Crossfire X and Windows XP

[VERTIGGO]

I did some benchmarking in XP with one X2 in and some with both in:

Opposing Fronts max @ 2560x1600 4xAA/16xAF

one 3870X2 38.1 [CrossfireX ON], [Catalyst AI ON]
two 3870X2 38.0 CFX ON, AI ON
two 3870X2 35.1 CFX OFF, AI ON

Opposing Fronts max @ 2560x1600 0xAA/16xAF

two 3870X2 75.0 CFX ON, AI ON
one 3870X2 51.5 CFX ON, AI ON
one 3870X2 46.2 CFX OFF. AI ON

Opposing Fronts max @ 1920x1200 4xAA/16xAF

one 3870X2 98.5 CFX ON, AI ON
two 3870X2 91.5 CFX ON, AI ON
two 3870X2 57.1 CFX OFF, AI ON

In Win XP supposedly CFX is not possible, but I still have a check box. It seems from the 3rd test that XP is only using one board anyway, since the single board is faster than two as long as CrossfireX is on (wait a minute, I thought that meant 3-4 cores), and when CFX is OFF it drops to 57.1 fps. However, in the second one, I see a 50% increase in performance with a second card in, though the 3rd and 4th cores are not supposed to work.

Does anyone else know about this? Why is there an option if it's not possible, and what is actually being used in Win XP?

 

[VERTIGGO]

This just in! I found that the 75.0 fps above was a fluke. However, it's not an accident. It's all about Catalyst AI. If I set it to ON and start the game, I get normal (50fps) frame rates, but if I change it (from ON to Advanced or vice versa) while the game is running, and then re-test, the frame rates shoot up to the 75 average. WTF mate!!! Is the software fooling the GPUs into working in CFX or something? It happens every time. If I restart the game, I have to mess with AI (back and forth) to get the frames up again.

I tried again with 4x AA and yet again, the 38.0 fps went up to about 45-50 fps!
Unless I'm missing something basic, this seems like another "Vista is necessary" "DX10 is amazing" charade. I though there was a fundamental reason that XP couldn't run 4 cores at once. Is that really true?

 

[Quiksilver]

I think you need to test it in more games to prove your theory...

 

[VERTIGGO]

Oh it's just a speculation, and BTW, I don't expect it would work in other games. I just can't think of any "reasonable reason" why the performance could recieve a 50% bonus. I checked carefully, and AA was still on! Weird.