Engineering:SCSI CDB

In SCSI standards for transferring data between computers and peripheral devices, often computer storage, commands are sent in a Command Descriptor Block (CDB). Each CDB can be a total of 6, 10, 12, or 16 bytes, but later versions of the SCSI standard also allow for variable-length CDBs. The CDB consists of a one byte operation code followed by some command-specific parameters. The parameters need not be a full byte long, and the parameter length varies from one command to another. The available commands, with links to articles describing the detailed structure of many of them, are listed in the article section List of SCSI commands.

Typical CDB structures, for the 6- and 16-byte SCSI Request Sense Command, opcode 3, are:

6-byte CDB:

Bit Byte	7	6	5	4	3	2	1	0
0	Operation code = 03h
1	LUN			Reserved
2	Reserved
3	Reserved
4	Allocation length
5	Control

16-byte CDB:

Bit Byte	7	6	5	4	3	2	1	0
0	Operation code = 03h
1	LUN			Service Action
2	Logical Block (MSB)
3
4
5	Logical Block (LSB)
6	Addition CBP information
7	Addition CBP information
8	Addition CBP information
9	Addition CBP information
10	Allocation length (MSB)
11
12
13	Allocation length (LSB)
14	Misc. CDB data
15	Control

An example with different allocation of bits to parameters is the 6-byte SCSI Mode Sense Command:

Bit Byte	7	6	5	4	3	2	1	0
0	Operation code = 1Ah
1	LUN			Reserved	DBD	Reserved
2	PC		Page code
3	Reserved
4	Allocation length
5	Control

The generic form of the 12-byte CDB is:

Bit Byte	7	6	5	4	3	2	1	0
0	Operation code
1	miscellaneous CDB information			SERVICE ACTION (if required)
2	Logical Block (MSB)
3
4
5
6	TRANSFER LENGTH (if required)
7	PARAMETER LIST LENGTH (if required)
8	ALLOCATION LENGTH (if required)
9
10	miscellaneous CDB information
11	CONTROL

See SPC-4 (http://www.t10.org/cgi-bin/ac.pl?t=f&f=spc4r34.pdf, free registration required) for more information.

See also

• LUN

References

• SCSI Command Reference Manual, Seagate Corp, October 2016.