HOWTO: Determine the Currently Installed Display Driver

• Windows 95 Device Driver Kit (DDK), version 4.0

SUMMARY

This article demonstrates a technique for an application to determine the currently installed display system device driver on Windows 95.

MORE INFORMATION

In most cases, it is recommended that applications not rely on the hardware- dependent information that this technique provides. There is no API available to provide this information directly, and parsing the Registry is problematic for several reasons. The most reliable method is to ask the display system itself for the information.

The display system drivers use a data structure called 'DISPLAYINFO' for various purposes, but you need to use it to obtain the Registry DevNode for the current display driver. Calling the Windows 95 Virtual Display Device (VDD) VDD_GET_DISPLAY_CONFIG service populates this structure.

To do this, your application can thunk to a 16-bit DLL that allocates a DISPLAYINFO structure. The 16-bit DLL then uses the Interrupt 2Fh function 1684h interface to call into the Virtual Display Device's VDD_GET_DISPLAY_CONFIG service. Some application developers might be able to use the DeviceIOControl() API to complete this step without thunking or using Interrupt 2Fh. Either way, the VDD will populate the DISPLAYINFO data structure which has DevNode for the current display system driver, as well as a lot of other information regarding the display system.

Display driver developers have the option of extending the DISPLAYINFO structure for their own purposes, so the calling DLL may want to allocate a larger memory block for the DISPLAYINFO structure because it might be bigger than what Microsoft documents. How much larger is something you will need to determine empirically on many different display system drivers.

Once you have the populated DISPLAYINFO structure, you have the DevNode for the display driver in the diDevNodeHandle member of DISPLAYINFO. You can now use the Registry APIs to look at the currently installed display drivers' registry entries. You may already have all the information you wanted in the DISPLAYINFO structure, and can therefore not bother with the Registry APIs.

Example code that demonstrates some of this technique is shown in Assembler code in the Windows 95 Device Driver Kit (DDK) directory: DDK\Display\Samples\Mini\Mini\Init.asm. This is the code that the ring-3 display driver uses to obtain the DISPLAYINFO structure, which is what you need to do. C code for this technique is not available. Implementation of this technique is left as an exercise for the advanced mixed-programming language developer.

REFERENCES

Windows 95 Device Driver Kit.

For more information on calling into VxDs from an application, please see the following article in the Microsoft Knowledge Base: