Date: 15 March 1991                   X3T9.2/91-___ Rev 0 
         
         
        To:   X3 Secretariat/CBEMA
              Lynn Barra, Administrator, Stds. Processing
              311 First Street, N.W.
              Suite 500
              Washington, DC 20001-2178
         
        From: Doug Hagerman
              Digital Equipment Corporation
              SHR 1-3/E29
              333 South Street
              Shrewsbury, MA  01545
              Phone: 508-841-2145  FAX: 508-841-2604
         
        Subject: Request for Interpretation of the SCSI-2 (X3.131-199x)
              document on the use of MODE SENSE data.

         



        1  PROBLEM STATEMENT

        The MODE SENSE command may be sent with the following values
        in order to determine which parameters are changeable.

        1A 00 7F 00 FF 00

        DBD (disable block descriptor) = 0
        PC = 1 (changeable)
        Page Code = 3F (all pages)
        allocation length = FF


        The description in section 7.2.10.2 (page 7-35) of the
        document clearly states that for the mode pages, the page
        code, page length, and mask bits for every page should be
        returned.  However the document does not specify what should
        be returned in the block descriptors (section 7.3.3, page
        7-73.)

        One possibility is that a mask for the changeable fields in
        the block descriptors should be returned.  Another possibility
        is that the actual values for the Number of Blocks and Block
        Length fields in the block descriptors should be returned.

        Digital has found that devices from different manufacturers
        handle this case in different ways.  In one case, the device






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        returns FFFFFF for the block size implying that the Block size
        in the Block Descriptor is changeable.  In another case, the
        device returns the same Block Descriptor it would have if the
        the PC had been set to CURRENT, implying that changeable
        fields are not reported for the Block Descriptor.



        2  REFERENCES

        The relevant sections of the document are quoted below.

        7.2.10.2. Changeable Values

        A PC field value of 1h requests the target to return the
        changeable parameter mask for the page code specified.  The
        page requested shall be returned containing information that
        indicates which parameters are changeable.  All bits of
        parameters that are changeable shall be set to one.  All bits
        of parameters that are target defined (not changeable by the
        initiator) shall be set to zero.

        NOTE:  An attempt to change a target-defined parameter (via
        MODE SELECT) results in an error condition (see 7.2.8).

        IMPLEMENTORS NOTE:  The initiator should issue a MODE SENSE
        command with the PC field set to 1h and the page code field
        set to 3Fh to determine which pages are supported, which
        parameters within the pages are changeable, and the supported
        length of each page prior to issuing any MODE SELECT commands.

        7.3.3. Mode Parameters 

        This section describes the block descriptors and the pages
        used with MODE SELECT and MODE SENSE commands that are
        applicable to all SCSI devices.  Pages specific to each device
        type are described in the third subsection of each device-type
        section (i.e., 8.3, 9.3, etc.).

        The mode parameter list shown in Table 7-60 contains a header,
        followed by zero or more block descriptors, followed by zero
        or more variable-length pages.  Parameter lists are defined
        for each device type.