Command descriptor block – HP Q153090901 User Manual
Page 35
Command descriptor block
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Command descriptor block
A SCSI command descriptor block (CDB) is a sequence of 6, 10, 12 or 16 bytes sent by a host to
a SCSI target with the bus in command phase. The CDB tells the drive what action should be
performed. The final byte is known as the Control byte.
There are a number of fields in a CDB which are common to all commands. These are shown in
the following table.
7
6
5
4
3
2
1
0
0
Group Code
Operation Code
1
Reserved (0)
2
(MSB)
Multi-Byte Parameter
n
−
1
(LSB)
n
Vendor Unique (0)
Reserved (0)
NACA(0) Flag (0) Link (0)
Group Code
and
Operation Code
The operation code uniquely identifies the command. The top three bits of the
operation code are known as the group code and these define the length of the
command descriptor block:
Group 0
Group 1
Group 2
Group 3
Group 4
Group 5
Group 6
Group 7
Six-byte commands
Ten-byte commands
Ten-byte commands
Six-byte commands
Sixteen-byte commands
Twelve-byte commands
not supported
not supported
Reserved
A reserved field should always be set to zero. The drive checks reserved fields, and if
one is non-zero then it will reject the command with
CHECK CONDITION
.
Multi-Byte
Parameter
A multi-byte parameter field in a command is “big-endian”, that is, bit 7 of the first
byte of this field is the most significant.
Control
The control field is mainly concerned with the use of linked commands. These are not
supported by the LTO SCSI Command Set, so a
CHECK CONDITION
will be generated
if this field is set to anything other than zero.
Vendor-Unique
This field is ignored by the firmware
NACA
0 The Normal ACA flag is 0, indicating that it is not supported.
Flag
0
Link
0 Linked commands are not supported.