PCI-E x16 (X16 and X8 confusion)

I'm looking for a motherboard, and so I think one of these two will be one of my picks:

But here's the problem: when I look under the specifications tab (it's above the description), the following comes up as a descriptor for its PCI Express 2.0 x16:

1 x PCI Express x16 slots, running at x16 (PCIEX16_1)
1 x PCI Express x16 slots, running at x8 (PCIEX8_1)


2x PCI Express 2.0 x16 slots with ATI CrossFireX support (single X16 or dual X8)

I don't really understand what this says, and when I look it up, I'm confused whether this means it has two PCI-E x8 slots or one PCI-E x16 slot...anyway, I can't figure it out. Can someone please explain what that means, and can you guys tell me if this old GFX card will fit into these new motherboards:

5 answers Last reply
More about confusion
  1. Quote:
    The first one means it has two x16 size slots with one of them running at x8 speeds.

    I can only assume that the board has a total of 24 pci-e lanes and chose to split them up like that.

    The second board has opnly 16 lanes and allows you to choose either to use one running at full 16 speed or use two slots for crossfire but running at only 8x speed each.

    The card will work fine.

    Okay, thank you very much for the response. What do those speeds mean? Does it restrict certain cards going into those slots, and does it mean that cards running at x16 speed will be faster than x8?
  2. Quote:
    Basically, pci-e lanes can only shove through so much data, so the more lanes you have the faster you can shove data through, it being a serial connection, each slot needs individual lanes rather than being able to share the same bus like a parralel one such as pci.

    So a slot with 16 lanes will hae a theoretical throughput twice as high as a slot with 8 lanes, however, it does not mean you will see an increase in performance unless the data travelling to and from the slot exceeded the capacity of the lanes if you see what i mean.

    That should be how it works.

    Alright. Thank you very much, now I get it, but do most video cards push enough info that the x8 is keeping it back?
  3. Quote:
    i do not know what causes bandwidth usage of the pci-e lanes myself, but have never seen a test that saturates pci-e 1 x16 or even 8x, pci-e 2 can handle twice as much as pci-e 1 so pci-e 2 8x is equal to pci-e 1 x16.

    Alright, thanks a bunch for the info!
  4. IIRC (its been a while) the 3870x2 came close to using all of a PCI-E v1 x8 slot.
  5. Tweaktown did a P45 vs x48 xfire comparison where they compared x16 and x8 PCIe modes for dual graphics cards. Sadly they only did a few tests but in most cases there was little of an impact on FPS, however, there was a FPS loss with Crysis and then with high AA/AF settings at the high resolution of 2560. I wish they would have tested with a lot more games, but it is what it is.


    If you are going to stick with a single GPU or lower resolutions with low AA settings, I wouldn't worry about it.
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