Command Descriptor Block - HP Q153090901 Technical Reference Manual

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.
7
0
1
2 (MSB)
n 1
Vendor Unique (0)
n
There are a number of fields in a CDB which are common to all commands. These are shown in
the following table.
Group Code and
Operation Code
Reserved
Multi-Byte
Parameter
Control
Vendor-Unique
NACA
Flag
Link
6 5
Group Code
Multi-Byte Parameter
Reserved (0)
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 Six-byte commands
Group 1
Ten-byte commands
Group 2 Ten-byte commands
Group 3 Six-byte commands
Group 4
Sixteen-byte commands
Group 5 Twelve-byte commands
Group 6 not supported
Group 7
not supported
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
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.
The control field is mainly concerned with the use of linked commands. These are not
supported by the LTO SCSI Command Set, so a
if this field is set to anything other than zero.
This field is ignored by the firmware
0 The Normal ACA flag is 0, indicating that it is not supported.
0
0 Linked commands are not supported.
4 3 2
Operation Code
Reserved (0)
NACA(0) Flag (0) Link (0)
CHECK CONDITION
CHECK CONDITION
Command descriptor block
1 0
(LSB)
.
will be generated
35