SCSI Description
5–6Quantum Atlas 10K II Ultra160/m SCSI Hard Disk Drives
Bit
Byte
76543210
0Operation Code
1Reserved (MSB) Logical Block Address
(LBA)
2 - 3 Logical Block Address
(LBA)
(LSB))
4Transfer Length, Parameter List Length, or Allocation Length
5Control
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.
Figure 5–1 Typical (6-Byte) Command Descriptor Block — Data FormatTable 5–2 Command Descriptor Block — Field Descriptions
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 7-5) and a Command Code
field (bits 4-0). The 3-bit Group Code field provides for eight groups of
command codes. The 5-bit Command Code field provides for 32
command codes in each group. A total, therefore, of 256 possible
operation codes exist. Operation 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
06-byte commands
110-byte commands
210-byte commands
3Reserved
416-byte commands
512-byte commands
6Vendor specific
7Vendor specific
The operation code specifies the command being requested. The list of
supported SCSI commands and their operation codes are contained in
Table 5-1.
(continued)