![](/images/backgrounds/146247/146247-25362x1.png)
5.2Command Descriptor Block
An initiator communicates with the drive by sending a
Table
Bit | 7 | 6 | 5 | 4 |
| 3 | 2 |
| 1 | 0 |
Byte |
|
| ||||||||
|
|
|
|
|
|
|
|
|
| |
0 |
|
|
| Operation Code |
|
|
|
| ||
|
|
|
|
|
|
|
|
|
|
|
1 |
| Reserved |
| (MSB) |
| Logical Block Address |
|
| ||
|
|
|
|
| (LBA) | |||||
|
|
|
|
|
|
|
|
|
| |
|
|
|
|
|
|
|
|
|
| |
2 - 3 |
|
|
| Logical Block Address |
|
|
|
| ||
|
|
|
| (LBA) |
|
|
|
| ||
|
|
|
|
|
|
|
|
| ||
|
|
|
| |||||||
4 |
| Transfer Length, Parameter List Length, or Allocation Length |
| |||||||
|
|
|
|
|
|
|
|
|
| |
5 |
|
|
|
| Control |
|
|
|
| |
|
|
|
|
|
|
|
|
|
|
|
Note: Reserved fields in CDBs and Parameters have a value of 0; Reserved fields in states and other parameters sent to an initiator are set to 0.
Table
Table
| Table | |||
|
|
|
| |
Field |
|
| Description | |
|
|
| ||
Operation Code |
| The first byte of a SCSI CDB contains an operation code. The operation code of the CDB has | ||
|
| a Group Code field (bits | ||
|
| provides for eight groups of command codes. The | ||
|
| command codes in each group. A total, therefore, of 256 possible operation codes exist. Oper- | ||
|
| ation codes are defined in the SCSI command standards. The group code for CDBs specified | ||
|
| therein correspond to the length of the command descriptor as below: | ||
|
|
|
|
|
|
| Group Code |
| Meaning |
|
|
|
|
|
|
| 0 |
| |
|
|
|
|
|
|
| 1 |
| |
|
|
|
|
|
|
| 2 |
| |
|
|
|
|
|
|
| 3 |
| Reserved |
|
|
|
|
|
|
| 4 |
| |
|
|
|
|
|
|
| 5 |
| |
|
|
|
|
|
|
| 6 |
| Vendor specific |
|
|
|
|
|
|
| 7 |
| Vendor specific |
|
|
|
| |
|
| The operation code specifies the command being requested. The list of supported SCSI com- | ||
|
| |||
|
|
|
|
|