PCIe slot speeds

Question

I'm really rather confused by it all, so I've come up with a question or two:

If I buy a motherboard with a PCIe x16 and PCIe x4 slot and put a graphics card in each, will they both run at x8 speeds due to the x16 being split in half, or will they run at x16 and x4?
If I buy two ATI 6850s (let's simplify it to "top-midrange graphics cards"), will there be a noticeable performance drop from running one of them at x4 speeds. They'll be in CrossfireX (linked).

Sorry if this is the wrong place to ask this question.

I think Superuser is the best place to ask this question.
– Oli Glaser
Aug 31, 2011 at 18:51 — Oli Glaser, Aug 31, 2011 at 18:51

Brian Carlton · Accepted Answer · 2011-08-31 18:29:03Z

2

The one in the x4 slot won't run at x8; it needs a x8 or x16 slot to do that. They will run at x16 and x4.
Look on a benchmarking site.

answered Aug 31, 2011 at 18:29

Brian Carlton

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Thanks very much Brian. This was my though too, but I read about bus sharing and thought that might have an impact.
– Bojangles
Sep 7, 2011 at 18:11
1

There is no bus sharing in PCIe - it's all point-to-point.
– LawrenceC
Mar 19, 2012 at 15:01

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MGZero · Accepted Answer · 2011-08-31 18:48:42Z

1

The one in the x4 slot can't run at x8 speeds. The x4 slot is limited to x4. The slots don't split their speed amongst each other like that.

answered Aug 31, 2011 at 18:48

MGZero

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2 Answers 2