Engineering:Command descriptor block

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Short description: Storage device command data structure

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 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[3]
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:[7]

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:[3]

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

References

  1. ↑ 1.0 1.1 Buchanan, William (2000). Computer Busses: Design and Application. Elsevier Science. pp. 167–172. ISBN 9780340740767. https://www.google.com/books/edition/Computer_Busses/KhcEcNRkBHgC?hl=en&gbpv=1&dq=%22Command+Descriptor+Block%22&pg=PA167&printsec=frontcover. 
  2. ↑ Warner, William C. (June 20, 1991). "Adapter and software simplify interface to SCSI peripherals". EDN (UBM Canon) 36 (13): 31–35. https://archive.org/details/bitsavers_EDNEDNV36N_46837082/page/31/. 
  3. ↑ 3.0 3.1 3.2 3.3 3.4 Rahmel, Ron; Dan Rahmel (1995). Interfacing to the PowerPC Microprocessor. Sams Publications. pp. 297–298. ISBN 9780672305481. https://www.google.com/books/edition/Interfacing_to_the_PowerPC_Microprocesso/X45QAAAAMAAJ?hl=en&gbpv=1&bsq=%22Command+Descriptor+Block%22+%226%22+%2212%22+%2216%22&dq=%22Command+Descriptor+Block%22+%226%22+%2212%22+%2216%22&printsec=frontcover. 
  4. ↑ Hufferd, John L. (2003). iSCSI: The Universal Storage Connection. Addison-Wesley. pp. 259–261. ISBN 9780201784190. https://www.google.com/books/edition/ISCSI/clx9Zrhr56sC?hl=en&gbpv=1&dq=%22+length%22+%22cdb%22+%22iSCSI%22&pg=PA260&printsec=frontcover. 
  5. ↑ Black, John, ed (1992). The System Engineers Handbook. Morgan Kaufmann Publishers. pp. 333–334. ISBN 9780080519029. https://www.google.com/books/edition/The_System_Engineers_Handbook/82ylrSRpdbcC?hl=en&gbpv=1&dq=%22command+descriptor+block%22+%2216-bit%22&pg=PA333&printsec=frontcover. 
  6. ↑ Glass, L. Brett (March 1990). "The SCSI Bus". Byte (McGraw-Hill) 15 (3): 291–298. https://archive.org/details/eu_BYTE-1990-03_OCR/page/n317/. 
  7. ↑ SCSI Commands Reference Manual. Seagate Corporation. October 2016. pp. 97–100. https://www.seagate.com/files/staticfiles/support/docs/manual/Interface%20manuals/100293068j.pdf#page=97. 




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