4-3.18.2 Read Reverse Check Condition Status - Fujitsu M2488 Product Manual

M2488 PRODUCT GUIDE
4-3.18.2 READ REVERSE CHECK CONDITION Status
If the fixed bit is one, then the Mode Parameter Block Descriptor Block Length must be non-zero.
Otherwise CHECK CONDITION status is reported with the sense key as ILLEGAL REQUEST.
See description of MODE SENSE for more details about the Mode Parameter Descriptor Block.
For data in EDRC format, it is impossible to transfer the bytes of blocks in reverse order for the
tape format being used because of data packeting and data compression. If a block is to be read in
the reverse direction, instead of transferring the block's bytes in reverse order, the tape unit spaces
backward over the block to be read (logically positions to the BOT side of the block to be read)
and then CHECK CONDITION status is reported. Within the sense data reported, the sense key is
ILLEGAL REQUEST, the ASC and ASCQ are 30h and 02h respectively (indicating 'Cannot
Read Medium - Incompatible Format'), and the Host ERPA code is 26. After receipt of this sense
data, the initiator is expected to read forward to transfer the bytes of the block and then space
backward one block (i.e. issue a READ command followed by a SPACE command, Code = 0 and
Space count of -1.) Note: Even if more than one block is requested to be transferred by the READ
REVERSE command in fixed block mode, the tape unit only spaces backward one block.
This procedure is performed regardless of the tape format (i.e., 18-
track clear data).
If the SILI bit is set to 1 and the Fixed bit is set to 0 then the tape unit checks for overlength condi-
tions; however since the tape unit will never transfer bytes for the READ REVERSE command, an
overlength condition for a READ REVERSE command will never occur. If the SILI bit is set to 1
and the Fixed bit is set to 1 then CHECK CONDITION status is reported. In the sense data
reported, the sense key is set to ILLEGAL REQUEST and the additional sense code is set to
INVALID FIELD IN CDB.
If a filemark is encountered when reading in the reverse direction then CHECK CONDITION sta-
tus is reported. In the sense data reported, the sense key is NO SENSE, the valid bit is 1 and the
information field will equal the requested Transfer length. Upon termination, the logical position
is the BOT side of the filemark encountered.
If the logical unit encounters BOT during a READ REVERSE command then CHECK CONDI-
TION status is reported. In the sense data reported, the sense key is NO SENSE, the EOM bit is set
to 1, the valid bit is 1 and the information field will equal the requested Transfer length.
April 1997
** NOTE **
CG00000-011503 REV. A
TAPE UNIT SCSI COMMANDS
4-81