Command descriptor block

In SCSI, the command descriptor block (CDB) is a structure used for sending commands between computers and peripheral devices. This is often particularly used as a standard for transferring data between computers and storage devices.

Description

In the initial SCSI-1 specification, the CDB can be a total of 6, 10, 12, or 16 bytes. Later versions of the SCSI standard such as iSCSI 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 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	4
0	Operation code = 03h
1	LUN	Reserved
2	Reserved
3	Reserved
4	Allocation length
5	Control

16-byte CDB
Bit Byte	7	4
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	5	4	3	2
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	4
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

References

1 2 Buchanan, William (2000). Computer Busses: Design and Application. Elsevier Science. pp. 167–172. ISBN 9780340740767 – via Google Books.
↑ Warner, William C. (June 20, 1991). "Adapter and software simplify interface to SCSI peripherals". EDN. Vol. 36, no. 13. UBM Canon. pp. 31–35 – via the Internet Archive.
1 2 3 4 5 Rahmel, Ron; Dan Rahmel (1995). Interfacing to the PowerPC Microprocessor. Sams Publications. pp. 297–298. ISBN 9780672305481 – via Google Books.
↑ Hufferd, John L. (2003). iSCSI: The Universal Storage Connection. Addison-Wesley. pp. 259–261. ISBN 9780201784190 – via Google Books.
↑ Black, John, ed. (1992). The System Engineers Handbook. Morgan Kaufmann Publishers. pp. 333–334. ISBN 9780080519029 – via Google Books.
↑ Glass, L. Brett (March 1990). "The SCSI Bus". Byte. Vol. 15, no. 3. McGraw-Hill. pp. 291–298 – via the Internet Archive.
↑ SCSI Commands Reference Manual (PDF). Seagate Corporation. October 2016. pp. 97–100.

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[buchanan-1] 1 2 Buchanan, William (2000). Computer Busses: Design and Application. Elsevier Science. pp. 167–172. ISBN 9780340740767 – via Google Books.

[warner-2] Warner, William C. (June 20, 1991). "Adapter and software simplify interface to SCSI peripherals". EDN. Vol. 36, no. 13. UBM Canon. pp. 31–35 – via the Internet Archive.

[rahmel-3] 1 2 3 4 5 Rahmel, Ron; Dan Rahmel (1995). Interfacing to the PowerPC Microprocessor. Sams Publications. pp. 297–298. ISBN 9780672305481 – via Google Books.

[4] Hufferd, John L. (2003). iSCSI: The Universal Storage Connection. Addison-Wesley. pp. 259–261. ISBN 9780201784190 – via Google Books.

[black-5] Black, John, ed. (1992). The System Engineers Handbook. Morgan Kaufmann Publishers. pp. 333–334. ISBN 9780080519029 – via Google Books.

[6] Glass, L. Brett (March 1990). "The SCSI Bus". Byte. Vol. 15, no. 3. McGraw-Hill. pp. 291–298 – via the Internet Archive.

[7] SCSI Commands Reference Manual (PDF). Seagate Corporation. October 2016. pp. 97–100.