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 driver determines the best rendering mode and applies it. Of course, these are still very early drivers, and a manual rendering mode configuration may yet appear in future versions.
Unlike SLi, you can't build a dual-GPU CrossFire system out of any two cards. Instead you have to purchase what ATI calls a master card and marry this up to a normal ATI graphics board. At present, the master cards come in two flavours, Radeon X850 XT and Radeon X800 XL. Although the slave card doesn't have to be an exact match to the master card, I suspect that the system will run at the speed of the slower card. Although ATI has hinted at the fact that cards can run asynchronously, I didn't have time to put this to the test.
In order to put CrossFire through its paces I needed some hardware, and Evesham was kind enough to supply me with some. Evesham built a complete PC based on the CrossFire platform - this consisted of an ATI reference motherboard for Socket 939 AMD Athlon 64, an FX-57 CPU and two Radeon X850 XT cards, one master and one slave.
When the Evesham machine arrived it fired up first time and proved to be rock solid throughout a complete run of benchmarks. However, when I came to actually play some games, things weren't quite so rosy. There seemed to be considerable tearing when panning around a scene, while turning on FSAA resulted in a horrible motion blur effect that produced a near instantaneous headache.
After much work and investigation we were unable to eradicate the tearing problem, although the motion blurring seemed to stop after I removed the graphics cards and re-installed them. Eventually we were supplied with a new driver revision by ATI which smoothed out the tearing and allowed me to get a better idea of what CrossFire is capable of. I re-ran all the benchmarks using the new driver.
Looking at the benchmark results first, there's no doubt that CrossFire works, and two X850 XT cards definitely produce enough grunt for pretty much any game you're likely to throw at them. Running our custom Far Cry demo turned in a very impressive 85fps at a resolution of 1,600 x 1,200 with 4x FSAA and 8x AF, while in Doom 3, a game that's traditionally nVidia friendly, the CrossFire machine managed 70fps at the same resolution and settings. Obviously I expected Half-Life 2 performance to be good and with a score of 81fps at 1,600 x 1,200 with 4x FSAA and 8x AF, I wasn't disappointed.