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What does it mean that a "PCI-Express 3.0 X16 Slot runs at X8"?

Thinking about buying the ASrock Extreme4 for my computer. However, under slots on the amazon page it says
"2X PCI-Express 3.0 X16 Slots (runs at X8)"

What does this mean? I don't want to buy with knowing what I'm buying.
Thanks.

PS here's the page for full context.
http://www.amazon.com/LGA1155-CrossFireX-Motherboard-Z77-EXTREME4/dp/B007KTY4A6/ref=wl_it_dp_o_pC_nS_nC?ie=UTF8&colid=LM67QDIAZZHH&coliid=I3555V339CXQ6L
9 answers Last reply Best Answer
More about what express slot runs
  1. It mean the slot are full length (i.e. video cards) but run at half that, because it's sharing bandwidth ("lanes") with other slots/devices. Pretty common.
  2. How do they run at half length?
  3. It probably meant 8x/8x dual mode, the pcie 3.0 slot single is at 16x, single at x16 (PCIE2) / x8 (PCIE3)
  4. Sorry, meant it's running at the speed as if it was half that length.
  5. So how much of an effect will that have on the video card's performance?
  6. Best answer
    Next to none. Until you get down into PCIe 1 or PCIev2 x4 you aren't going to see any drops.
    Here's a link that shows the effects,

    http://www.techpowerup.com/reviews/Intel/Ivy_Bridge_PCI-Express_Scaling/23.html
  7. +1
  8. Gpu set up look like this. 1x gpu=x16; 2x gpu=x16+x16=(x32); 3x gpu=x16+x8+x8=(x32); 4x gpu=x16+x8+x4+x4=(x32). so 2xgpu=4xgpu. So insert the gpu in x16 slots.
  9. Best answer selected by RS13.
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