PCI Express Voltage Question

Computer Forum CPU, Motherboards and Memory
1

As I’m new here let me start by saying ‘Hello All’.

I’ve read the sticky by Big B about PCI-X, PCIe, PCI and AGP: Differences but I still have a question. I’ve built a couple PCs using PCIe and either this is new or I missed it and got lucky, but I was looking for a new motherboard and on a description for an Intel D975XBX2 it says under

‘PCI-E x16 Slots: 3 (2 routed as x8, 1 as x4)’

This Mobo is suppose to support Crossfire. My previous MoBo only came with 1 PCIe slot (bought them before SLi or Crossfire) so I don’t know crap about the dual vid card deal. I can guess that this board will run 2 ATI cards, so I guess my questions are: (after all this babbling) What does it mean by ‘2 routed as x8, 1 as x4’? Would PCIe16 card run at peak performance in a routed as x4 slot?

Guess I’ll stop there, this is becoming a book.

Thanks in advance for clearing up my confusion.

2

No it wouldn’t. Each PCIe lane is 500MB/s (250MB/s one way). You only have 25% of the lanes. Now, wether it’s a decernable difference to the end user is an entirely different story. It has been shown that running less than 8 PCIe lanes to an x16 PCIe card does give a performance hit.

Originally, most chipsets sported 20-24 PCIe lanes, total. When running dual GPU’s, the system would be configured by the user (or automatically in some cases) to route 8 lanes to the physical x16 slots, and that worked pretty well. However, that trick wasn’t going to work forever, so newer high-end chipsets sport 32-40 PCIe lanes.

3

Oh ok, I see now. Thank you for your help.

Very nice, and informative forums here by the way. Wish I would’ve found you all sooner.

4

Glad to hear it.