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CrossFire Sneek Peek

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If you’re into your graphics you’ll know that the talk of the town at the moment is ATI’s CrossFire technology. Well, a couple of days ago I got a call asking if we’d like to take a look at a working CrossFire system and as I hadn’t yet seen one 'in the flesh' I grabbed the chance. The only downside was that we wouldn't be allowed to run any benchmarks - it would be a strict ‘look but don’t touch’ scenario. But as we found out it proved to be a moot point anyway.

In case you’d missed it, CrossFire is ATI’s answer to nVidia’s SLI technology. It essentially enables you to use two graphics card in your system for 3D at the same time, delivering a potentially huge performance increase in most situations. This could be a crucial technology for ATI as it hopes to claw back some of the market share and mind-share it has lost out to nVidia since the launch of SLI.

The system box that was brought round to our offices was one that had made an appearance at Computex last week for Sapphire, so it was a touch travel weary. Inside were no less than two X850 XT cards. These were standard two-slot cards, with large fans and it has to be said they do make quite a lot of noise especially when going full pelt. However, it was actually less intrusive than some of the nVidia based SLI systems we’ve reviewed. And with cards such as the GeCube Radeon X850XT available it shouldn't be too hard to keep the noise level down. However, remember that of the two cards, one has to be a special CrossFire Editon card, and we’ll have to wait and see if any low noise versions of these cards become available.



Just to recap, a current X800 or X850 PCI Express card will work in CrossFire mode. The second card however, has to be a new CrossFire Edition card. This will be available in three flavours, X800 128MB, X800 256MB and X850 256MB. The X800 will run at X800 XL clocks speeds, while the X850 is an XT part. Both cards in the system will have to have the same amount of RAM. If your existing card is slower than the CrossFire Edition card both will be able to work at their own native clocks speeds but the faster card will have to wait for data from the slower.

The CrossFire Edition card acts as the master as it contains a new chip that's responsible for the blending of the images from each board (pictured above). One of three techniques can be used; Alternate Frame Rendering (AFR), where each board renders one frame each, Scissor Mode, which splits the screen into two with each board responsible for its part of the image, and finally a ‘Supertile’ mode, which divides up the screen into a series of small chequers or tiles, which are mixed together to make the final image. The graphics driver will automatically detect which mode to use, but Supertiling is the default method under DirectX, due to it being the most effective at load balancing itself across the two cards.

As you can see the motherboard that the cards were sitting in was one based on the Radeon Xpress chipset for AMD, powering an Athlon 64 3500+. While, ATI has stated that Radeon Xpress is the optimal platform for CrossFire it has been non committal over whether it will work in other twin slot PCI Express motherboards, such as say, just perhaps, an nForce4 SLI. No doubt ATI wishes to promote its own chipset motherboards, which could well do with a boost due to the tough market. It has to be said that the competition's offerings are better featured right now, though ATI’s board partners could easily remedy this by adding third party chips for integrated LAN and SATA2.

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