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Crossfire bandwidth

Last response: in Graphics & Displays
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July 4, 2009 7:56:31 AM

hi, i have the asus m3a78-t mobo and i am running 2 HD4870's in CF. my mobo only runs at x8/x8 in crossfire so im wondering how much of a difference i would see with one that runs at x16/x16. thinking about the M4a79 deluxe but if it's really not much of a performance boost then i will try to be content lol.

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a c 191 U Graphics card
July 4, 2009 12:02:22 PM

First check you have the 'board set for X8/X8, it can be set for X16/X4 with dual cards.
This MB is PCI-E 2.0 so it's actually running at PCI-E 1.0/1.1 X16 speeds and that is far more than the HD4870 needs.
The only reason to change is if you are using a HT 3.0 CPU (AM3) but check a few reviews first, it may not be worth the effort for the increased memory performance.

A review not unlike this, in fact:
http://www.cpu3d.com/review/7161-1/asus-m3a78-t-am2-mot...
July 7, 2009 9:14:12 AM

so x16/x4 is better or worse than x8/x8?
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a c 191 U Graphics card
July 7, 2009 12:33:56 PM

I have not read the MB manual but the tech specs say it can set to X16X4 if it is set to those speeds I suggest you read the manual and reset them to X8X8.
The HD4870 cannot use more bandwidth than PCI-E 2.0 X8 can provide.
July 7, 2009 11:27:55 PM

I understand that a single pci-e 2.0 uses x16 on the first slot and with 2 cards it drops to x8 in the first slot and x8 in the third slot The original question is would a board that allows x16/x16 in dual mode be an improvement over my board which is x8/x8. You have said that the 4870 cannot use more bandwidth than 2.0 x8. so why are they making boards at 2.0 x16 and for that matter 2.0 x16/x16 in dual if it isnt even being utilized. I would think that having both cards using x16 would give u double the performance.
a c 191 U Graphics card
July 8, 2009 12:07:47 PM

The situation is that currently the PCI-E interface can flow more data than even the fastest card can provide, the whole idea being to have a standard that lasts more than a few years.
So, to clear up any further confusion: Your MB supports PCI-E 2.0 at X8/X8 and that is fast enough for any card on the market today. You will see no improvement by changing to a PCI-E X16/X16 MB at all, be it PCI-E 1.0, 1.1, or 2.0.
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