[Solved - can't read] Strange pcie lane sharing on Asus prime x370-pro?

Hardware Motherboards
1

For the first time ever I will probably be using all slots and all lanes on a motherboard, so I took a look at the board manual. :open_mouth: I know, right :grin:

Well, I have a hard time understanding how pcie lanes are shared between slots and devices on this board. Here is the link to the user manual:


I am looking at page 20 and 21 of the pdf. As far as I understand, on the x370 platform with Ryzen, as soon as I use both slot 1 and 4 (the first two full length slots) these two slots will be running with pcie gen3 x8 right? However, when I look at the bottom table on page 21 the second x16 slot shares bandwidth with the chipset, the M.2 slot and the Asmedia connector (I am guessing sata?). So does this mean the M.2 slot is connected via the chipset and somehow shares bandwidth with the second pcie slot that is usually used for dual gpu setups? This is completely different than on every other board that I have seen.

I just want to arrange my expansion cards in a way that makes most use of the bandwidth without bottlenecking. I don’t use all simultaniously so I am trying to make sense of the pcie lane sharing on this board.
Here is the table I am talking about:

prime-x370-pro_pcie_lane_sharing
prime-x370-pro_pcie_lane_sharing.PNG1461x965 156 KB

2

Ryzen has 24 PCIe 3.0 lanes. On X370 they’re, 16x for GPUs which can be split to 8x/8x, 4x for NVMe and 4x for chipset.
Chipset also provides 8 PCIe 2.0 lanes for mostly 1x devices.

3

I know, right? But the table in the manual completely contradicts this layout, at least how I understand it. This is why I am so confused.

4

Have I missed something or am I just interpreting the table wrong?

5

Well, IRQ =/= PCIe.

The page before the one you are looking at has the PCIe 3.0 slot details, the first x16 slot will run at x16, or x8/x8 with the second x16 if cards are in both slots.

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image.png665x904 59.6 KB

Ryzen has x4 PCIe 3.0 lanes dedicated to M.2 or other PCIe based storage (like SATA Express). The M.2 slot on your motherboard is not shared with any other slots since there are no PCIe 3.0 slots other than the x16 slots nor is there SATA Express.

The PCIe 2.0 slots run off of the x8 PCIe 2.0 lanes that Ryzen processors have for other/general expansion cards.

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6

:man_facepalming: I am an idiot. How did I not read that?
Thanks a lot!

7

It happens to the best of us!

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