Save Data Pointer questions

Miller, Jim jmiller at
Fri Mar 7 06:37:00 PST 1997

* From the SCSI Reflector (scsi at, posted by:
* "Miller, Jim" <jmiller at>


According to the SCSI-2 rev. H copy of the standard, section 5.6.6 2nd para. 
seems to say there SHALL (on disconnecting devices) be a SAVE DATA POINTER - 
DISCONNECT message after each successful connection (except possibly the 
last).  This would say that after the command phase there should be one.  If 
one is to look at what the message 02 SAVE DATA POINTER message does, it 
only saves the DATA POINTER not the COMMAND or STATUS pointer, section 
5.6.20, so it does not make sense for a target to do a 0204 sequence after 
the COMMAND phase, but that's not what the standard says.  Generally you 
don't see the SAVE DATA POINTER after
the COMMAND phase, but it seems I have seen this before.  Because there is 
an implied RESTORE pointer message on a reconnect, the COMMAND and STATUS 
(and DATA) pointers are restored to the last saved values, so there is not a 
need to do the SAVE DATA pointer message after the COMMAND phase, but SCSI 
standard does say SHALL send 0204 sequence after all successful connections. 
 Your call.

                         Jim Miller
>From: scsi-owner
>To: 'SCSI reflector post'
>Subject: Save Data Pointer questions
>Date: Thursday, March 06, 1997 4:26PM
>* From the SCSI Reflector (scsi at, posted by:
>* "Koller, James F             TR" <JFK1 at>
>I am working on a multi-processing platform that interfaces to a mainframe
>host as a SCSI target device.  I use the Save Data Pointer - Disconnect
>message-in sequence to allow target disconnects on Read commands.  When it
>occurs, the disconnect sequence immediately follows the command phase in
>which the Read is issued.  The target follows with a reselection to 
>the command when data is available.  There is never more than one of these
>disconnect-reselect pairs per Read command.
>My questions involve the need for a Save Data Pointer message.  Our
>proprietary SCSI host adapter apparently does not require it in the manner
>that I do disconnects.  A commodity host adapter I have recently interfaced 

>to also does not require it.  In the latter case, however, the Save Data
>Pointer actually has an occasional adverse effect and is therefore
>Does the industry standard not require a Save Data Pointer message in the
>way that I use it (i.e. sent once before any data is transferred) ?
>How can I be sure that, if I omit the Save Data Pointer message, no other
>commodity host adapters I try will have problems?
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