page 1 of 1


Date: May 9, 1988                                       X3T9.2/88-055R0

To: X3T9.2 Committee (SCSI)   

From: James McGrath (Quantum)

Subject: Advisory vs Mandatory Bit in the Proposed Read Caching Page




  In order to insure ease of testing and integration, and in general
to increase customer confidence in caching products, there should be
some way to force the prefetch quantities described in the read cache
mode page to be always used precisely as requested by the initiator.
If this is impossible, then an error should be raised to the
initiator.  This allows the initiator to always know, based upon its
knowledge of the commands given, the mode parameters, and the cache
size, what data should and should not be in the cache.  This
knowledge can then be used to verify the cache implementation during
testing.

  In table 8-56 (Read Caching Parameters), designate byte 2, bit 2
(currently reserved) as the AM bit.  Insert the following section
between the sections describing the ABSOLUTE/MULTPLE SELECTION (AMS)
bit (paragraph 3) and the DEMAND READ RETENTION PRIORITY field
(paragraph 4):

  The ADVISORY/MANDATORY (AM) bit is a control parameter.  An AM bit
of zero indicates that the target shall consider all advisory
parameters on this page not to constrain the target in its cache
management, but only to advise it.  An AM bit of one indicates that
the target shall consider all advisory parameters on this page to be
control parameters.  Thus the retention priorities, when to prefetch,
and how much to prefetch, are all determined by the page parameters
and the parameters for the current READ command.  The target shall
not do any other anticipatory prefetching, and shall report an error
if the prefetching requested cannot be accomplished.