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.

Brian Carlton · Answer 1 · 2011-08-31 18:29:03Z

up vote 2 down vote accepted

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 '11 at 18:29

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

1

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

feedback

MGZero · Answer 2 · 2011-08-31 18:48:42Z

up vote 1 down vote

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 '11 at 18:48

feedback

migrated from electronics.stackexchange.com Aug 31 '11 at 18:59