ATI Crossfire

[blu4]

Hello!

I have two cards- ATI Radeon 4850 (512mb) and ATI HD 4870 (1gb). I was wondering if i could hook them up together for crossfire?

thanks

[Blín D'ñero]

Yes you can.
By doing so, your 1GB 4870 will downgrade to 4850 512 MB.
Effectively resulting in 4850 512 MB crossfire.

[blu4]

Well, the I guess i am getting another 4870

Thanks for your help. Close.

[caveman-jim]

Originally Posted by Blín D'ñero

Yes you can.
By doing so, your 1GB 4870 will downgrade to 4850 512 MB.
Effectively resulting in 4850 512 MB crossfire.

No, it won't. the 4870 will run as if it has 512mb. Everything else will perform as stock.

A better idea than getting another 4870 is a new 7800 series card.

[Blín D'ñero]

Originally Posted by caveman-jim

No, it won't. the 4870 will run as if it has 512mb. Everything else will perform as stock.

[...].

HD4870 "as stock" =
750 mhz core clock
1800 MHz GDDR5 memory clock

HD4850 "as stock" =
625 MHz core clock
1000 MHz GDDR3 memory clock

from this follows that the 4870 -which is faster as stock- has to slow down, or throw away buffer contents; anyway the tandem would not perform the same as 2x 4870 "as stock" but likely the same (effectively) as 2 x 4850 "as stock". Effectively.
That is what logic dictates.

[caveman-jim]

Originally Posted by Blín D'ñero

HD4870 "as stock" =
750 mhz core clock
1800 MHz GDDR5 memory clock

HD4850 "as stock" =
625 MHz core clock
1000 MHz GDDR3 memory clock

from this follows that the 4870 -which is faster as stock- has to slow down, or throw away buffer contents; anyway the tandem would not perform the same as 2x 4870 "as stock" but likely the same (effectively) as 2 x 4850 "as stock". Effectively.
That is what logic dictates.

Your premises are faulty. There is no requirement for both cards to perform identically. The workload is scaled onto the greater number of SIMD's in the 4870 than the 4850, and the clocks are held at their BIOS stock settings.

You are correct that 4870 + 4850 won't be as fast as 4870 + 4870. This will produce microstutter in cases where the 4850 is bottlenecked by it's design and the 4870 is not. These circumstances are quite few, however. The first bottleneck to be run in to will be VRAM, all cards are limited to the frame buffer of the smallest equipped card. 4850 + 4850 won't be as fast as 4870 + 4850, either. However you have the most opportunity for microstutter (uneven render times) with 4870 + 4850 instead of the other configurations.

CrossFireX is not concurrent, it is consecutive - Alternate Frame Rendering - , meaning both cards don't work on the same scene at the same time. They work on alternate scenes, with processing on the second scene beginning while the first scene is still being composed.

Look at AMD dual graphics on APU's, they have significantly different GPU configurations that work together in CrossFireX. This is because there is no requirement for both GPU's to become identical for CrossFireX.

It doesn't matter what your logic suggests, the fact is the above method is how CrossFireX operates, per AMD. You do not need identical cards, and they do not automatically speed match.