excerpt from "Hello I/O Kit: Creating a Device Driver With Xcode"
talking about virtual device
============================================================
Important: IOMatchCategory is a special property list element that
allows multiple drivers to match on a single nub. The presence of
IOMatchCategory allows HelloIOKit to match on IOResources (a special
nub at the root of the I/O Registry that provides system-wide
resources) without preventing other drivers from matching on it. When
you write a functional driver following the guidelines in this
tutorial, you should not include the IOMatchCategory element in your
driver's property list at all unless there is a valid reason for your
driver to match on the same device nub at the same time as another
driver (for example, a serial port with multiple devices attached to
it).
The vast majority of drivers should not match on IOResources. A rare
exception to this is a driver for a virtual device, because a virtual
device does not produce its own nub in the I/O Registry. If you're
developing such a driver, you must include the IOMatchCategory
property to make sure your driver doesn't claim IOResources and
prevent other drivers from matching on it. To ensure your driver's
IOMatchCategory value is unique, use your driver's class name, in
reverse-DNS notation and with underbars instead of dots (for example,
com_MyTutorial_driver_HelloIOKit).
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a.d
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