MODE SENSE(6) command

3.11.1 MODE SENSE(6) command introduction

The MODE SENSE(6) command (see table 73) provides a means for a device server to report parameters to an application client.

It is a complementary command to the MODE SELECT(6) command. Device servers that implement the MODE SENSE(6) command shall also implement the MODE SELECT(6) command.

DBD (disable block descriptors) bit

0 A disable block descriptors (DBD) bit set to zero specifies that the device server may return zero or more block descriptors in the returned MODE SENSE data.

1 A DBD bit set to one specifies that the device server shall not return any block descriptors in the returned MODE SENSE data.

PC (Page Control) field

The page control (PC) field specifies the type of mode parameter values to be returned in the mode pages. The PC field is defined in table 74.

The PC field only affects the mode parameters within the mode pages, however the PS bit, SPF bit, PAGE CODE field, SUBPAGE CODE field, and PAGE LENGTH field should return current values (i.e., as if PC is set to 00b). The mode parameter header and mode parameter block descriptor should return current values.

Some SCSI target devices may not distinguish between current and saved mode parameters and report identical values in response to a PC field of either 00b or 11b. See also the description of the save pages (SP) bit in the MODE SELECT command.

PAGE CODE and SUBPAGE CODE fields

The PAGE CODE and SUBPAGE CODE fields specify which mode pages and subpages to return (see table 349).

ALLOCATION LENGTH field

The ALLOCATION LENGTH field is defined in 2.2.6.

Table 73 MODE SENSE(6) command Bit

Byte

7 6 5 4 3 2 1 0

0 OPERATION CODE (1Ah)

1 Reserved DBD Reserved

2 PC PAGE CODE

3 SUBPAGE CODE

4 ALLOCATION LENGTH

5 CONTROL

Table 74 Page control (PC) field

Code Type of parameter Reference

00b Current values 3.11.1.1

01b Changeable values 3.11.1.2

10b Default values 3.11.1.3

11b Saved values 3.11.1.4

An application client may request any one or all of the supported mode pages from the device server. If an application client issues a MODE SENSE command with a page code or subpage code value not implemented by the logical unit, 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.

If an application client requests all supported mode pages, the device server shall return the supported pages in ascending page code order beginning with mode page 01h. If mode page 00h is implemented, the device server shall return mode page 00h after all other mode pages have been returned.

If the PC field and the PAGE CODE field are both set to zero, the device server should return a mode parameter header and block descriptor, if applicable.

The mode parameter list for direct access devices for MODE SELECT and MODE SENSE is defined in 5.3. Parts of the mode parameter list are specifically defined for each device type. Definitions for the parts of each mode parameter list that are unique for each device-type may be found in the applicable command standards.

3.11.1.1 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 (see 3.7); 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 (see 3.7).

3.11.1.2 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 (see 3.7).

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.

3.11.1.3 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.

3.11.1.4 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.

3.11.1.5 Initial responses

After a logical unit reset, the device server shall respond in the following manner:

a) If default values are requested, report the default values;

b) If saved values are requested, report valid restored mode parameters, or restore the mode parameters and report them. If the saved values of the mode parameters are not able to be accessed from the nonvolatile vendor specific location, the command shall be terminated with CHECK CONDITION status, with the sense key set to NOT READY. If saved parameters are not implemented, respond as defined in 3.11.1.4; or

c) If current values are requested and the current values have been sent by the application client via a MODE SELECT com-mand, the current values shall be returned. If the current values have not been sent, the device server shall return:

a) The saved values, if saving is implemented and saved values are available; or b) The default values.