February 20, 1989

To:     X3T9.2 Committee

From:   Dan Davies     Cipher

Subj:   Tape Read Overlength Detection w/SILI Bit


The  following proposal offers revised wording for the sequential-access  READ 
command to allow for block overlength detection during read operations,  using 
the  SILI  bit.   This method is submitted as an alternative to  the  new  bit 
definition proposed in document 89-021r0.

Revise the wording of the sequential-access READ command (9.2.5) on page  9-17 
of SCSI-2 Revision 7, as follows:


READ Command

     Peripheral Device Type:  Sequential-Access
        Operation Code Type:  Mandatory

                            READ Command

==============================================================================
  Bit|   7    |   6    |   5    |   4    |   3    |   2    |   1    |   0    |
Byte |        |        |        |        |        |        |        |        |
==============================================================================
 0   |                           Operation Code (08h)                        |
-----|-----------------------------------------------------------------------|
 1   |   Logical Unit Number     |        Reserved         |  SILI  | Fixed  |
-----|-----------------------------------------------------------------------|
 2   | (MSB)                                                                 |
-----|---                                                                 ---|
 3   |                           Transfer Length                             |
-----|---                                                                 ---|
 4   |                                                                 (LSB) |
-----|-----------------------------------------------------------------------|
 5   |                           Control Byte                                |
==============================================================================

  The  READ command (Table 9-6) requests that the target transfer one or  more 
block(s) of data to the initiator beginning with the next block on the logical 
unit.

  The  fixed bit specifies whether fixed-length or variable-length blocks  are 
to  be  transferred.   Refer  to the READ BLOCK  LIMITS  command  (9.2.6)  for 
additional information about fixed and variable block mode.

  If the fixed bit is one, the transfer length specifies the number of  fixed-
length  blocks to be transferred, using the current block length  reported  in 
the  mode  parameters block descriptor (7.5.1).  If the fixed bit is  zero,  a 
variable-length  block  is requested with the transfer length  specifying  the 
maximum number of bytes allocated for the returned data.

  A  successful  READ  command  with a fixed bit of  one  shall  transfer  the 
requested  transfer  length  times the current block length in  bytes  to  the 
initiator.  A successful READ command with a fixed bit of zero shall  transfer 
the requested transfer length in bytes to the initiator.  Upon completion, the 
logical  position shall be after the last block transferred  (end-of-partition 
side).

    If the suppress incorrect length indicator (SILI) bit is one,  the  target 
shall not return CHECK CONDITION status if the only error is that the transfer 
length  is  not equal to the actual block length.  This option is  only  valid 
when the fixed bit is zero.

  If  the suppress incorrect length indicator (SILI) bit is one and the  fixed 
bit  is  zero, the target shall not report CHECK CONDITION when  an  incorrect 
length  block is read if the only error is that the requested transfer  length 
does  not  match  the  actual block length.  The  target  shall  report  CHECK 
CONDITION status for the incorrect length condition if the length of the block 
exceeds  the current value of the block length field (in the  mode  parameters 
block  descriptor, see 7.3.3).  The ILI and valid bits shall be set to one  in 
the  sense  data  and the information field shall be  set  to  the  difference 
(residue)  of  the requested transfer length minus the  actual  block  length.  
Targets that do not support negative residues shall set the information  field 
to  zero if the actual block length is larger than the requested  length.   If 
the  current  value of the block length field is zero, the  target  shall  not 
return CHECK CONDITION status for this condition.

  IMPLEMENTORS  NOTE:  Since the residue information normally provided in  the 
  information  field of the sense data may not be available when the SILI  bit 
  is set, other methods for determining the actual block length should be used 
  (e.g., length information could be included in the data block).

  If the SILI bit is one and the fixed bit is one, the target shall  terminate 
the  command  with CHECK CONDITION status and the sense key shall  be  set  to 
ILLEGAL REQUEST with an additional sense code of INVALID FIELD IN CDB.

  If  the  SILI  bit  is zero and an incorrect length  block  is  read,  CHECK 
CONDITION status shall be returned and the ILI and valid bits shall be set  to 
one in the sense data.  Upon termination, the logical position shall be  after 
the  incorrect length block (end-of-partition side). If the fixed bit is  one, 
the  information  field  shall  be set to  the  difference  (residue)  of  the 
requested  transfer  length  minus  the actual  number  of  blocks  read  (not 
including  the  incorrect  length  block).  If the  fixed  bit  is  zero,  the 
information  field shall be set to the difference (residue) of  the  requested 
transfer  length minus the actual block length.  Targets that do  not  support 
negative residues shall set the information field to zero if the actual  block 
length is larger than the requested length.