VESA DDC Support Overview

The VDD has the ability to detect DDC compatible monitors at boot time. However, in the retail upgrade version of Windows 95, this support is disabled by default. To enable DDC, the value "DDC=1" must be added to the Windows 95 registry in the software key for the current display driver. The installation disk for the display driver should add this value if the driver supports refresh rates via DISPLAYINFO or EDID, as described in the section Enabling Refresh Rate Support.

When DDC support is enabled, Windows 95 detects and installs DDC monitors as follows:

Note  Because it is the Windows 95 VDD that calls the DDC BIOS, Windows 95 DDC support requires a Windows 95 compatible display driver. Display drivers and virtual display drivers written for Windows 3.1 cannot use the DDC support in Windows 95.

1. At boot time, the main VDD makes the VESA DDC BIOS function call to check for a DDC compatible monitor.
2. If the ReadEDID call succeeds, a hardware key for the monitor is written to the Windows 95 registry, using the hardware ID generated from the manufacturer and product ID values found in the EDID. The hardware key is created as follows:

   HKEY_LOCAL_MACHINE
     \Enum
       \Monitor
         \<Device ID, e.g. ABC1234>
           \<instance value, e.g. 0001, 0002, etc.>
           
             HardwareID=<device id>, *PNP09FF
             EDID=<128-bypte structure returned from VESA BIOS>
    

3. The VDD calls the Plug and Play Configuration Manager to create a dev node for the monitor in the current Windows session. "Dev nodes" are internal data structures which represent a device and contain information such as their ID, resource usage (null for monitors), and registry location.
4. If the monitor has previously been detected and installed, the enumeration process is complete at this step. If this is a new ID not previously seen on this computer, then the installation process begins. Configuration Manager calls the Device Installer to create a software key for the monitor.
5. A "New Hardware Found" dialog will appear while the Windows 95 Device Installer searches all of the INF files in the Windows 95 \INF subdirectory (hidden) to find a match for the monitor's device ID. If it finds the ID in one of its INF files, the dialog will be updated to indicate the name of the monitor, and Windows 95 will automatically create the monitor's software key in the registry using information from that INF file, including its manufacturer and model names, maximum resolution, DPMS support, and horizontal and vertical frequency ranges.

Because not many DDC monitors shipped at the time of Windows 95 retail release, it is likely that a monitor's ID will not be recognized by Windows 95. To address this issue, when the VDD created the hardware key for the DDC monitor, it appended "*PNP09FF" to the HardwareID value. This is a secondary ID that is used to install unrecognized DDC monitors. This ID matches against a generic monitor type in the Windows 95 MONITOR.INF file called "Plug and Play Monitor (VESA DDC)". In the Display control panel, users can change to a more specific monitor type by selecting from the list of supported monitors, or by using a Windows 95 configuration disk supplied by the monitor manufacturer.