2.1.1 Points to Consider about User I/O Requests
Keep the following points in mind when designing an NT driver:
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NT drivers can be layered, and more than one NT driver can process a single
I/O request (IRP).
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NT drivers communicate the success or failure of a requested I/O operation in
the I/O status block of the IRP. The I/O Manager communicates the success or
failure of a requested I/O operation to a user-mode requestor.
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NT drivers need not and should not be designed to provide application-specific
support. A protected subsystem or its subsystem-specific, user-mode drivers
supply this kind of support. There is one exception to this rule: an
MS-DOS® application that relies on an application-dedicated device can
require an NT driver to control the device and a closely coupled Win32
user-mode virtual device driver (VDD). See the Virtual DOS Drivers
documentation in the DDK for more information about VDDs..