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I'm looking at a cheap Radeon HD 8570 for an older SFF PC, and I looked it up and saw it used PCIe 3.0 x8. However, I saw that it had pins like an PCIe x16 card. I think there were traces going to the back half as well. Since there are pins there and the speed for PCIe 3.0 x8 and PCIe 2.0 x16 are roughly the same, could this card have some sort of backwards compatibility to run at PCIe 2.0 x16 (rather than running at x8 on PCIe 2.0 slots)? If it helps, the GPU is from early 2013.

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If the GPU is only x8 then that means it only has the physical wires, the electrical connection, for an x8 connection. That it has a connector for x16 is irrelevant.

An x8 chip on an x16 card in an x16 slot will only ever establish an x8 connection.

The PCIe revision is also irrelevant. Just because PCIe is faster does not mean that the lanes somehow split, they don't.

The slot size does not tell you anything about how many lanes are wired up, only what could potentially be wired up.

PCIe speed has two limitations

  1. You can only work at the maximum number of lanes supported by the lowest device. If you have an x8 card and an x16 motherboard then you can only work at x8
  2. You can only work at the highest PCIe revision supported by both sides. If you have a PCIe 2 card in a PCIe 3 slot then it will work at PCIe 2 speeds.

Your card will work at either PCIe3 x8 or PCIe2 x8. Nothing more and nothing less. If it works at x16, then it is a true x16 card.

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  • Would you happen to be able to estimate how much PCIe bandwidth a GPU of this era would need at full load?
    – TheYootz Media Group
    Commented Nov 13, 2021 at 1:49
  • @TheYootzMediaGroup A single lane of PCIE v2 is about 500MB/s so you would have about 4GB/s of bandwidth. How much you "need" is down to the game and whether it loads all textures at the start (most games) or if it loads them when used and even then it depends on how large they are and how many are being loaded. Chances are that the bandwidth you have is enough to completely refill the memory once every second or two which is the extreme use case and not likely to be seen by most games and even if it were then usually your hard disk is going to be the worse bottleneck.
    – Mokubai ♦
    Commented Nov 13, 2021 at 7:15

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