To:  The SCSI Committee                       X3T9.2/88-116 Rev 0

From: Jeff Stai, Western Digital Corp.

Subject: WDTR Disable Residue function

     I seem to recall that during one of our recent working 
groups we decided to add a bit to disable the IGNORE WIDE RESIDUE 
message. Since this thought disappeared, I thought I would bring 
it up again for more formal consideration. The proposed text 
follows the style of the wording of the DiscPriv bit in the 
IDENTIFY message, and as such should be non-controversial (!).

     This new bit is useful for cases when the initiator does 
everything as word or double-word values, and never needs to be 
bothered with the IGNORE message. These initiators would either 
have information in the data itself as to what is invalid, or it 
would be a situation where it doesn't care. I do not propose any 
"implementors note" to define when it is appropriate to use this 
bit, but I would certainly welcome such a thing. Also, I do not 
care where the function goes; if it belongs in byte 4, so be it.

     As a side note: In order to get the disable residue function 
in as a bit, I had to use the following table format for an 
extended message. I believe this new format should be adopted for 
all extended messages, since it matches all of the other tables 
in the document and is therefore more "Architecturally Pure".


                          WIDE DATA TRANSFER MESSAGE
==============================================================================
  Bit|   7    |   6    |   5    |   4    |   3    |   2    |   1    |   0    |
Byte |        |        |        |        |        |        |        |        |
==============================================================================
 0   |                           Extended message code (01h)                 |
-----|-----------------------------------------------------------------------|
 1   |                           Extended message length (02h)               |
-----|-----------------------------------------------------------------------|
 2   |                           WIDE DATA TRANSFER REQUEST code (03h)       |
-----|-----------------------------------------------------------------------|
 3   | DisRes |                  Reserved                  | Transfer Width  |
==============================================================================

[additional text:]

     A disable residue message bit (DisRes) bit of zero indicates 
that the target shall send the IGNORE WIDE RESIDUE message when 
it is appropriate, as defined under that message. When the DisRes 
bit is one, the target shall never send the IGNORE WIDE RESIDUE 
message to the initiator. This bit is not defined and shall be 
set to zero when a WDTR message is sent by a target. The DisRes 
bit shall not be rejected by a target that supports the WDTR 
message. [this last sentence should be fun...!]

88-116r0.ws