// Workers AI · dad joke modeWhat did the command descriptor block say? "I'm a block above the rest.

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.^[1]^[2]

Description

In the initial SCSI-1 specification, the CDB can be a total of 6, 10, 12, or 16 bytes.^[3] Later versions of the SCSI standard such as iSCSI allow for variable-length CDBs.^[4] The CDB consists of a one-byte operation code followed by some command-specific parameters.^[3] The parameters need not be a full byte long, and the parameter length varies from one command to another.^[1] 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^[5]^[3]
Bit Byte ^[6]	7	4
0	Operation code = 03h
1	LUN	Reserved
2	Reserved
3	Reserved
4	Allocation length
5	Control

16-byte CDB^[3]
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:^[7]

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:^[3]

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.

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