New features (continued)

And finally, the one feature, which makes AGP 3.0 the best implementation of accelerated graphics ever: The possibility of having multiple AGP ports. This AGP 2.0 enhancement allows motherboard manufacturers, to create motherboards with more than one AGP slot. Due to the fact, that the use of a PCI card for a second or third display can slow down graphics big time, professionals have been waiting for this feature since AGP 1.0 was specified in the first place.

Even multiple displays, connected to one AGP video card, can – under given circumstances – cause a slowdown. By using a second AGP card, these issues wouldn't exist anymore, even the use of four displays without causing a loss of performance is possible.
Also, SMP computers will benefit from this big time. Graphics performance is a bottleneck of SMP computers, since all available microprocessors have to share one single AGP-Bus and AGP video card. In case, more than one microprocessor accesses the AGP video card simultaneously, at least one of them has to stall, which results in poor performance.
By implementing more than one AGP-Bus, each microprocessor could have its own AGP video card, meaning performance won't drop, if both processors try to access the video card simultaneously. Given, that each AGP-Port has its own AGP-Aperture, as well as its own GART, conflicts and stalls are prevented before they can even occur.

Last, but not least, the GART has been revised and thus optimized. The GART is the re-mapping table of translations for accesses to the AGP-Aperture. Not only does it now support multiple page-sizes, it can also be supported by (multiple) GTLBs (Graphics translation look-aside buffers). These small, but very fast and efficient buffers have mainly been used by microprocessors (CPUs or GPUs) until today. Their job, is to speed up the translation of AGP 3.0 aperture pages to system memory locations, by looking up the values in large tables. Basically, GTLBs are very similar to the GART, but in contrast to it, they are not defined by the driver but by the AGP core logic, hence a lot faster.
Unfortunately, this might be another feature remaining unused in home users products, mainly because the AGP aperture itself remains unused. The decision, whether the GART is supported by all available GTLBs or not, is to be made by the driver.