User's Guide

Page 96 DDS-4 SCSI Interface Manual, Rev. B
Byte
Bits
7 6 5 4 3 2 1 0
8 Decompression Algorithm
9 Decompression Algorithm
10 Decompression Algorithm
11 Decompression Algorithm
12 0 0 0 0 0 0 0 0
13 0 0 0 0 0 0 0 0
14 0 0 0 0 0 0 0 0
15 0 0 0 0 0 0 0 0
DC Control Page Field Descriptions
The following table describes the fields in the DC Control page.
Field Name Bytes Bits Description
DCE 2 7 The DCE (Data Compression Enable) bit controls data
compression. This bit contains the last value set by the host with
a MODE SELECT command. If the value is 1, the drive
compresses data received from the host during a WRITE
command before it writes the data to tape in the DDS-DC format.
If the value is 0, the drive does not compress data sent during a
WRITE command, and the host data is written to tape in the
uncompressed DDS format. This value is used for drives without
data compression capabilities.
DCC 2 6 The DCC (Data Compression Capable) bit values are as follows:
1 The drive is capable of doing compression
0 The drive cannot do compression
NOTE: This bit is ignored during MODE SELECT.
DDE 3 7 The DDE (Data Decompression Enable) bit contains the last
value set by the host with a MODE SELECT command. For either
value the drive decompresses data that has been compressed on
the tape before it sends the data to the host during a READ
command.
The default value is 1.
RED 3 5-6 The default value is 0.
Compression
Algorithm
4-7 The Compression Algorithm byte specifies the algorithm that is to
be used to compress data with the DCE bit set to 1. A value of
20h in this byte specifies the DCLZ algorithm, which is the default.
If the host selects a value of 01h, for the default algorithm, a value
of 20h is returned by the MODE SENSE data.
Decompressi
on Algorithm
8-11 The Decompression Algorithm byte allows the host to specify the
algorithm that is to be used to decompress data. The byte is valid
whether or not the drive decompresses the data. The byte
defaults to a value of 20h to indicate DCLZ decompression.
The following table shows the algorithm values and meaning.
Algorithm Definition
00 The data last sent to the host was uncompressed.
20 The data last sent to the host was compressed using the DCLZ algorithm.