Fw: Proposal to add a new modifier to SCSI Write commands.

Edward A. Gardner gardner at acm.org
Wed Nov 27 13:14:53 PST 1996

* From the SCSI Reflector (scsi at symbios.com), posted by:
* "Edward A. Gardner" <gardner at acm.org>
>  I would like to propose an alternative to the Write Long mechanism to
>  provide the "forced error" behavior.  The idea is to add a bit to the
>  Write commands that instructs the drive to force an ECC error for all
>  subsequent reads of the block.  The drive is free to implement this
>  command in any convenient way, provided that all subsequent reads
>  result in a "Medium Error", "Unrecovered Read Error".  A subsequent
>  to the block with the "force error" bit clear would return the block to
>  normal status.  Write commands with the "force error" bit set will be
>  restricted to one block transfer lengths.

>  In the interest of full disclosure:
>  ---------------------------------
>  Digital holds a patent on a technique for forcing errors that is similar
>  to the one described here.  It has been determined that this proposal
>  does not infringe on the Digital patent.  This determination must be
>  reassessed if the current proposal is modified.   

As one of the inventors listed on the above patent, as well as having spent
far too much time discussing this particular patent with lawyers in the
context of infringement litigation, I must say that I disagree with this
statement.  I think the proposal probably does infringe (assuming the
patent is valid).  If Gene Milligan's request for a distinct sense code is
incorporated (which I agree is highly desirable), then I've no doubt
whatsoever that it would infringe.

If Digital wants this feature added to SCSI, then I think it needs to
commit to complying with the ANSI patent policy.

re question from "Joseph Glider/Almaden/IBM" <gliderj at almaden.ibm.com>

> Any kind of 'forced error' implementation needs to include the capability
> seeing that the resulting error is 'logically bad' rather than physically
> bad.  Does the DEC patent entirely cover the concept of 'logically bad'
> just using the mechanism of using the ones complement of CRC to signal
> logically bad?

It was certainly intended (by the inventors) to cover the entire concept of
'logically bad', regardless of encoding or representation.

Edward A. Gardner               gardner at acm.org
Ophidian Designs                719 593-8866 voice
1262 Hofstead Terrace           719 593-8989 fax
Colorado Springs, CO  80907
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