Using Fixed And Variable Length Blocks; Variable And Fixed Length Blocks; Writing - Tandberg Data SLR7-140 PROGRAMMER Manual

Tandberg Data
2.6.

Using Fixed and Variable Length Blocks

2.6.1.

Variable and Fixed Length Blocks

The Drive is capable
variable length logical blocks. The concept of fixed or
variable mode for
indicates the method by which the Initiator specifies
the size of the logical block for transfer and not the
method of recording physical blocks on the medium.
When writing the Drive transforms logical blocks passed
from the Initiator into physical blocks to be stored on
the tape. The length of a logical block may vary while
the length of a physical block is defined by the tape
format in use (see [9] - [14]). When reading the Drive
transforms physical
logical blocks to be transferred to the Initiator,
When executing read and write commands two parameters
must be specified; the block length and the number of
blocks to read or write. As the SCSI read and write
commands only have room for a single parameter, only the
block length or the number of blocks can be specified
directly. For this
versions of each command capable of transferring tape
data to or from the Initiator. The fixed length type
commands can specify
processed. For these commands all blocks will be of
equal size. The actual size is given by the current
value in the Block Size field in the Block Descriptor
List of the MODE SELECT command. The variable length
type commands can specify the length of the individual
blocks, but the number of blocks written is always only
one (for each command).
When the Block Size field has a value different from
zero, the Drive is said to be in fixed block mode. In
this mode both the fixed length type commands and the
variable length
Block Size field is zero, the Drive is said to be in
variable block mode. In this mode only the variable
length type commands
length blocks of size zero bytes does not make any
sense).
Note that the
command differs
fixed/variable mode set by the MODE SELECT command.
2.6.2.

Writing

When writing, the Drive groups the data transferred from
the Initiator into blocks. Data blocks may be written
with two different versions of the WRITE command; one
with the fixed (FIX) bit set and one with the fixed
(FIX) bit cleared.
When the FIX bit is cleared, a WRITE command will write
a single block. The block length may be specified on a
block-by-block basis. This is useful when writing blocks
of varying length.
2-17 Tandberg SLR Product Line SCSI Functional Specifications
of supporting
writing and
blocks from
reason there
the number
type commands are
are allowed
illegal length
slightly depending
About Tape Streamers
both fixed
reading blocks
the tape back
are two different
of blocks
allowed. When
(as writing
handling in the
on the current
and
only
into
to be
the
fixed
READ