Some nice chaps from ATI have informed us that the current Catalyst 8.1 supports a mixed HD3870/HD3850 Crossfire mode. We set about checking these claims as they will no doubt make for some very interesting reading for all owners of HD3800 series cards as well as those who are considering getting them in the near future. The Catalyst driver allowed us to activate Crossfire with an HD3870 and an HD3850 card and we were eagerly awaiting the first test results. It's worth noting that Nvidia doesn't have a comparable feature with its SLI mode.
We tested the unusual card mix using all we had at hand. On default resolutions and 3DMark settings you can hardly tell the difference, so we'll get back to this test at a later date, as soon as ATI launches Catalyst 8.2.
We used the following cards:
HD 3850 256MB, 720MHz GPU, 830MHz memory
HD 3850 1024MB, 720MHz GPU, 830MHz memory
HD 3870 512MB, single and dual card setup, both running at 777MHz GPU and 1126MHz memory
HD 3870 512MB + HD 3850 256MB in Crossfire mode
HD 3870 512MB + HD 3850 1024MB in Crossfire mode
HD 3850 1024 + HD 3850 256MB
Processor: AMD Phenom 9600;
Memory: Kingston 5-5-5-15 800MHz
Catalyst 8.1 correctly identifies the cards. We could see one HD3870 and one HD3850 card and we could access them separately in the Overdrive section. You can overclock them and the driver allowed us to change GPU and memory clocks on each card regardless of the other card's settings. However, this didn't give us much in terms of performance, but we're still pleased to see that you can mix these R670 based cards and set the clocks independently.
The Crossfire mix works and here are the results:
Combining one reference clocked HD3870 card with an overclocked HD3850 265MB card seems to be the reasonable choice. As expected, mixing an HD3870 with an HD3850 1GB card gives you slightly better results but at a higher price.
One of the games that simply doesn't like Radeon cards these days is Crysis on Windows XP. The CPU used in the test might have also contributed to these unimpressive scores. We're expecting Catalyst 8.2 to go public any day now and we're hoping that it will provide us with slightly better performance.
In Half Life EP2 we can see that the cad mixing pays off and the overclocked HD3850 cards managed to score close to non overclocked HD3870 cards in Crossfire. Half life 2 loves the HD3870/HD3850 combination which gets you more than a 50 percent performance boost and this is what we expected from it.
In this test we can see that ATI has done a great job, but it's really up to you to consider if this interesting feature of mixing affordable HD3850 with HD3870s is really worth it. This new capability might also be very interesting when upgrading your rig.
As we've already said, we will get back to this topic as soon as the new CrossfireX driver becomes available so stay tuned.