Information Technology Reference
In-Depth Information
When communicating with direct-access block devices (PDT = 00h), the
Windows mass-storage driver and other hosts commonly request these
mode pages:
PAGE CODE = 3Fh requests all supported mode pages.
PAGE CODE = 1Ch requests the informational exceptions control mode
page. This page contains information about how the device reports excep-
tion conditions due to vendor-specific events and conditions that result
from scans or self-tests a device performs in the background (using no bus
bandwidth).
PAGE CODE = 08h requests the caching mode page, which defines param-
eters that relate to a device's use of its cache.
PAGE CODE = 05h is the flexible disk mode page (FDMP), which con-
tains parameters needed to convert between LBA and CHS addressing.
Some hosts refuse to try booting from devices that lack this page, possibly
because the boot code uses CHS addressing. The USB bootability specifica-
tion requires support for this page in all bootable devices with a PDT other
than 05h (CD/DVD drive). The bootability specification redefines mode
page 05h as a simplified version of the page defined in the original SBC
specification. (Note that the mode page is listed as obsolete in SBC-2 and
later but the USB version of the page is required for bootable USB devices.)
A response to a MODE SENSE command begins with a mode parameter
header (Table 6-5). Note that in byte 2, bit 7, an SBC device informs the
host whether the media is write-protected.
Following the header, an SBC device may send a mode parameter block
descriptor that specifies the number of logical blocks in the media and the
block size. A response can also contain one or more mode pages and/or
mode subpages.
On receiving a MODE SENSE command for an unsupported mode page, a
device should fail the command and return SENSE KEY = ILLEGAL
REQUEST and ADDITIONAL SENSE CODE = INVALID FIELD IN
COMMAND PACKET. Some devices with no data to send respond by
sending just the header. In response to a MODE SENSE(6) command, a
device with no data to return can return the header with MODE DATA
LENGTH = 3 and BLOCK DESCRIPTER LENGTH = 0.
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