SCSI CDB

In SCSI 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.

A typical 6 byte CDB structure is:

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

A typical 10 byte CDB is

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	Misc. CDB data
7	Allocation length (MSB)
8	Allocation length (LSB)
9	Control

A typical 12 byte CDB is

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	Allocation length (MSB)
7
8
9	Allocation length (LSB)
10	Misc. CDB data
11	Control

A typical 16 byte CDB is

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

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