Information Technology Reference
In-Depth Information
Flash drives that fail this command when PREVENT = 01 have improved
performance. When the command fails, Windows doesn't enable write cach-
ing. Write caching causes Windows to launch multiple threads that cause
random writes, which result in slow write performance on flash media.
REPORT LUNS
The REPORT LUNS command requests a list of the numbers of all logical
units that are present and that match the peripheral device type returned in
response to an INQUIRY command. The response can also optionally
include the numbers of logical units that currently have no device present. A
device should be able to respond to this command without having to access
the media and even when the device isn't ready to respond to other com-
mands. The SBC-2, SBC-3, and SPC-3 specifications all list this command
as mandatory. The command is optional in SPC-2, however.
REQUEST SENSE
The REQUEST SENSE command requests a structure containing sense
data. The specification says that the ALLOCATION LENGTH parameter
in the request should always equal 252 bytes to enable devices to return all
of their sense data, including vendor-specific data. Windows hosts typically
request just the first 18 bytes, however. For this reason, device vendors often
use vendor-specific control requests to obtain additional sense data if
needed. The REQUEST SENSE command is mandatory for SBC devices.
The command descriptor block for this command is 6 bytes. However, the
Windows driver sets bCBWCBLength in the CBW to 0Ch instead of 06h.
This incorrect value doesn't affect the number of bytes that transmit on the
bus because the CBW is always 31 bytes with 16 bytes reserved for the com-
mand descriptor block. Device firmware should accept the incorrect value
and ignore the additional declared 6 bytes.
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