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The LOAD UNLOAD command tells the target to load or unload the tape media
in the tape cartridge. If no cartridge is in the tape drive, both LOAD and UNLOAD
return a CHECK CONDITION status with a NOT READY sense key set.
Likewise, if the drive has received an UNLOAD command with the Immediate bit
set and then it receives another command that would require tape motion or if it
receives a TEST UNIT READY command, the drive returns a CHECK
CONDITION STATUS with a NOT READY sense key set.
127(
Operation of the UNLOAD version of this command is different if a media
loader is present.
Two modes of operation are possible if a media loader is configured. If
none of the media loader-specific commands have been issued, the
device operates in the sequential mode of operation described below.
Once a media loader-specific command has been issued, however, the
sequential mode of operation (described below) is disabled and the
UNLOAD command becomes a NO OPERATION.
If the tape drive is in the default sequential mode of operation and an
UNLOAD command is received by the subsystem, the current cartridge is
unloaded and automatically moved to the magazine slot from which it was
received. The cartridge from the next magazine slot (if the slot is not
empty) automatically moves from the magazine into the drive, is loaded,
and made ready. If the next magazine slot is empty, no CHECK
CONDITION status is created.
When the cartridge is unloaded into the last magazine slot, the subsystem
does not cycle back to slot 0. This prevents accidental overwriting of data
when using a media loader subsystem in sequential auto-loading mode.
The next cartridge in the cycle must be selected and loaded manually, or
with a SCSI MOVE MEDIUM command.
The sequential loading feature of the loader can be enabled/disabled by
modifying the ENALDRAUTOLD and DISLDRAUTOLDMC parameters of
EEPROM (mode page 3Eh of the MODE SELECT command).
A media loader does not affect the tape drive’s processing of the LOAD
portion of the LOAD UNLOAD command.