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ATI CrossFire multi-GPU platform

Worth the wait?

Review If you have even a passing interest in 3D graphics, you've probably been waiting for the appearance of ATI's dual-GPU solution. It seems like aeons ago that ATI announced that it would produce a platform to rival Nvidia's tremendously successful SLi, but now, finally I have a CrossFire system in front of me and it's time to see if it was worth the wait, writes Riyad Emeran.

What I'm talking about is the ability to install two graphics cards in a single PC so the two cards share the load when it comes to 3D rendering - resulting in a significant performance boost when you fire up your favourite game.


My first foray into the dual-GPU scene was when I got my hands on two 3dfx Voodoo2 cards. I remember having to keep pretty quiet about it in the office - having one Voodoo2 at the time was considered lucky, but installing two in the same machine was positively greedy! This was the birth of SLi, although as I remember, back then it stood for 'Scan Line Interleave', rather than the 'Scalable Link' Interface that Nvidia now attributes to the acronym.

It was a long time before I gave up my dual Voodoo2 cards, despite advances in 3D technology from other companies. But eventually 3dfx went the way of the Dodo, and it was time to move on. I had to wait quite a while before I saw a PC with two graphics cards again.

The barren spell ended about a year ago when Nvidia started to ship SLi kit into the market. Although there were cards available quite early, it took a while for the mainstream motherboards to appear, but when they did the flood gates opened. Now, dual-graphics card systems are commonplace, even at the low end of the market.

ATI didn't want Nvidia to have everything its own way, and it wasn't long before rumours of a platform to compete with SLi started to appear. But it was at the beginning of June that those rumours started to take shape, and ATI announced the CrossFire branding. Hardware was expected soon, allowing ATI to hit the streets with CrossFire before the launch of Nvidia's next generation 3D solution, the GeForce 7800GTX. Unfortunately, CrossFire didn't appear.

But all good things come to those who wait, so they say, and now that wait is over.

One of the most obvious physical differences between SLi and CrossFire, is the method of connection between the two cards. With SLi a small bridge is employed between the two cards that links them internally, while CrossFire uses a daisy-chained DVI cable that links the cards externally. There's no doubt that the Nvidia solution is more elegant from a PC building point of view, but looks aren't everything.

Another big difference between the two systems is that ATI has promised that CrossFire will work with every single game out there, whereas for SLi to weave its spell there needs to be a driver profile for each game you want to play. Of course, this doesn't mean that CrossFire will enhance the performance of every game, but just that every game should play in CrossFire mode.


Now, saying that SLi needs driver profiles and CrossFire doesn't isn't entirely true. CrossFire does need profiles depending on the rendering method that's employed - whereas SLi uses each graphics card to render alternate frames, CrossFire can employ one of three different rendering methods. The most basic rendering mode is tiling, which cuts the scene up into loads of squares and splits the load between the cards. Tiling doesn't require any kind of profile, but there's also no guarantee that there will be a performance increase over using a single card. The second rendering mode is scissor, which splits the screen in half, with each card rendering each half of the scene. The third and final method is alternate-frame rendering, which just like SLi allows each card to create alternate frames ' for this method, just like SLi, CrossFire needs a game profile.

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