Specifications

ATAPI For Streaming Tape QIC-157 Rev B
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A Data Compression Enable (DCE) bit of one indicates that the data compression is to be enabled.
When this bit is set, data sent to the Device by the Host shall be processed using the selected
compression algorithm before being written to the medium. A DCE bit of zero indicates that data
compression is disabled.
A Data Compression Capable (DCC) bit of one indicates that the Device supports data compression
and shall process data sent to it for transferal to the medium using the selected compression algorithm
when the DCE bit is one. A DCC bit of zero indicates that the Device does not support data
compression. This shall be a non-changeable field.
A Data Decompression Enable (DDE) bit of one indicates that data decompression is to be enabled. A
DDE of zero indicates that data decompression is disabled.
Implementor's Note: When the DDE bit is zero, all decompression algorithms are deemed
unsupported by the Device.
The Report Exception on Decompression (RED) field indicates the Device's response to certain
boundaries it detects in the data on the medium. There are a number of boundaries that may occur on
the medium between compressed and uncompressed data. These boundaries are shown in Table 6-49.
Table 6-49 Possible data compression boundaries
Prior Data Current Data
uncompressed compressed
(unsupported algorithm)
uncompressed compressed
(supported algorithm)
compressed uncompressed
(supported algorithm)
compressed compressed
(supported algorithm) (unsupported algorithm)
compressed compressed
(supported algorithm A) (supported algorithm B)
compressed uncompressed
(unsupported algorithm)
compressed compressed
(unsupported algorithm) (supported algorithm)
compressed compressed
(unsupported algorithm A) (unsupported algorithm B)
A RED field of zero indicates that the Device shall return a “Check Condition” status when data is
encountered on the medium during a read operation that the Device cannot decompress. This is the case
at the boundaries shown in Table 6-50.
Table 6-50 Boundaries that generate a “Check Condition” when RED field is zero
Prior Data Current Data Sense Key
uncompressed compressed MEDIUM ERROR
(unsupported algorithm)
compressed compressed MEDIUM ERROR
(supported algorithm) (unsupported algorithm)
compressed compressed MEDIUM ERROR
(unsupported algorithm A) (unsupported algorithm B)
A RED field of one indicates that the Device shall return a “Check Condition” status when data in
encountered on the medium during a read operation that requires different handling by the Host than the