Related Links:

AGP - A New Interface for Graphic Accelerators

AGP - The Practice

Benchmark Comparison PCI vs. AGP

3D Graphic Accelerator Review


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AGP - The Practice

After my short theoretical essay about AGP the practical part is following now. It is meant to explain how to use AGP on current systems and what performance advantage it really gives us. The Intel 440LX chipset is released now and many people are looking into buying motherboards with this chipset. After buying a board that supports AGP, you obviously want to be able using it.

The Software Issue

Although neither Windows 95 nor Windows NT 4 were ever designed for the usage of AGP, you are certainly able to run both operating systems on Intel 440LX boards inclusive AGP graphic cards. It takes a little bit more however if you want to take advantage of this new bus.

First of all we have to remember that AGP in one way is just an extension of the PCI specifications 2.1. This means that AGP can be dealt with as if it is another PCI slot. Therefore in most cases AGP graphic cards can be dealt with via the same driver as for the corresponding PCI card. In case of the Diamond Fire GL Pro you just have to install the Windows 95 and Windows NT 4 drivers once and can work with both cards AGP or PCI. Neither Windows 95 nor Windows NT complain in any way if you should just swap the AGP card with the PCI card or vice versa. This is good news for all people that were afraid that their OS wouldn't work with AGP.

Taking advantage of AGP's DIME (Direct Memory Execute, see "AGP - A New Interface for Graphic Accelerators") however is somewhat more difficult. DIME needs to allocate some system RAM via the OS to access large textures via AGP outside the local graphic memory of the card. The OS has to know what it's doing and hence definitely needs an extension that enables this procedure. Unfortunately there isn't any such extension available for Windows NT 4 yet, but if you realize, that NT is currently anyhow not the right platform for playing 3D games (no Direct3D support, only DirectDraw and others with SP3) you will understand why NT users will have to wait for a decent AGP implementation until NT 5 is released. For the majority of users however which are using Windows 95 (especially for games) there are three things necessary:

  • Check if you are already running Windows 95 OSR 2.1. You will need the program "USBSUPP.EXE" from Microsoft, which includes the USB supplement and a new memory manager (VMM32.VxD) needed for the AGP DIME feature.
  • Get DirectX5 from Microsoft. DirectX5 is the first DirectX version that supports AGP's DIME.
  • The graphics card vendor should supply you with a virtual device manager called "VGARTD.VXD", which usually will be installed in the installation procedure of the Windows 95 card driver. This virtual device manager is the key to the DIME feature, without it your AGP card is unable to use DIME. As the name says, "VGART" stands for Virtual GART Driver, where GART is the Graphics Address Remapping Table you know about from reading "AGP - A New Interface for Graphic Accelerators".
After setting up your Windows 95 with these things your system is ready for taking advantage of AGP's DIME feature.

The Performance Issue

The expected increase in system performance is certainly the most interesting thing to all of us. This question can be answered fairly simple in the first place - there doesn't seem to be any!!! Shocking!! Well, it isn't quite like that, but you have to try hard finding any improvement in performance from using AGP. Using any real world applications or benchmarks won't get you anywhere. It is difficult to believe, but any popular benchmark program will show you the same results with the PCI card as well as with the corresponding AGP card. As to my knowledge there is currently only one benchmark available that can show you the advantage of AGP and its DIME feature, which is the 3D Winbench 97 from Ziff-Davis. Now hold your horses before you start benchmarking with 3D Winbench, because if you are using the benchmark just as it comes, it won't show you any difference between PCI and AGP either. What you will need in addition to 3D Winbench is a special scene called "Station 2 Large Textures Scene". This scene can be downloaded from ZDLabs as 3d97agp.exe. This "Large Scene" got its name because it's using very large textures of up to 6.1 MB. Even new cards with 8 MB onboard memory won't be able to hold the complete textures in their local (onboard) RAM in case you are using high resolutions that need a lot of local memory for frame buffering and Z-buffering.This means that the texture has to be swapped to local memory. Here is where DIME comes in for AGP cards that are able to use it. You can see a benefit of about 200% by using the AGP card, when running the "large scene" at a resolution of 1024x768. The benefit from AGP decreases whilst using lower resolutions and depending on the amount of local card memory the results will finally be the same at some low resolution, e.g. 640x480 in case of a 8 MB graphic card.

You invoke the "large scene" by selecting 'user scene' in the 'test settings' of 3D Winbench 97. Select 'Stations2LargeTextures.scene' as user scene name and tick the options you want to run the test with. Choose the resolution you want to run the test at and run the test via 'selected' in the 'Run' menu, choosing '3D Scene/User Defined'.

To summarize the findings regarding system performance with AGP graphic cards compared to the corresponding PCI graphics card there are the following things to be considered:

  • I can say that so far there is no real world application available that benefits from AGP's DIME feature. 
  • Normal applications, like e.g. business applications don't benefit from AGP at all. In Winstone 97 the Business as well as the HighEnd benchmark are showing identical results between AGP and corresponding PCI card.
  • Current 3D applications are also not benefiting from AGP. The textures used by current 3D games and 3D benchmarks are either not large enough or the games and benchmarks don't run at high enough resolutions for forcing the graphic card to swap parts of the textures outside their local (onboard) RAM into main memory. 
  • As new graphics cards are coming with more local (onboard) RAM, the textures can get larger than used currently and still fit into the local RAM. Only applications that use particularly large textures or graphic cards with only 4 MB local RAM or less will see an advantage from AGP, at least for now.

If you want to see some hard numbers which prove the above said, please use the following link:

Benchmark Comparison PCI vs. AGP