ATI CrossFire

But all good things come to those who wait, so they say, and now that wait is over. But the big question is whether CrossFire was worth the wait. Well, before I answer that question, let me cover a little background.

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 up 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.

I had initially thought that the CrossFire driver would allow the user to choose which rendering method to use, but when I got my hands on the hardware I found out that this wasn’t the case. It seems that the Catalyst AI determines what the best rendering mode is and applies it. Of course these are still very early drivers, and a manual rendering mode configuration may yet appear in future versions.

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