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TAPE UNIT SCSI COMMANDS M2488 PRODUCT GUIDE
4-80 CG00000-011503 REV. A April 1997
4-3.18 READ REVERSE command 0Fh
The READ REVERSE command requests that the tape unit transfer blocks of data to the initiator.
Any buffered write data or filemarks are written before this operation is executed. This command is
similar to the READ command except that the direction of the read is reversed; blocks are transferred
starting from the current position and progressing towards BOT.
** NOTE **
It is recommended that this command not be used extensively. The
overall execution time of the READ REVERSE command is exces-
sive due to the extra tape positioning involved.
4-3.18.1 READ REVERSE CDB Description
READ REVERSE is a six-byte command. The bytes are shown below and described in Table 4-
74. Common fields are described in paragraph 4-3.1 on page 4-4.
BITS
BYTES 7 6 5 4 3 2 1 0
0
0 0 0 0 1 1 1 1
1
LUN Reserved SILI Fixed
2
MSB
Transfer Length
3
4
LSB
5
Reserved Flag Link
Note: Changeable fields in the CDB are shaded.
Table 4-74. READ REVERSE Field Description
BYTE BIT VALUE DESCRIPTION
0 0-7 0Fh Operation code.
1 0 1 or 0 The Fixed bit specifies both the meaning of the transfer length field and
whether fixed-length or variable-length blocks are to be transferred.
If the Fixed bit is zero, a single block is transferred with the bytes transferred
being the lesser of the actual block length or the requested transfer length.
If the Fixed bit is one, the transfer length specifies the number of blocks to be
transferred to the initiator.
1 1 1 or 0 Suppress Incorrect Length Indication (SILI) flag.
2-4 The Transfer Length indicates the number of bytes or blocks to transfer. The
block length used is the current block length specified in the mode parame-
ters block descriptor (refer to the Mode Select Block Descriptor in Table 4-
47 on page 4- 53.)
When the transfer length is zero, no data is transferred and the current posi-
tion on the logical unit is not changed. This condition is not considered an
error.