Quantum 10K II - Manual (Page 73)

SCSI Description
5–6 Quantum Atlas 10K II Ultra160/m SCSI Hard Disk Drives
Bit
Byte
7 6 5 4 3 2 1 0
0 Operation Code
1 Reserved
(MSB)
Logical Block Address
(LBA)
2 - 3 Logical Block Address
(LBA) (LSB))
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.
Figure 5–1 Typical (6-Byte) Command Descriptor Block — Data Format
Table 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
0 6-byte commands
1 10-byte commands
2 10-byte commands
3 Reserved
4 16-byte commands
5 12-byte commands
6 Vendor specific
7 Vendor 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)