Video BIOS Shadowing

Common Options : Enabled, Disabled

Quick Review

This BIOS feature allows faster access to the video BIOS by shadowing or making a copy of it in the system memory. This appears quite an attractive feature since it results in at least a thousand-fold improvement in video BIOS performance and the only price you pay is losing the small amount of system memory used to mirror the video BIOS. Unfortunately, the truth is not so simple.

Modern operating systems do not even use the video BIOS. They bypass the BIOS completely and use the graphics card's driver instead. Therefore, absolutely no benefit can be realized by shadowing the BIOS.

In addition, shadowing the video BIOS can sometimes cause conflicts to occur. There is always a risk of certain software writing to the RAM region used to shadow the video BIOS. When this happens, a conflict occurs and the system will crash.

What could be a bigger issue would be the shadowing of just a portion of the video BIOS. Newer video BIOSes are generally much larger than 32KB in size. But most motherboards shadow only a 32KB block from C0000 to C7FFF. If only this region of the video BIOS is shadowed and the rest left unshadowed, applications may have trouble accessing the video BIOS properly.

Finally, all graphics cards now use Flash ROM which allows easy upgrading of the firmware by a simple BIOS flash. However, if the video BIOS is shadowed, any attempt at flashing the video BIOS will likely result in a system crash. It could be even worse if only a portion of the video BIOS had been shadowed when the video BIOS upgrade was attempted.

With all that said, there may still be a use or two for this BIOS feature. For one thing, most real-mode DOS games use the video BIOS's VGA functions because they cannot directly access the graphics processor. Such games will benefit from the shadowing of the video BIOS.

Shadowing of the video BIOS also provides performance benefits when it comes to the fail-safe mode of certain operating systems (for example, Safe Mode in Microsoft Windows XP). These operating systems fall back on the video BIOS because all video BIOSes contain the same, standardized VGA functions.

If this BIOS feature is enabled, the video BIOS will be shadowed in system memory. This improves graphics rendering performance if the VGA functions of the video BIOS are used.

If this BIOS feature is disabled, the video BIOS will not be shadowed in system memory. Any access to the video BIOS will have to go through the XT or LPC bus.

Since drivers have replaced the video BIOS as the interface between the graphics hardware and the operating system, it is recommended that you disable Video BIOS Shadowing. The risk of crashes and BIOS corruptions due to this BIOS feature is not worth the benefits it provides in certain circumstances.

However, if you do play a lot of old real-mode DOS games or work a lot in safe-mode Windows, then you should shadow the video BIOS for improved performance.

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