Introducing AGP 8X
The AGP bus connects the graphics card to the rest of the computer, and it provides a speedy, dedicated connection for transferring all the data a GPU needs to make movie magic—vertex data to describe scenes, texture data to give surfaces their essence, and instructions about how to manipulate these things.

At its heart, AGP 8X is a simple step up from AGP 4X: it provides twice the bandwidth of AGP 4X. Under the surface, things are a little more complex. Both AGP 4X and 8X (along with the older 1X and 2X standards) have a common clock rate of 66MHz, but AGP 4X "strobes" four times per common clock cycle to provide an effective data rate of 266MHz. AGP 8X strobes eight times per common clock cycle to achieve a 533MHz data rate. Since AGP is a 32-bit bus, the effective bandwidths of AGP 4X and 8X are 1.06GB/s and 2.1GB/s, respectively.

An intimidating drawing from the AGP 3.0 spec paper The AGP 3.0 standard, which defines AGP 8X mode, does several things. It describes the changes required to double AGP bandwidth, of course. The standard also removes some (apparently unneeded) features and complexity from the previous AGP 2.0/4X standard. The AGP 3.0 spec defines how chips that implement AGP 8X can maintain backward compatibility, as well. (All the core-logic chipsets that currently support AGP 8X have "Universal" AGP implementations capable of supporting both AGP 2.0 and 3.0 signaling.) Finally, AGP 3.0 adds some new capabilities, including isochronous mode operation for improved data streaming and—get this—support for multiple AGP 3.0 ports, with multiple AGP devices per port, in a single system.

These improvements should be much needed for driving next-generation GPUs to produce cinematic quality rendering in real time. The more immediate question is: what can AGP 8X do for a GeForce4 chip running current games? That's tough to say, because nearly every game out there is written and tuned for graphics cards with 64MB of memory or less. Game developers have to keep a working set of data that will fit into most graphics cards' local memory, so their games won't choke on all but the very fastest high-end systems. As a result, current games don't seem likely to show off the benefits of shuffling data back and forth from main memory faster via AGP 8X.

That said, we're going to try our best to see how AGP 8X affects performance in these new GeForce4 chips. Let's see what we find.