To:       John Lohmeyer                          X3T9.2/87-188
          Chairman, X359.2

From:     Robert Snively
          Adaptec
          580 Cottonwood
          Milpitas, CA 95037

Date:     October 22, 1987

Subject:  Explicit selection of target functions

In  general,  the proper view of a peripheral device from a  host 
system  is  as a set of logical units.   Each logical unit has  a 
logical model which it implements.   The logical model of a  disk 
device  is  typically a set of constant length blocks  which  are 
addressable  by  a  contiguous  set of  logical  block  addresses 
ranging  from  zero  to the maximum block on  the  logical  unit.  
Other logical models are provided for sequential devices, optical 
memory devices,  and other devices.  The target is conceptually a 
path  processor for a set of logical units.   The target provides 
only an electronic interface and an arbitration entity.   As  far 
as a normal operating system environment is concerned, the target 
is only a transparent node on the way to the logical unit.

In practice,  certain diagnostic and system maintenance utilities 
may  occassionally  require  access to processes running  on  the 
target.  One such requirement has been identified, but others may 
later appear.   The particular example is the use of the  INQUIRY 
command  to  obtain model and revision level information  and  to 
obtain  Vital  Product  Data (VPD)  unique  to  the  target.   By 
providing  a mechanism for explicitly accessing the target,  this 
requirement  and  any related requirements in the future  can  be 
met.   It  is  possible that the target may eventually  become  a 
separate  peripheral type,  but for the  present  proposal,  only 
certain commands are supported by a target.   The target is  only 
allowed  to support vendor unique commands and the INQUIRY,  READ 
BUFFER, READ LOG, RECEIVE DIAGNOSTIC RESULTS, REQUEST SENSE, SEND 
DIAGNOSTIC, TEST UNIT READY, and WRITE BUFFER commands

The following text is added to the SCSI-2 document to include the 
explicit target selection capability.
  
The  text  on  page  5-18 describing  the  IDENTIFY   message  is 
modified in the following manner.

IDENTIFY   80h to FFh.    These messages are sent by  either  the 
initiator or the target to establish the physical path connection 
between  an  initiator and target for a particular logical  unit.  
The logical unit number addresses one of up to eight physical  or 
virtual  devices attached to a target.   The IDENTIFY message  is 
also  used  to establish a connection explicitly to one of up  to 
eight  processes  or  functions  in  the  target  itself.    Such 
target functions have no access to user data and are expected  to 
be used for maintenance and diagnostic purposes.

Bit  7.   This  bit  shall be set to  one  to  distinguish  these 
messages from the other messages.

Bit  6.    This bit is only set to one by the initiator to  grant 
the target the privilege of disconnecting.   If this bit is zero, 
the target shall not disconnect.   This bit shall be set to  zero 
if sent by the target.

Bit 5.    Reserved

Bit 4.    This bit,  the Target/LUN bit, shall be set to identify 
a target process.   If the bit is set to one, the operation to be 
executed is intended to be executed strictly by and on behalf  of 
the  target.   If the bit is set to zero,  the operation is to be 
executed  on behalf of the Logical Unit,  using services of  both 
the target and LU.  If a target process is not supported, and the 
bit is set to one,  the message shall be rejected.  In that case, 
the target will not continue the operation but will return to the 
BUS FREE phase after the MESSAGE REJECT is transferred.

Bit 3    Reserved

Bit  2-0    These  bits  specify a logical  unit  number  if  the 
Target/LUN bit is zero.    If the Target/LUN bit is one, the bits 
specify a process in the target.

Only  one  logical unit number or target process number shall  be 
identified  for  any one selection sequence;  a  second  IDENTIFY 
message with a new logical unit number shall not be issued before 
the  bus has been released (BUS FREE phase).   The initiator  may 
send  one or more IDENTIFY messages during a selection  sequence.  
However,  the logical unit number or target process number in any 
additional  IDENTIFY  messages shall be the same as  the  logical 
unit  number specified in the first IDENTIFY message sent by  the 
initiator.  

When  sent from a target to an initiator during reconnection,  an 
implied  RESTORE  POINTERS  message shall  be  performed  by  the 
initiator prior to completion of this message.

If any target processes are supported,  at least target process 0 
is required.  The support of other processes is optional.  Target 
processes are allowed to execute only the following commands:

                    Vendor Target Unique maintenance and 
                         diagnostic commands
                    INQUIRY
                    READ BUFFER
                    READ LOG
                    RECEIVE DIAGNOSTIC RESULTS 
                    REQUEST SENSE
                    SEND DIAGNOSTIC
                    TEST UNIT READY
                    WRITE BUFFER



The above text should completely describe and clarify the concept 
of accessing a target directly and independently of the LU.  Note 
that all subsequent references to a target/LUN bit,  specifically 
in VPD of the INQUIRY command should be removed.


Sincerely,




Robert N. Snively