Multiple-Monitor Support in the Display Driver
Multiple-monitor support is provided by Windows 2000 and later; therefore, display driver writers must not implement any special code to provide this support.
Display drivers must be implemented without using global variables. All state must exist in the PDEV for a particular display driver. GDI will call DrvEnablePDEV for every hardware device extension that is created by the video miniport driver.
To track window changes in a multiple-monitor system, a driver can request GDI to create WNDOBJ objects with desktop coordinates. The driver does this by calling EngCreateWnd using the flag WO_RGN_DESKTOP_COORD. See Tracking Window Changes for more information.
In a multiple-monitor system, GDI stores the device's desktop position in the dmPosition member of the DEVMODEW structure.