Date: Nov 15, 1990                            X3T9.2/90-183 Rev 0


    To: X3T9.2 Committee (SCSI)

    From: George Penokie (IBM)

    Subject: Increasing LUN Addressability


    Problem:

    In the near future array controllers are going expand to the point
    where the number of addressable devices attached to a single controller
    will exceed the number of addressable LUNs allowed in SCSI.

    Solution:

    SCSI-3 should increase the number of addressable LUNs to 32 by
    reclaiming bits 3 and 4 of the Identify message.  The only other place
    where the LUNs are still used is in the copy commands segment
    descriptors.  The current layout of those segment descriptors does not
    allow any bits to expand the LUN addressability.  However, the new
    segment descriptors which have been defined in document 90-048 do allow
    for an expansion of the LUN addressability.

    Proposal:

    -Make bits 3 and 4 of the Identify message part of the LUN field.
    -Modify the copy commands new segment descriptors in document 90-048
     to allow for 32 LUNs.