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SCSI Commands
5-6 Quantum DLT 7000 Tape System
Bit
Byte
7 6 5 4 3 2 1 0
5 Vendor Specific Reserved Flag Link
Figure 5–2 Command Descriptor Block Control Field - Data Format
Table 5–3 Command Descriptor Block Control Field - Field Descriptions
Field Description
Vendor Specific Bits These bits must be 0.
Flag Bit The Flag bit is used in conjunction with the Link bit to notify the initiator in
an expedient manner that a command has been completed.
Link Bit A Link bit set to 1 indicates that the initiator requests continuation of a task
(an I/O process) across two or more SCSI commands.
If the Link bit is 1 and the Flag bit is 0, and the task completes successfully,
the drive continues the task and returns a status of INTERMEDIATE and a
LINKED COMMAND COMPLETE message.
If the Link bit and the Flag bit within a Control word are both set to 1, and
the drive completes a command with a status of INTERMEDIATE, the drive
returns a LINKED COMMAND COMPLETE message (with Flag).
5.1.3 Status/Error Reporting
SCSI message-level errors are communicated by messages that are defined
specifically for that purpose (for example, MESSAGE PARITY ERROR,
MESSAGE REJECT). Message-level errors are also handled by drive-managed
retries. Refer to Chapter 4 for more detailed message-handling information.
SCSI command-level errors are communicated by a status code that is returned
by the drive during the STATUS phase. This phase occurs at the end of each
command, unless the command is terminated by one of the following events:
• ABORT message
• BUS DEVICE RESET message
• Hard reset condition
• Unexpected disconnect