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SCSI Commands

5-4 Quantum DLT 4000 Tape Drive

In response to the SAVE DATA POINTER message, the initiator stores the

value of the current data pointer into the saved data pointer for that I/O

process. The drive can restore the current pointer from the saved pointer value

for the active I/O process by sending a RESTORE POINTERS message to the

initiator. The initiator then copies the set of saved pointers into the set of

current pointers. Whenever a drive disconnects from the SCSI Bus, only the set

of saved pointers is retained in the initiator. The set of current pointers is

restored from the set of saved pointers when the I/O process is reconnected.

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

1 2 ... 99 100 101 102 103 104 105 106 107 108 109 ... 299 300

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Quantum DLT 4000 Handbuch Seite 104