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SCSI Commands

5-6 Quantum DLT 8000 Tape System

Table 5–2 Command Descriptor Block — Field Descriptions (continued)

Field Description

Control Field

The Control Field is the last byte of every command descriptor block.

Its format is shown in Figure 5–2 and it contains the Flag and Link bits.

Use of these bits is initiator-dependent. Setting the Link bit = 1

provides an automatic link to the next command, bypassing the usual

ARBITRATION, SELECTION, and MESSAGE OUT phases that would

normally occur between commands. Other bits in the Control Field are

considered to be reserved.

Relative Address (RelAdr) Must be 0 (not supported).

The final byte of the CDB is called the Control byte. Its contents are defined in the next

figure and table. In the remainder of this chapter, the Flag and Link bytes will not be

redefined each time they appear as part of a command.

Bit

Byte

7 6 5 4 3 2 1 0

--- Unused 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

Unused Bits 6 & 7 of the Control Byte are reserved for vendor-specific use.

They are not used by this tape drive, so the value in each of these

bits is always 0.

Flag 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 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).

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