Product Manual - Disc Drive SCSI-2/SCSI-3 Interface (Vol. 2; Ver. 2), Rev. E
43
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4.2.1
Operation code
The operation code (Table 4.2-1) of the Command descriptor Block has a group code field and a command
code field. The three bit group code field provides for eight groups of command codes. The five bit command
code field provides for thirty two command codes in each group. Thus, a total of 256 possible operation
codes exist. Operation codes are defined in Section 5.
For the disc drive the group code specifies one of the following groups:
Group 0 - Six byte commands (see Table 4.2-2).
Group 1 - Ten byte commands (see Table 4.2-3).
Group 2 - Ten byte commands (see Table 4.2-3).
Group 3 - Reserved
Group 4 - Reserved
Group 5 - Twelve-byte commands
Group 6 - Vendor specific
Group 7 - Vendor Specific
Table 4.2-1. Operation Code Format for CDB
Bit
7
6
5
4
3
2
1
0
Byte(s)
0
Group Code
Command Code
Table 4.2-2. Typical Command Descriptor Block for Six Byte Commands
Bit
7
6
5
4
3
2
1
0
Byte(s)
0
Operation Code
1
Logical Unit No.
Logical Block Address
0
0
0
(if req.) (MSB)
2
Logical Block Address (if required)
3
Logical Block Address (if required )(LSB)
4
Transfer Length (if required)
5
Control Byte
