p55 boards - more than 24 pci-e lanes possible?

[darren1394]

According to this block diagram http://icrontic.com/uploads/2009/09/P55-blockdiagram.gif
p55 supports total of 24 pci-e lanes.

However both the gigabyte GA-P55-UD4P and MSI P55-GD65 seem to use more the 24 lanes.

the gigabyte states:
1 pci-e x16 slot running at x16
1 pci-e x16 slot running at x8
3 pci-e x1 slots

that's 27 lanes not counting the lanes used for LAN and the extra SATA chip.

I want to know will these boards run two graphics cards (one for physX, NOT in SLI) at x16 and x8 simultaneously, and if so, where do the extra pci-e lanes come from?

 

[Lead Butthead]

I think when the x8 slot is occupied, the x16 slot downgrade itself to x8. You can see this in the board's manual (download PDF from Gigabyte website.)

 

[footballrunner800]

some lanes come from the southbridge aka the P55 chipset. I believe the x4 slot that some motherboards have and some of the extra chips get connected to the p55 also.

 

[Meaker10]

You will notice that sometimes slots will downgrade, or lanes will switch off (eg in crossfire 1x slots will not be usable) etc.

 

[BushLin]

some lanes come from the southbridge aka the P55 chipset. I believe the x4 slot that some motherboards have and some of the extra chips get connected to the p55 also.

Bingo, the P55 is connected by 2GB/s DMI and in theory can offer up to four PCI-E 2.0 lanes. However, onboard USB, Ethernet, Sound and Disk controllers are sharing this too.