Monitor class function driver
A device node is used to represent each video output on a display adapter that has a connected monitor. The device node is a child of the display adapter's device node.
Typically, there are only two device objects in the device stack that represent a video output/monitor pair:
- The physical device object (PDO).
- The functional device object (FDO).
In some cases, there's a filter device object (DO) associated with a vendor-supplied filter driver. This filter device object sits above the FDO.
For integrated monitors, such as the built-in flat panel on a laptop computer, there might be a filter DO associated with the Advanced Configuration and Power Interface (ACPI) driver. This filter DO sits above the PDO.
The following table shows the device stack for a video output that has a connected monitor.
| Device object | Required/Optional | Driver |
|---|---|---|
| Filter DO | Optional, typically not needed | Filter driver supplied by monitor vendor |
| FDO | Required | Monitor class function driver (Monitor.sys) supplied by Microsoft |
| Filter DO | Required only for integrated ACPI display panels | ACPI driver (Acpi.sys) supplied by Microsoft |
| PDO | Required | Bus driver (display miniport/port pair) supplied by display adapter vendor |
User-mode applications use WMI to invoke the services of the monitor class function driver. Those services include exposing a monitor's identification data. For an ACPI display, services include setting the brightness of the display.
A monitor stores its identification and capability information in an Extended Display Identification Data (EDID) structure. EDID is a metadata format that lets the display supply the host with information about its identity and capabilities independent of the communications protocol used between the monitor and host. The FDO processes a request from a user-mode application to read a monitor's EDID in that monitor's device stack. When the FDO receives a request to retrieve the monitor's EDID:
- The FDO sends a request to the PDO at the bottom of the monitor's device stack.
- The PDO uses the Display Data Channel (DDC) protocol to read the monitor's EDID over the I²C bus, which is a simple two-wire bus built into all standard monitor cables.
The EDID can be obtained using the ACPI_METHOD_OUTPUT_DDC method whose alias is defined in Dispmprt.h. This method is required for integrated LCDs that don't have another standard mechanism for returning EDID data.
For more information about communication between display adapters and monitors, see I2C Bus and Child Devices of the Display Adapter.
For details about EDID structures and the DDC protocol, see the following standards published by the Video Electronics Standards Association (VESA):
Enhanced Display Data Channel Standard (E-DDC)
Enhanced EDID Standard (E-EDID)
You can download these standards from vesa.org in the Free Standards section.
For details about the I²C bus, see the I²C Bus Specification published by Philips Semiconductors.