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130 Fibre Channel Interface Manual, Rev. D
11.2.1 Command Descriptor Block (CDB)
A request by an initiator to a disc drive is performed by sending a Command Descriptor Block (CDB) to the disc
drive. For several commands, the request is accompanied by a list of parameters sent in FCP DATA frames.
See the specific commands for detailed information.
The Command Descriptor Block always has an operation code as the first byte of the command. This is fol-
lowed by command parameters (if any) and a control byte. For all commands, if there is an invalid parameter in
the Command Descriptor Block, the disc drive terminates the command without altering the medium. The for-
mat description for the Command Descriptor Block as supported by the disc drive is shown in Tables 77 and
78.
11.2.1.1 Operation Code
The Operation Code (Table 76) 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 32 command codes in each group. Thus, a total of 256 possible operation codes exist.
Operation codes are defined in Section 12.0.
For the disc drive, the group code specifies one of the following groups:
Group 0 - Six-byte commands (see Table 77)
Group 1 - Ten-byte commands (see Table 78)
Group 2 - Ten-byte commands (see Table 78)
Group 3 - Reserved
Group 4 - Sixteen-byte commands
Group 5 - Twelve-byte commands
Group 6 - Vendor specific
Group 7 - Vendor specific
Table 76: Operation Code format for CDB
Bit
Byte
76543210
0 Group Code Command Code