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SCSI Commands
5-4 Quantum DLT 7000 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.
Figure 5–1 Typical Command Descriptor Block - Data Format