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I'm looking at the Asus Maximus XII Hero. Its webpage says it supports the following:

MULTI-GPU SLI®/ CFX SUPPORT​

  • 2 x PCIe 3.0 x16 Safeslots (x16, x8/x8)​
  • 1 x PCIe 3.0 x16 slot (x4)

I understand the first bullet point, but it's the latter line that is confusing me. Why does it say x16 and also x4? Other Asus pages also use the terminology x4 mode. So which is it, x16 or x4?

My web research showed unclear results, but you're the brilliant one. Thanks.

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    It's a slot of x16 size (meaning it accepts cards of x1, x4 or x16) but electrically x4 (meaning any card in it runs at maximum x4 speed). This should be a dup of somewhere because these configurations has been around since the first year of PCIE. Dec 25, 2020 at 3:28
  • Ohhh, wow, I had on idea. I think that PCIe slot uses the chipset's pci-e lanes rather than the CPU's. Is there much of a difference in performance between the two? Dec 25, 2020 at 5:14

1 Answer 1

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16x is physical slot size, that is capable of wiring up to 16 PCIe lanes, but on your motherboard that slot has only 4 lanes connected to it.

For the former slot with 16x or 8x/8x that means that when only one slot is occupied - it will get all 16 lanes, but when two slots are used - these 16 lanes will be divided by them.

This happens because CPUs have limited PCIe lanes count. To wire more lanes CPU need to have more pins in socket, more layers and copper on the motherboard etc. This is more expensive and most of buyers will not use more than 1-2 PCIe devices anyway. Thus for some applications where these lanes are actually needed - one will have to install a top-tier server CPU with lots of cores that may not actually be used.

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  • Why does a PCIe slot which has only 4 lanes physically has 16-slot size? Why not make it have only 4-slot size physically?
    – Bruce
    Mar 14, 2023 at 8:48
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    @Bruce this way it is possible to install a 16x-capable device and still have it working (albeit at 4x), if the slot is physically 4x - device will not fit at all
    – Vasfed
    Mar 15, 2023 at 9:25

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