The VDD returns monitor values in the DISPLAYINFO structure only if the RefreshRate value is present in the display driver's software registry key. If your display driver supports setting refresh rate based on DISPLAYINFO or EDID, the driver's INF file should include the following lines in its AddReg section:
HKR,DEFAULT,RefreshRate,,-1 HKR,DEFAULT,DDC,,1
The first line sets the RefreshRate value for the driver to -1, which tells the VDD to return a pre-computed refresh rate based on the EDID or registry timing ranges for the current monitor. If RefreshRate is set to a positive integer value, the VDD will return this override value instead (intended for future use with a refresh rate user interface).
The second line tells VDD to make the VESA DDC BIOS function call at boot time to attempt to detect the monitor and store the EDID returned by it, if any, in the registry. If DDC=1 is not present, VDD will not attempt to read the monitor EDID structure.
For more information about the INF file format, see the "Display Driver Install Disk" chapter.