95-103r2 -- Attached Medium Changer Operation Codes

ROWEBER at acm.org ROWEBER at acm.org
Sat Mar 11 14:52:45 PST 1995


                                                   X3T10/95-103R2

To:       Membership of X3T10

From:     Ralph O. Weber           ROWeber at ACM.org
          ENDL Associates

Date:     March 11, 1995

Subject:  Attached Medium Changer Operation Codes


As has been noted for sometime now, there is a conflict between
operation code usages for CD-ROM devices and attached medium
changers.  This proposal, if approved, will resolve that
conflict.

I believe that approved document X3T9.2/92-006r1 (Enhancement of
Command Sets for Removable Media Devices with Medium Changers)
intended that CD-ROM device be capable of having attached medium
changers.  My belief was confirmed (by lack of any opposing
comment) at the January 1995 X3T10 Working Group meeting.

However, even in SCSI-2 R10C the MOVE MEDIUM command shared
operation code A5h with the PLAY AUDIO(12) command.  During
preparation of the SCSI-3 Multi-Media Commands, the problem was
compounded by making the READ ELEMENT STATUS command share
operation code B8h with the SET CD-ROM SPEED command.

As a result, CD-ROM devices cannot have attached medium changers. 
In particular, the MOVE MEDIUM and READ ELEMENT STATUS read
element status commands cannot be directed to a LUN with peri-
pheral device type code 05h.  The setting of the MChngr bit in
the INQUIRY data is irrelevant.  The LUN will not be able to
distinguish MOVE MEDIUM from PLAY AUDIO(12) or READ ELEMENT
STATUS from SET CD-ROM SPEED.

N.B. CD-ROM devices may still implement a medium changer as a
separate LUN.  However, the requirement to do this is absolutely
counter to the intent of approved document X3T9.2/92-006r1 and,
apparently, counter to the current wishes of the X3T10 Working
Group.  Furthermore, the X3T10 Working Group has shown no
interest whatsoever in reversing its prior approval of 92-006r1.

I propose that the conflict be fixed by assigning different
operation codes to the MOVE MEDIUM and READ ELEMENT STATUS
commands for their attached medium changer usage.  This proposal
was generally endorsed at the January 1995 X3T10 Working Group
meeting.  However, a couple of conditions were attached to that
endorsement.  First, the usage of operation code A5h for the MOVE
MEDIUM command and B8h for the READ ELEMENT STATUS command must
continue to be sanctioned for medium changers attached to tape
devices.  Second, the new attached changer operation codes for
the MOVE MEDIUM and READ ELEMENT STATUS commands must be
completely unique (not shared with any device type).

With these requirements in mind, I propose that Table B.2 of the
SCSI-3 Primary Commands be updated as follows:

    OP  DTLPWRSOMCA  Description
    A5h  O      M    MOVE MEDIUM
    A7h OO  OO OO    MOVE MEDIUM (attached)
    B4h OO  OO OO    READ ELEMENT STATUS (attached)
    B8h  O      M    READ ELEMENT STATUS

If approved, this proposal also requires equivalent changes in
the Table 5 of the SCSI-3 Primary Commands, changes in all the
SCSI-3 command documents that list the attached medium changer
commands, and changes in the SCSI-3 Medium Changer Commands
document.

In all cases (particularly the SPC and SMC), the A7h and B4h
operation codes must be described as the preferred usage for the
MOVE MEDIUM and READ ELEMENT STATUS commands, respectively, for
attached medium changer implementations.  However, usage of the
A5h and B8h operation codes must be described as permissible for
older tape device implementations.




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