Specifications

ATAPI For Streaming Tape QIC-157 Rev B
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6.5. Unit Attention Condition
The ATAPI Device shall generate a unit attention whenever the Device has been reset by a hard
reset condition, or by a power-on reset. The Device shall also generate a unit attention condition
whenever one of the following events occurs:
1. A removable medium may have been changed;
2. The version or level of microcode has been changed;
3. The mode parameters in effect have been restored from non-volatile memory;
4. Any other event occurs that requires the attention of the Host.
The ATAPI Device may queue unit attention conditions. After the first unit attention condition is
cleared, another unit attention condition may exist (e.g. a power on condition followed by a
microcode change condition).
The unit attention condition shall persist, until the Host clears the condition as described in the
following paragraphs.
If an INQUIRY command or any non packet ATA command is received from the Host with a
pending unit attention condition, the ATAPI Device shall perform the INQUIRY command and
shall not clear the unit attention condition.
If a REQUEST SENSE command is received from the Host with a pending unit attention condition,
then the ATAPI Device shall:
1. report the unit attention condition, may discard any pending sense data, and clear the unit
attention condition.
If the Host issues a command other than INQUIRY or REQUEST SENSE while a unit attention
condition exists, the ATAPI Device shall not perform the command and shall report “Check
Condition” status unless a higher priority status as defined by the ATAPI Device is also pending
(e.g. BUSY).
6.6. Commands and Parameters
The ATAPI Streaming Tape commands are derived from the QIC-121 command set.
The interface uses logical rather than physical addressing for all data blocks. Each Device may be
interrogated to determine how many blocks it contains.
Commands are classified as mandatory, optional, or vendor-specific. ATAPI Devices are required
to implement all mandatory commands and may implement other commands as well. ATAPI
Devices contain commands that facilitate the writing of self-configuring software Drivers that can
discover all necessary attributes without prior knowledge of specific peripheral characteristics.
6.7. ATAPI Media Access Command Considerations
ATAPI media access commands are a class of commands that normally return good completion
status to the Host system before they are finished executing the command. The purpose of this is to
allow the Host to execute more than one command at a time on the same IDE cable.
ATAPI Devices use the DSC bit to indicate the completion of commands. No INTRQ is issued by
these Devices when the DSC bit is set, so it the responsibility of the ATAPI Driver to poll this bit to
determine the completion. See also 2.15 ATAPI Command Protocol and DSC Handling on page 15.