Specifications

ATAPI For Streaming Tape QIC-157 Rev B

Implementor's Note: When compressed data is encountered on the medium that the Device cannot

decompress, the Device should treat the data as uncompressed. In the sense data, the Valid bit, the ILI

bit, and the Information field should be set accordingly.

A RED field of 3 is undefined and shall result in a “Check Condition” status with the Sense Key set to

ILLEGAL REQUEST.

The Compression algorithm field indicates the compression algorithm the Device shall use to process

the data sent to it by the Host when the DCE bit is set to one. If the Host selects an algorithm the Device

does not support the Device shall return a “Check Condition” status. The Sense Key shall be set to

ILLEGAL REQUEST. A value of zero shall indicate that no compression algorithm is currently

selected.

For the MODE SELECT command, the Decompression Algorithm field indicates the decompression

algorithm selected by the Host for use in subsequent decompression of data encountered on the

medium. For Devices capable of the automatic recognition of the compression algorithm used to

process data encountered on the medium, the decompression algorithm selected by the Host may be

ignored, or overridden by the Device for a subsequent read operation if the selected value does not

match the compression algorithm detected by the Device, which was used to process the data

encountered on the medium.

For the MODE SENSE command, the Decompression Algorithm field reflects the algorithm selected

by the Host. For some Devices, the Decompression Algorithm value returned in response to a MODE

SENSE command may change dynamically to match the compression algorithm, detected by the

Device, which was used to process the data most recently encountered on the medium, during a read

operation. A value of zero shall indicate the data encountered on the medium during the most recent

read operation was uncompressed. Table 6-53 shows the compression algorithm identifiers registered

under the International Register of Processing Algorithms [to be] established by ISO/IEC JTC1.