What do the "modes" of the PCI-e specs mean?

[PanSola]

One mother board I'm looking at has: 3 x PCIe 2.0 x16 (at x16/x16/x1 or x16/x8/x8 mode),
another one says: 3 (@ x16/x16/x4 mode)
and one mentioned: 1 x PCIe 2.0 x16 slot (at x4 mode)

What do they mean and how do they impact performance?

 

[4745454b]

There are two parts of the PCIe slot. One is the physical size of the slot, the other is how many lanes are electrically connected to that slot. The 16x means there are 3 full size PCIe slots on that motherboard. The x16/x16/x1 means the first slot will have the full 16 lanes connected, as will the second. The third slot will only have one lane, meaning that card will not be running as fast as it could. I can also appearently be set up to run 16 lanes on the primary card, with the second and third cards getting 8 lanes each.

16/16/1 is good for a two card SLi setup with the third slot running an Nvidia card as a PhysX card. 16x/8x/8x is better for a tri SLI setup. 16/16/4 is more like the first option, but with 4 lanes going to the PhysX card instead of just one. A 16x slot can run a 4x card, so this last one might be better if you want to run high end SATA/SCSI cards.

 

[PanSola]

There are two parts of the PCIe slot. One is the physical size of the slot, the other is how many lanes are electrically connected to that slot. The 16x means there are 3 full size PCIe slots on that motherboard. The x16/x16/x1 means the first slot will have the full 16 lanes connected, as will the second. The third slot will only have one lane, meaning that card will not be running as fast as it could. I can also appearently be set up to run 16 lanes on the primary card, with the second and third cards getting 8 lanes each.

16/16/1 is good for a two card SLi setup with the third slot running an Nvidia card as a PhysX card. 16x/8x/8x is better for a tri SLI setup. 16/16/4 is more like the first option, but with 4 lanes going to the PhysX card instead of just one. A 16x slot can run a 4x card, so this last one might be better if you want to run high end SATA/SCSI cards.

Thanks for the response. So the best thing for a tri SLI setup would be 16x/16x/16x right? Just double-checking my understanding (-:

And if a motherboard simply says "PCI Express 2.0 x16 : 3", what would be the worst-case assumption about that board? 16x/1x/1x ?

 

[4745454b]

Best might be 16x/16x/16x, but I don't think any are made with that many lanes. If you want tri SLI, 16x/8x/8x is probably the best you can do.

Technically worst would be 1x/1x/1x, but I don't think you'd see any of those either. 16x/16/1x pretty bad. You really don't want to run a GPU at 1x. And SLIing the others will slow them down to match the speed of the 1x.

 

[gkay09]

Well you do get full x16/x16/x16 on the current mobos...
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131358

And the best the AMD camp is X16/X8/x8 or x16/x16
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131363

 

[4745454b]

Ok, the newest x58 boards can do this, I'll be sure to update with this fact. I doubt 16x/16x/16x will be real world faster then 16x/8x/8x. I doubt running 8x/8x will really slow down the other cards.

 

[gkay09]

Yes you are right...those are just marketing points to sell costlier products...
Actually like you said the current graphics cards in crossfire/ SLI cannot take full advantage of those bandwidth...