6-8-90                                              X3T9.2/90-086

TO:     X3T9.2 / SCSI-3

FROM:   Thomas Wicklund / Ciprico

RE:     SCSI-2 caching page problem


In examining the caching page, I find that some of the parameters
are poorly specified.  I'm not sure that this can be corrected in
SCSI-2 but should be cleaned up in conjunction with any
modifications made for SCSI-3.

All comments relate to section 8.3.3.1.  I will refer to
page numbers and paragraphs in the Rev 10c document distributed
with the April mailing.

On page 8-66 the 3rd paragraph states:

     "A minimum pre-fetch field of zero indicates that
  pre-fetching should be terminated whenever another command is
  ready for executing.  This consideration is ignored when the
  minimum pre-fetch is equal to the maximum pre-fetch."


1.  At the very least, change "ready for executing" to "ready to
execute".

2.  As written, the logic a device must implement when a new
command comes in is:

  A.  Minimum pre-fetch = 0:
        Abort pre-fetch.
  B.  Minimum pre-fetch == maximum pre-fetch:
        Complete pre-fetch before executing new command.
  C.  Pre-fetched less than minimum pre-fetch:
        Continue pre-fetch at least until minimum pre-fetch is
        read.
  D.  Pre-fetched more than minimum, less than maximum pre-fetch:
        Undefined.

In particular, if minimum pre-fetch is not 0, there is no
requirement that pre-fetch be aborted when a new command is ready
to execute (though any reasonable implementation would do so).

I think that the paragraph stated above should be changed to:

     "If pre-fetch has read more than the amount of data
  specified by the minimum pre-fetch then pre-fetching should be
  terminated whenever another command is ready to execute.  This
  consideration is ignored when the minimum pre-fetch is equal to
  the maximum pre-fetch."