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The reason for writing this guide is that many people still ignore the importance of AAS or set it incorrectly. Generally measured in megabytes (MB), AAS is the amount of system memory (RAM) shared with an AGP graphics card in order for it to have more memory to process textures and other visual data.
Some people think that the more RAM you share with the graphics card, the larger the performance boost. In some cases, this might be true. In others, it might result in a performance drop. There are two common outcomes from setting the AAS too high: (1) If you share too much RAM with your graphics card and an application takes advantage of that, you will have less RAM for other functions of the software, and possibly for background applications. (2) If the graphics card does not utilize the memory, it is not taken from system memory. In this case, there are no problems with AAS sucking all of your system RAM, despite its setting in the BIOS configuration utility. The latter is the most common case, since most people have enough memory now to compensate for a high AAS, and most applications would not take all of the memory anyway.
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und hier der link zum vollst. artikel:
Tweak3D - AGP Aperture Size