Operation Code - Seagate SCSI Interface Product Manual

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4.2.1

Operation code

The operation code (Table 13) 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 5.0.
For the drive, the group code specifies one of the following groups:
Group 0—Six byte commands (see Table 14)
Group 1—Ten byte commands (see Table 15)
Group 2—Ten byte commands (see Table 15)
Group 3—Reserved
Group 4—Reserved
Group 5—Twelve byte commands
Group 6—Vendor specific
Group 7—Vendor specific
Table 13. Operation code format for CDB
Bit 7
Byte
0 Group Code
Table 14. Typical Command Descriptor Block for six byte commands
Bit 7
Byte
0
1 Logical Unit No.
0
2
3
4
5
6 5
6 5
0 0
Logical Block Address (if required)
Logical Block Address (if required)
Transfer Length (if required)
SCSI Interface Product Manual, Rev. J
4 3
Command Code
4 3
Operation Code
Logical Block Address (if required) (MSB)
Control Byte
2 1
2 1
0
0