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Bandwidth For The Masses! VIA P4X333 with DDR333 and AGP 8x

Ready For Graphics: AGP 8x Support

The P4X333 is the first Pentium 4 chipset to support AGP 8x (or AGP 3.0, to be more precise). Though the standard has been defined since late 2000, it is not yet introduced through the industry. The upcoming Intel chipsets i845E and i845G both do not support AGP 8x, neither does the just released 850E version. In addition, there are no AGP 8x graphics cards available now, so this may not even be so tragic.

You may wonder why it could ever be necessary to have such a huge bandwidth between the graphics card and the system. On the one hand, the graphics adapter always has the possibility to swap textures and other graphic data to the main memory. Most BIOSes have an item called "aperture size"; here you can define the maximum memory capacity that can be used by the graphics adapter. Machines running with on-board graphics and unified memory architecture (no dedicated video memory available) obviously will benefit tremendously from the bandwidth doubling. But there is quite a lot of traffic on the AGP bus anyway, so we should expect a performance gain in most benchmarks.

The bandwidth doubling from AGP 4x to AGP 8x was mainly achieved by running the AGP at octuple-pumped 66 MHz (resulting in effective 533 MHz) rather than quad-pumping. Doesn't that sound familiar? Yes, the Pentium 4 does pretty much the same with its system bus. So far, it has been running at 100 MHz quad-pumped (= 400 MHz), while the latest chipsets (850E, 845E) raised the clock speed to 133 MHz. Thanks to this, the FSB and the AGP keep running pseudo-synchronous.

The following table shows the differences between all AGP standards:

AGP 1.0AGP 2.0AGP 3.0
NameAGP, AGP 2xAGP 4xAGP 8x
Signaling3.3 V1.5 V0.8 V
Clock Speed66 MHzdouble-pumped66 MHzquad-pumped66 MHzoctuple-pumped
Bus Width32 Bits32 Bits32 Bits
Bandwidth533 MB/s1066 MB/s2133 MB/s
Backwards Compatibleyesyesonly to AGP 4x

AGP 8x uses the same connector as AGP 4x, the only difference is that some pins have been reassigned in order to support the new signaling. As a result, you will be able to run all AGP 8x and AGP 4x graphics cards (at 0.8 V and 1.5 V) - but not AGP 2x! This means that you won't be able to use graphics adapters that were made before mid-1999 . So once again, you will have to sacrifice backwards compatibility in order to get a faster platform.

Please also see:

The Impact of AGP Speed , dated February 2000.
Exploration into Overclocked AGP Graphics , dated March 2000.