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Jon Peddie June 2005: ATI drives CrossFire to market

by Bob Crabtree on 20 June 2005, 00:00

Tags: ATi Technologies (NYSE:AMD)

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ATI drives CrossFire to market

No, ATI hasn’t brought DaimlerChrysler and neither are all the executives driving new sport cars.

Jon Peddie Research
In a move that caught no one by surprise at Computex ATI announced CrossFire, the company’s new multi-card graphics architecture designed to competed with arch-rival Nvidia's multi-card SLI technology.

Like Nvidia, ATI used the migration to PCI Express to enable multi-card graphics. Motherboard manufacturers can use the Radeon Xpress 200 CrossFire Edition chipsets, to provide dual slots for graphics, enabling two PCI Express cards to populate the same system.

Unfortunately, like Nvidia’s Nforce 4 SLI chipset, the Radeon Xpress 200 CrossFire Edition appears to support dual x16 physical connectors with only 8 electrical lanes underneath, an approach call downshifting which technically violates the PCI Express Electromechanical Specification.

ATI and Nvidia chipsets (and probably SIS and Via too) can dynamically configure the arrangement of PCIe lanes. So Crossfire and SLI use the most optimal configuration which is two, 8-lane slots. Intel doesn't have this flexibility and so they must use their fixed configuration of one, 16-lane slot and one, 4-lane slot.

Testing has shown that 8-lanes is enough for 99% of today’s apps to not be bandwidth limited, but 4-lanes does impact performance due to the bandwidth limitation. Bus bandwidth is definitely a build it first and the apps will follow kind of thing. The good news is that most of the apps available now need AGP 8X (or at least more than AGP 4X). So use of AGP 16X (PCIe 16-lane) bandwidth should be coming along.

While Nvidia has never denied that SLI’s speedup is application and usage dependent, we were impressed with ATI architects’ willingness to look for ways to make the 2nd GPU useful when applications, user demands, and/or host systems make it difficult for AFR or Scissor mode to produce a solid performance increase. As a result, CrossFire architects added support for two additional modes that SLI' have. (or at least not yet): Supertile and Super AA modes.

In Supertile mode, the screen is divided into a checkerboard of 32x32 tiles, with one GPU rendering even tiles and the other odd.

Super AA mode won’t increase frame rates, but will increase image quality at the same rate.

One place where ATI clearly has an advantage over SLI is in the use of multiple displays. Nvidia can currently only support one display with SLI, although they are working on this and will soon have a dual display capability we’re told. However, because of ATI's approach, their built in mutli display capability (which Nvidia also has) is not disabled. That means with two AIBs, ATI could drive four displays. And, if the OEM/ODM chose to not turn off the graphics in a one-size-fits-all motherboard when an AIB is plugged, then there could be five active displays.

One of the criticisms of multi displays was the performance hit, now with dual AIBs, as well as very powerful AIBs, that complaint goes away and new immersive level of gaming can be enjoyed.

One other point that ATI's approach offers is the ability to mix and match. CrossFire-enabled Radeon board can be connected to any existing model, configuration, or brand of PCI Express graphics card that uses a Radeon X800 or X850 series GPU. This provides an easy upgrade path to existing owners. This is investment protection and easy upgrading.

SLI and CrossFire are not novel ideas, but they are really exciting and these guys have a really hot product. Nether company will sell a zillion boards for it, but it will crate a zillion impressions, and that will sell AIBs elsewhere. And then the ATI execs can go buy one of those nifty new sports cars.

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will people downgrade to an ati motherboard ?
or can ati get crossfire working on a nvidia motherboard ?