4-3.1 Command Description Block Format; Table 4-2. Cdb Field Description - Fujitsu M2488 Product Manual

TAPE UNIT SCSI COMMANDS
4-3.1 Command Description Block Format
A command is executed by sending a Command Descriptor Block (CDB) to a target. See the exam-
ple for a six-byte command below. The CDB may be 6, 10 or 12-bytes in length depending on the
type of command. Some commands have additional fields which are described in the individual com-
mand. Use the description in Table 4-2 to interpret the common fields of the commands that follow.
7
BYTES
0 Group Code
1
2
3
4
5
Note: Changeable fields in the CDB are shaded.
FIELD
Operation Code (Op Code)
Command Code
Group Code
LUN (Logical Unit Number)
Reserved
Control
Link Bit
Flag Bit
4-4
6 5 4
LUN
Reserved

Table 4-2. CDB Field Description

All commands have one Op Code in byte 0. This is the operation to be performed.
Consists of a command code and a group code.
Identifies the command to be executed.
Specifies a CDB format, length in bytes, and classifies the type of operation.
For CDBs using reserved group codes 3 and 4, the controller will accept only one byte
of CDB, then go to the Bus Free phase. The host system should not send more than one
byte of CDB in this case.
The LUN is defined in the IDENTIFY message. The target ignores the logical unit
number specified within the CDB. The LUN in the CDB should be zero.
The LUN field is included in the CDB for compatibility with some SCSI-1 devices.
This field may be reclaimed in SCSI-3. New implementations should use the outbound
IDENTIFY message, which is mandatory in SCSI-2, to establish the I_T_L nexus.
Set aside fields for future use. Default value is 0 for all bits.
Last byte of all commands. Used for linked commands. Consists of a link bit, a flag
bit, and reserved bits.
Used to continue I/O processes across multiple commands. Creates an automatic link to
the next command upon successful completion of the current command. If Link=0, then
no link to next command. If Link=1, then an automatic link to the next command
occurs.
Interrupts the initiator between linked commands. Used with the link bit to specify
which message is returned to the initiator when the command completes without error;
if Flag=0, LINKED COMMAND COMPLETE message; if Flag=1, LINKED COM-
MAND COMPLETE (with Flag) message.
CG00000-011503 REV. A
M2488 PRODUCT GUIDE
BITS
3 2
Command Code
Reserved
Reserved
DESCRIPTION
1 0
Flag Link
April 1997