The SCSI Mode Select command is used to modify device information contained in mode pages

SCSI mode pages

SCSI target devices provide a number of SCSI mode pages. These can be interrogated by a Mode Sense command and set by a Mode Select command. The Mode Sense and Mode Select commands include a 6-bit address field, allowing for 64 possible mode pages. A description of many of these page codes is...

 in a SCSI

SCSI

Small Computer System Interface is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, and electrical and optical interfaces. SCSI is most commonly used for hard disks and tape drives, but it...

 target device. There are two different versions of the command, a 6 byte version and a 10 byte version.

The CDB

SCSI CDB

In SCSI computer storage, commands are sent in a Command Descriptor Block .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...

structure for the Mode Select(6) command is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 15h
1	LUN			PF	Reserved			SP
2	Reserved
3	Reserved
4	Parameter list length
5	Control

The CDB structure for the Mode Select(10) command is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 55h
1	LUN			PF	Reserved			SP
2	Reserved
3	Reserved
4	Reserved
5	Reserved
6	Reserved
7–8	Parameter list length
9	Control

The special control fields in the CDB have the following meaning:

• PF - Page Format:
  • 0 - SCSI-1 compliant (vendor-specific)
  • 1 - SCSI-2 compliant
• SP - Save Pages - if this bit is one then the target will save any saveable updates to non-volatile memory or media area.