Proposal for Control Mechanism for Reserved Handling

John Lohmeyer jlohmeye at ncr-mpd.FtCollinsCO.NCR.COM
Wed Jun 1 23:54:07 PDT 1994


Accredited Standards Committee
X3, Information Processing Systems
                                          Doc. No.:  X3T10/94-130 r0
                                              Date:  June 1, 1994
                                           Project:  995-D
                                         Ref. Doc.:  
                                          Reply to:  Mr. John Lohmeyer
                                                     AT&T/NCR Microelectronics
                                                     1635 Aeroplaza Dr.
                                                     Colo Spgs, CO 80916
                                                     (719) 573-3362

To:            X3T10 Membership
Subject:       Proposal for Control Mechanism for Reserved Handling

At the May 1994 X3T10 meeting, a motion to reverse the sense of the current
reserved field checking requirements in SCSI-3 passed by a vote of 16:10. 
The voting margin was too close to believe that the action represents a real
consensus on the topic.  I believe that many of those in the minority on this
action (including me) could be pursuaded to join the majority if there were
some explicit mechanism to control this behavior.  Consequently, I propose
that the section on "Reserved" be revised and a DRC bit be added to bit 3 of
byte 4 in the Control Mode Page in SPC as follows:

x.x.x Reserved

Reserved bits, fields, bytes, and code values are set aside for future
standardization.  Their use and interpretation may be specified by future
extensions to this standard.  A reserved bit, field, or byte shall be set to
zero, or in accordance with a future extension to this standard.  A target
that receives a reserved bit, field, or byte that is not zero or receives a
reserved code value shall:

     a) If the DRC bit in the control mode page is one, the target shall
        ignore nonzero reserved bits, fields, and bytes.  The target may,
        at its discretion, map reserved code values to supported code values
        or terminate the command with CHECK CONDITION status and a sense key
        of ILLEGAL REQUEST.  

     b) If the DRC bit in the control mode page is zero, the target shall
        terminate the command with CHECK CONDITION status and a sense key
        of ILLEGAL REQUEST.

It shall also be acceptable for a target to interpret a bit, field, byte, or
code value in accordance with a future extension to this standard. 

                               Control mode page
+=====-========-========-========-========-========-========-========-========+
|  Bit|   7    |   6    |   5    |   4    |   3    |   2    |   1    |   0    |
|Byte |        |        |        |        |        |        |        |        |
|=====+========+========+=====================================================|
| 0   |   PS   |Reserved|         Page code (0Ah)                             |
|-----+-----------------------------------------------------------------------|
| 1   |                           Page length (06h)                           |
|-----+-----------------------------------------------------------------------|
| 2   |                           Reserved                           |  RLEC  |
|-----+--------------------------------------------------------------+--------|
| 3   |     Queue algorithm modifier      |    Reserved     |  QErr  |  DQue  |
|-----+-----------------------------------+-----------------+--------+--------|
| 4   |  EECA  |              Reserved    |   DRC  | RAENP  | UAAENP | EAENP  |
|-----+-----------------------------------------------------------------------|
| 5   |                           Reserved                                    |
|-----+-----------------------------------------------------------------------|
| 6   | (MSB)                                                                 |
|-----+---                        Ready AEN holdoff period                 ---|
| 7   |                                                                 (LSB) |
+=============================================================================+

A disable reserved checking (DRC) bit of one specifies that the target shall
not perform checking of reserved bits, fields, and bytes.  The target may
perform checking of reserved code values or may map reserved code values into
supported code values, at its discretion.

A DRC bit of zero specifies that the target shall perform checking of
reserved bits, fields, bytes, and code values as described in x.x.x.

-- 
John Lohmeyer                      E-Mail:  John.Lohmeyer at FtCollinsCO.NCR.COM
NCR Microelectronics                Voice:  719-573-3362
1635 Aeroplaza Dr.                    Fax:  719-597-8225
Colo Spgs, CO 80916              SCSI BBS:  719-574-0424 300--14400 baud




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