note:  DOCUMENT X3T9/87-093 R0

.fo left
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Date: June 19, 1986              

To:   John B. Lohmeyer, X3T9.2 Chair
      NCR Corporation
      Wichita, Kansas

From: Gary R. Stephens
      IBM Corporation
      Tucson, Arizona

Subject: Sequential Access Device Position Definition

Enclosed please find on the diskette containing revised text for
the Tape Model another file containing text proposed for addition
to SCSI-2 to define the physical and logical positions after
certain commands or actions by the target.
The diskette is formatted for an IBM PC, DOS, Release 2.0,
in a double sided format.

The file of interest is SCSISPOS.SCT.

The SCSI architecture does not crisply define the logical and
physical positions of a sequential access device after certain
commands or actions.  The attached pages are requested for
review by the committee for inclusion into the SCSI-2 standard.

Attached is a print of the file for inclusion
in the next mailing after you assign an appropriate number.





Gary R. Stephens



.*
.*
:h2.Positioning for Motion-Related Commands
.*
.*
:p
Positioning is relative to recorded objects or the beginning of a
partition (BOPx).
.*
:h3.Mount or Rewind
.*
When a volume is first mounted and made ready, or is rewound, the
medium shall be positioned at BOM (also BOP1) such that the
first recorded object of the default data partition can be read
or written.
.*
:h3.Partition Transition
.*
A command to change partitions without specifying a location
within the partition shall position at BOPx.  If the command
contains valid position information in the partition then the
device shall attempt to position before the object.
(See Normal Position Operation Completion.)
If either the partition or the location within a partition cannot
be located then the new position is indeterminate.
:p
A partition transition defines a semi-permanent state change similar
to a physical volume change.  Once the transition is complete,
all subsequent commands shall execute in the specified partition.
:p
When a transition is made to a new partition, all non-partition
specific attributes and operating conditions associated with the
mounted volume shall persist in the new partition.
.*
:h3.Normal Data Operation Completion
.*
After successful completion of a read, write, or space operation,
the logical position shall be to process the next recorded object
in the same direction as the completed operation.
.*
:h3.Abnormal Data Operation Completion
.*
After an unsuccessful read (including spacing),
or write operation for which all
specified automatic error recovery (within the device) has been
performed, the logical position shall be such that the recorded object
that caused the exception can be read in a direction opposite from
that which was used when the exception occurred.
.*
:h3.Normal Position Operation Completion
.*
After successful completion of an explicit position operation
to a recorded object (locating) , the logical position
shall be before the object.  If the recorded object cannot be
found, it is sufficient to position after the previous recorded object.
.*
:h3.Abnormal Position Operation Completion
.*
If an error is detected while performing an explicit positioning
operation, the final logical position is defined in the vendor
specification.
.*
:h3.Filemark Detected
.*
If a filemark is detected while performing a read or space operation,
the logical position shall be such that the next sequential recorded
object in the same direction can be processed.
.*
:h3.BOPx Detected (Reverse Operations)
.*
If BOPx is detected while reading, spacing, or positioning backward
to a specific recorded object, the logical position shall be at BOPx.
.*
:h3.EMW or EEMW detected
.*
Position relative to EMW and/or EEMW is command dependent and
defined with the individual command descriptions.
.*
:h3.PEOPx Detected
.*
If PEOPx is detected while moving the medium in a forward direction,
the logical and physical position is defined in the vendor
specification (the medium may have left the tape path).
.*
:h3.Interface Error During Data Transfer
.*
In each instance below, the device has attempted to transfer at
least one byte between the device and the initiator.  The error
may occur on the first data byte transfer attempted.
:p
For buffered write operations or unbuffered write operations with a
buffered device, if an error is detected on the
interface between the initiator and the data buffer, the logical
position shall be before the data block being transferred
(i.e., as if the attempt to transfer it had not been performed).
The initiator may begin recovery on a data block boundary.
Note: This position implies a SAVE DATA POINTERS message is sent
to the initiator
at the end of each data block received from the initiator.
:p
For buffered read operations, if an error is detected on the
interface between the initiator and the data buffer, the logical
position shall be before the data block being transferred
(i.e., as if the attempt to transfer it had not been performed).
The initiator may begin recovery on a data block boundary.
Note: This position implies a SAVE DATA POINTERS message is sent
to the initiator
at the end of each data block received from the initiator.
:p
For unbuffered write operations with an unbuffered device,
the position may be before or
after the data block in error depending on whether write activity
had commenced.  For devices with a storage area that does not
qualify as a data buffer, some data may be transferred to the
device before physical motion is started for the medium.
Refer to the vendor specification.
:p
For unbuffered read operations with an unbuffered device,
the position is
after the data block in error.  At least some portion of the
data block must be read to attempt to transfer the first
byte of data.