?

If your answer is correct, WHY?  Why would we want to send 1 to 7 quadlets 
of useless information over the bus?  Are these useless bytes also stored 
on the media when recording?  If so, why have the qpc in the CIP header at all?
But it in the sph header...

This may sound like the start of another endless argument, but I don't have my 
heels dug in.  I just want closure.

steve finch

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