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 G raphics A ddress R emapping T able 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.