Information Technology Reference
In-Depth Information
MODE SELECT
A host can use the MODE SELECT command to specify parameters relat-
ing to the storage media, a logical unit, or the device itself. The information
is in structures called block descriptors and mode pages. There are two com-
mands that differ in the size and format of the mode-parameter header that
precedes any block descriptors and mode pages being sent. MODE
SELECT(6) has a 4-byte header, and MODE SELECT(10) has an 8-byte
header. Support for MODE SELECT is optional unless the device also sup-
ports MODE SENSE as explained below.
Mode page 08h is the caching mode page. The Windows mass-storage
driver attempts to read the page and if supported, send a page to disable
caching unless the user has selected
optimize for performance
for the volume
in Device Manager > Properties > Policies.
MODE SENSE
The MODE SENSE commands are the complements to the MODE
SELECT commands. A host can use MODE SENSE to request parameters
relating to the storage media, a logical unit, or the device itself. As with
MODE SELECT, there are two commands that differ in the size and format
of the mode-parameter header that precedes any block descriptors and mode
pages being returned. MODE SENSE(6) has a 4-byte header, and MODE
SENSE(10) has an 8-byte header.
The SPC specification says that a device that supports a MODE SENSE
command should support the corresponding MODE SELECT command,
and a device that supports a MODE SELECT command should support
the corresponding MODE SENSE command. The MODE SENSE com-
mand is optional for many SBC devices, but Windows and other hosts use
this command to request information from devices. Bootable devices must
support MODE SENSE(10).
Each MODE SENSE command block contains a page code and a subpage
code that together specify what information the host is requesting. The SBC
specification defines a variety of mode pages. When SUBPAGE CODE =
00h, the mode page uses the page_0 format. The page includes fields for the
page code, page length, and mode parameters. For other subpage codes, the
mode page uses the subpage format, which adds a field for a subpage code.
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