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SCSI Commands

5-6 Quantum DLT 4000 Tape Drive

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).

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 complete a command with a status of INTERMEDIATE, the drive

returns a LINKED COMMAND COMPLETE message (with Flag).

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Quantum DLT 4000 Manual Pagina 106