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100

101

SCSI Commands

5-4 Quantum DLT 8000 Tape System

5.1.2 Command Descriptor Block

An initiator communicates with the drive by sending a 6- or 10-byte Command

Descriptor Block that contains the parameters for the specific command. The SCSI

command's operation code is always the first byte in the Command Descriptor Block and

a control field is the last byte. For some commands, the Command Descriptor Block is

accompanied by a list of parameters sent during the DATA OUT phase. Figure 5–1

shows the format of a typical 6-byte Command Descriptor Block. Table 5–2 contains a

description of the Command Descriptor Block fields.

Bit

Byte

7 6 5 4 3 2 1 0

0 Operation Code

1 Logical Unit Number

(LUN)

(MSB)

Logical Block Address

(LBA)

2 - 3 Logical Block Address

(LBA) (LSB)

4 Transfer Length, Parameter List Length, or Allocation Length

5 Control

NOTE: Unless otherwise specified, all reserved bits indicated in the commands are 0.

The Control byte is described on Page 5-6.

Figure 5–1 Typical 6-Byte Command Descriptor Block — Data Format

1 2 ... 143 144 145 146 147 148 149 150 151 152 153 ... 407 408

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