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Multimonitor Support in WDDM

On Windows Vista, older XPDM drivers still work and the multi-monitor behavior with XPDM drivers hasn't changed, because the operating system uses the legacy graphics stack.

However, the Windows Vista Display Driver Model (WDDM) brings fundamental changes to the management of multiple graphics adapters and external displays. This includes a new restriction, because WDDM drivers do not support "heterogeneous multi-adapter" multi-monitor implementations. Specifically:
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All graphics adapters in a system must use the same display driver model. That is, all of them should either be running XPDM or WDDM. The driver models are mutually exclusive, and Windows Vista does not allow the simultaneous loading of both an XPDM driver and a WDDM driver.

If a system has one graphics adapter with a XPDM driver and another with a WDDM driver, then Windows Vista will choose the POST device, which is the one with VGA resources. This is commonly referred to as the "VGA adapter."