To:    John Lohmeyer, Chairman X3T9.2

From:  Dan Davies
       Cipher Data Products

Subj:  PF Bit Requirement

The Page Format (PF) bit was incorporated into SCSI-2 to address a potential 
compatibility problem between SCSI-2 targets and initiators designed prior to 
SCSI-2.  This bit is required in the MODE SELECT command to allow a SCSI-2 
target to determine whether or not the data sent by an initiator, following 
the header and any block descriptors, complies to page format.  Without a PF 
bit in the CDB, it was possible for a target complying to page format to 
mistake vendor unique data as a page descriptor.  The PF bit was also included 
into the Common Command Set, Revision 4B, since CCS devices introduced the 
concept of page format and were exposed to the same potential problem.

The requirement for a PF bit in commands using page format is essential for 
those commands involving initiator-supplied data (e.g. MODE SELECT, SEND 
DIAGNOSTIC) in order to meet the compatibility objective of SCSI-2.  However, 
it is not clear that the PF bit is required for commands using page format 
involving target-supplied data (e.g MODE SENSE, RECEIVE DIAGNOSTIC RESULTS, 
ACCESS LOG).

In the case of MODE SENSE, the PC and Page Code fields in byte two of the CDB 
provide any explicit selection of page format.  If both fields are set to 
zero, as was the case in SCSI-1, vendor unique format is explicitly requested 
and any data following the block descriptors may or may not conform to page 
format.

In the case of RECEIVE DIAGNOSTIC RESULTS, since this command must follow a 
SEND DIAGNOSTIC  command no confusion exists over the nature of the returned 
data format.

In the case of the ACCESS LOG command, no problem exists since this is a new 
command that was reserved under SCSI-1.

Based on these conclusions, the following proposal is suggested:

1. Delete PF bit requirement for MODE SENSE command.
2. Delete PF bit requirement for RECEIVE DIAGNOSTIC RESULTS command.
3. Delete PF bit requirement for ACCESS LOG command.

As a final note, I would like to point out that requiring a PF bit in MODE 
SENSE creates an inherent incompatibility between SCSI-2 and CCS, since CCS 
only defines the PF bit for MODE SELECT and maintains the location in MODE 
SENSE as a reserved field.  This incompatibility with CCS may be of lesser 
concern, but I point it out in case it has been overlooked by other concerned 
parties.