Page Control -- Mode parameters

Page Code: A field within a page whose value is unique within a command. The page code provides the means to interpret the remaining fields in a page.
The Page Control (PC) field defines the type of mode parameter values tobe returned in the mode pages.

Page control field

Code	Type of parameter
00b	Current values                       (volatile)
01b	Changeable values
10b	Default values                       (not user changeable)
11b	Saved values                         (non-volatile)

* The only MODE SELECT conditions(parameters) that are influenced by the hard reset are the current parameter values.

Current values

A PC field value of 00b requests that the device server return the current values of the mode parameters. The current values returned are:

a) The current values of the mode parameters established by the last successful MODE SELECT command;

b) The saved values of the mode parameters if a MODE SELECT command has not successfully completed since the mode parameters were restored to their saved values or

c) The default values of the mode parameters if a MODE SELECT command has not successfully completed since the mode parameters were restored to their default values.

Changeable values

A PC field value of 01b requests that the device server return a mask denoting those mode parameters that are changeable. In the mask, the bits in the fields of the mode parameters that are changeable all shall be set to one and the bits in the fields of the mode parameters that are non-changeable (i.e., defined by the logical unit) all shall be set to zero.

If the logical unit does not implement changeable parameters mode pages and the device server receives a MODE SENSE command with 01b in the PC field, then the command shall be terminated with CHECK CONDITION status, with the sense key set to ILLEGAL REQUEST, and the additional sense code set to INVALID FIELD IN CDB.

An attempt to change a non-changeable mode parameter using the MODE SELECT command shall result in an error condition.

The application client should issue a MODE SENSE command with the PC field set to 01b and the PAGE CODE field set to 3Fh to determine which mode pages are supported, which mode parameters within the mode pages are changeable, and the supported length of each mode page prior to issuing any MODE SELECT commands.

Default values

A PC field value of 10b requests that the device server return the default values of the mode parameters.

Unsupported parameters shall be set to zero. Default values should be accessible even if the logical unit is not ready.

Saved values

A PC field value of 11b requests that the device server return the saved values of the mode parameters. Mode parameters not supported by the logical unit shall be set to zero. If saved values are not implemented, the command shall be terminated with CHECK CONDITION status, with the sense key set to ILLEGAL REQUEST, and the additional sense code set to SAVING PARAMETERS NOT SUPPORTED.

The method of saving parameters is vendor specific. The parameters are preserved in such a manner that they are retained when the device is powered down. All saveable mode pages should be considered saved when a MODE SELECT command issued with the SP bit set to one has returned a GOOD status or after the successful completion of a FORMAT UNIT command.