Quantum DLTtape DLT 4000, Product Manual

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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