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ULTIMATE Video Card Guide

ULTIMATE Video Card Guide - PCSTATS
Abstract: New games demand more of video cards and their memory; bigger textures, more triangles, and various other new means of hardware acceleration to look good and still run at playable speeds.
Filed under: Video Cards Published:  Author: 
External Mfg. Website: Various Jul 18 2000   D. Dee  
Home > Reviews > Video Cards > Various

Interfaces



Currently there are two video card interfaces on PCs: PCI and AGP. PCI (Peripheral Component Interconnect) is a 32-bit, 33MHz bus that has a maximum bandwidth of only 132 megabytes per second (33 MHz x 32 bits / 8 bits per byte = 132 megabytes/second). Clearly, that's very slow compared to the memory speed of today's video cards, some of which have throughputs of over 2 gigabytes per second. In addition, the bandwidth of the PCI bus is shared with other devices, so the disk controller(s), sound card, network card, and any other PCI devices all take a chunk out of that number. That doesn't leave very much for the video card. For this reason, PCI video cards are not common since most new motherboards have AGP slots. AGP (Accelerated Graphics Port) is a dedicated video card interface that is actually a specialized version of PCI. AGP has four speeds: 1x, 2x, and 4x. AGP 1x's base bandwidth is twice that of PCI since it runs at 66MHz. AGP 2x and 4x use other tricks (like sideband addressing) to increase bandwidth further.

The maximum theoretical bandwidth of AGP 4x is a little over a gigabyte per second (4 x 66MHz x 32 bits / 8 bits per byte = 1,056 megabytes per second). That's respectable, but it's still less than half of the speed of the fastest video cards' memory. AGP has some other selling points that sound nice in theory but aren't so great in practice. One is known as DiME (Direct Memory Execute), or simply AGP texturing. It allows a video chip to render textures directly from system memory without having to go through the video card's memory.

The idea behind this was that video cards would need less video memory since cheaper main memory could be used for textures. In practice, AGP texturing is quite slow. This is due partly to the lack of bandwidth of the interface and mostly to the slowness of the system memory. However, it does allow scenes with extremely large and/or numerous textures to be used, where a PCI video card would be slowed to a crawl by all the texture swapping between the video and system memory. AGP also allows for direct communication with the CPU, which allows video data to go straight from the processor to the video card, thereby bypassing main memory, saving bandwidth, and increasing speed. Again, in practice this doesn't seem to make much of a difference. Ultimately the main benefit of AGP over PCI is its bandwidth.


I have read recently that Intel is working on a new 8x AGP implementation, but unless main memory speed increases drastically in the near future, this won't be much more of a help than AGP 4x is now. While the speed increase going from 1x to 2x AGP is significant, there are few situations in which AGP 4x actually makes a difference, so I don't expect 8x to be much of an improvement.


I'd like to see Intel make a 64-bit AGP interface, but since main memory is still so slow in comparison to video memory, I doubt it'd help either, and in the near future would probably just add cost since it'd need a physically larger interface and more circuitry. What I'd really like to see is better methods of connecting the video card to the CPU, since the current ones don't help and main system memory is just too slow. Overall I don't expect any significant improvements in interface technology for a while.

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Contents of Article: Various
 Pg 1.  ULTIMATE Video Card Guide
 Pg 2.  NVidia's chips
 Pg 3.  Diverging Paths
 Pg 4.  Present and future hardware features
 Pg 5.  More present and future hardware features
 Pg 6.  — Interfaces
 Pg 7.  Keeping up
 Pg 9.  More players

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