PCI-E Lanes - CPU + Chipset

[Grumbledook]

Just been looking at AM5 motherboards and I'm a bit confused about PCI-E Lanes.

For example the Asus ROG STRIX X670E-A GAMING WIFI has a x16 PCI-E Gen 5 slot and then also supports 2x M.2 PCIe 5.0 x4 mode slots (all from the CPU rather than the chipset). Am I right in thinking that would be 24 PCI-E Gen 5 lanes?

If so, that's more than the 20 PCI-E lanes for the X670E motherboard. I can't see anything on the specifications page or in the manual suggesting that filling all of those will cause slowdown, or a drop to x8 on the PCI-E slot.

Are those 20 lanes in addition to extra ones on the CPU?

 

[mickyflinn]

24 pcie5 lanes

Read this

AMD AM5 chipsets explained (X870E, X870, B850, B840 & all the others)

Are you planning to build a PC using an AMD Ryzen processor? Maybe you want to buy one of the processors from the latest Ryzen 9000 series, or maybe you prefer an older Ryzen 8000 or Ryzen 7000 series CPU. No matter which processor you're choosing, you need a motherboard based on the AMD AM5 […]

www.digitalcitizen.life

 

[Grumbledook]

Ah nice one, looks like the top result when I tried googling this beings up a page with the wrong info on it.

For anyone else confused by this, the Ryzen 7000 series CPUs have 28 lanes, of which 24 are useable (the site I found said 20). 4 lanes are reserved for communicating with the chipset, 16 for the GPU (or 8+8 depending on the motherboard), 4 for a NVME drive and then 4 spare the motherboard manufacturer can use for what they want. In the case of that Asus motherboard I mentioned above, it's for a second NVME M.2 drive.

So that explains why there isn't any mention of slowing down or restricting another port if you fill all of those up on this particular board.

Whether PCI-E Gen 5 lanes are even worth having at this moment in time for the extra price you have to pay is another matter.

Useful article to explain the differences compared to everything else I've read to date, cheers micky!

 

[Tetras]

For anyone else confused by this, the Ryzen 7000 series CPUs have 28 lanes, of which 24 are useable (the site I found said 20). 4 lanes are reserved for communicating with the chipset, 16 for the GPU (or 8+8 depending on the motherboard), 4 for a NVME drive and then 4 spare the motherboard manufacturer can use for what they want. In the case of that Asus motherboard I mentioned above, it's for a second NVME M.2 drive.

Just one caveat to that, typically they only make all of these lanes (at least, operating at PCI-E 5.0) available with a X670E board, but theoretically even a low-end board could have 2x PCI-E 5.0 M.2 drives with no impact on the GPU.