SCSI Mode Sense Command

The SCSI Mode Sense command is used to obtain current device information from mode pages in a SCSI target device. There are two different versions of the command, a 6 byte version and a 10 byte version.

The CDB structure for the Mode Sense(6) command is:

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 CDB structure for the Mode Sense(10) command is:

bit→ ↓byte	7	6	5	4	3	2	1	0
0	Operation code = 5Ah
1	Reserved			LLBAA	DBD	Reserved
2	PC		Page code
3	Reserved
4	Reserved
5	Reserved
6	Reserved
7–8	Allocation length
9	Control

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

• LLBAA - Long LBA Accepted - if "1" then the target is allowed to return parameter data with the LONGLBA bit set to "1"
• DBD - Disable Block Descriptors - if "1" then the target must not return any block descriptors
• PC - Page control -
  • 00 - return current values
  • 01 - return changeable values
  • 10 - return default values
  • 11 - return saved values
• Page Code - see the separate Wikipedia page for a list of SCSI mode pages.

References

• SCSI Command Reference Manual, Seagate Corp, April 2010.