Added: 01/22/2006 |
Principles of AGP graphics cardsAn accelerated graphics port (AGP) is a direct connection between a graphical sub-system and RAM. This interface is required to process big volumes of 3D graphics data. Before the appearance if AGP graphics cards, PCI graphical adapters were in use. Therefore, growing requirements to the volume of the processed data in modern games was the reason for increasing the data volume from the processor to a graphical chip.
A bus maximal capacity (133 MB/sec at 33 MH) became insufficient for providing the needed speed of the information exchange. As a result, the bus appeared to be a bottle's neck in the computer's graphical sub-system. AGP graphics cards solved this problem. The AGP bus work with the clock rate 66MHz and it is able to pass data with the speed 264 MB/sec in the 1x mode. Additionally, the usage of a new slot takes loading from the PCI bus.
It was only the beginning of significant changes in the video cards industry. Some time later graphical chips of the new generation appeared. New AGP graphics cards include a GPU (Geometric Processor Unit). This achievement makes the AGP capacity actual again. Actually, powerful chips, such as the GeForce2 GTS, are not able to demonstrate all its abilities with the AGP 4x. The reason is that the graphical chip requires a higher volume of passed data for calculating the geometry. At the same time, the capacity of RAM is restricted with 800 MB/sec. This capacity is enough for the video processing, but this channel is required by sound functions as well. The things became worth due to the side-band addressing (SBA). This feature is included to the AGP standard for the speed increasing and it does not work for all NVIDIA based chip cards. Thus, the users of the AGP slot card can increase its performance, controlling the side-band addressing function.
In fact, the SBA is the additional 8-bit addressing, built into the AGP. AGP graphics cards are able to request the information via the SBA and receive 32-bit data stream via the AGP bus. Due to this functionality, video cards get 20 bytes for 4 tacts instead of 16.
Switching off the SBA leads to looses of the performance for AGP graphics cards, but there are some reasons to do it. The AGP bus with an activated SBA is very sensitive to the increase of the system bus frequency. Thus, switching off the SBA increases the stability of the AGP work on non-standard frequencies. Over lookers use this feature to accelerate the graphical processor by the FSB frequency increasing.
Some tests with the activated and deactivated SBA on computers with different configurations were made to affect the influence of the side-band addressing to the games performance. The QuakeIII (OpenGL) was used for the tests in modes 800x600x32, 1024x768x32, HQ setting, 4/4 textures, bilinear filtering and sound off.
Essentially, the result of switching off the SBA is minimal. TNT2/TNT2 Pro video cards are not affected with this feature, since the AGP 2x usage. Simultaneously, the SBA mode should affect GeForce video cards, but it does not. Most likely that the reason is in the specific Quake 3 mode without the full T&L support. OpenGL games, which will support the T&L in full measure, will make the situation with the feature much clearer.
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