User Manual

SCSI Commands Reference Manual, Rev. J 40
www.seagate.com General Concepts
2.1.3 The variable length CDB formats
The first byte of a variable length CDB shall contain the operation code 7Fh. The CONTROL byte is the second byte in the variable
length CDB (see table 6)
ADDITIONAL CDB LENGTH field
The ADDITIONAL CDB LENGTH field specifies the number of additional CDB bytes. This value in the ADDITIONAL CDB LENGTH field shall be a
multiple of 4. If the number of CDB bytes delivered by the service delivery subsystem is not sufficient to contain the number of bytes specified
by the ADDITIONAL CDB LENGTH field, then the command shall be terminated with CHECK CONDITION status, with the sense key set to ILLE-
GAL REQUEST, and the additional sense code set to INVALID FIELD IN CDB.
SERVICE ACTION field
The SERVICE ACTION field specifies the action being requested by the application client. The SERVICE ACTION field is required in the variable
length CDB format and is described in 4.3.4.2. Each service action code description defines a number of service action specific fields that are
needed for that service action.
Table 6 Typical variable length CDB
Bit
Byte
7 6 5 4 3 2 1 0
0
OPERATION CODE (7Fh)
1
CONTROL
2
Miscellaneous CDB information
3
Miscellaneous CDB information
4
Miscellaneous CDB information
5
Miscellaneous CDB information
6
Miscellaneous CDB information
7
ADDITIONAL CDB LENGTH (n–7)
8
(MSB)
SERVICE ACTION
9
(LSB)
10
(MSB)
Service Action specific fields
. . .
n
(LSB)