A function driver is the main driver for a device (see the Possible Driver Layers). The PnP manager loads at most one function driver for a device. A function driver can service one or more devices.
A function driver provides the operational interface for its device. Typically the function driver handles reads and writes to the device and manages device power policy.
The function driver for a device can be implemented as a driver/minidriver pair, such as a port/miniport driver pair or a class/miniclass driver pair. In such driver pairs, the minidriver is linked to the second driver, which is a DLL.
If a device is being driven in raw mode, it has no function driver and no upper or lower-level filter drivers. All raw-mode I/O is done by the bus driver and optional bus filter drivers.
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