PCIe lane allocation and speed with older standard

[Sharpman85]

Hi All,

 

I have a question on how PCIe works with older standards as I am planning to get a new X570 motherboard (ROG STRIX X570-I) and judging from the previous models I assume that the second nvme storage will be connected to the CPU lanes used for the GPU and thus cut down the actual availability to 8. I hope that they will connect it this time to the chipset and allow usage with sata m.2 drives, but I digress.

My question is, how does PCIe work with older standards. Will using a PCIe 3 GPU and only 8 lanes (PCIe 4) allow for full PCIe 3 x16 bandwidth or will it be limited to PCIe 3 x8 as only that many will be available? Logic dictates that it will be at full speed, but I want to be sure and I do not have much experience with lane sharing.

 

All input highly appreciated!

[GoldenLag]

31 minutes ago, Sharpman85 said:

judging from the previous models I assume that the second nvme storage will be connected to the CPU lanes used for the GPU and thus cut down the actual availability to 8.

No?

 

AM4 has 24 CPU lanes. 16 to slots and 4 to NVMe and 4 to chipset. 

 

32 minutes ago, Sharpman85 said:

Will using a PCIe 3 GPU and only 8 lanes (PCIe 4) allow for full PCIe 3 x16 bandwidth or will it be limited to PCIe 3 x8 as only that many will be available?

No the lanes will run at the slowest PCIe revision of the device its connected to. Only PCIe 4.0 will run at 4.0.   PCIe 3.0 will run at 3.0 speed. 

 

Just like how PCIe 1-3 worked 

34 minutes ago, Sharpman85 said:

hope that they will connect it this time to the chipset and allow usage with sata m.2 drives, but I digress.

There were allready chipset NVMe slots and Sata slots. 

 

Depending on the board, X570 will bring more of that

[Sharpman85]

44 minutes ago, GoldenLag said:

No the lanes will run at the slowest PCIe revision of the device its connected to. Only PCIe 4.0 will run at 4.0.   PCIe 3.0 will run at 3.0 speed. 

 

So the 8/16 lanes will run at the speed of the device connected? That cleares a lot of confusion from my side, thank you.

 

46 minutes ago, GoldenLag said:

There were allready chipset NVMe slots and Sata slots. 

 

Depending on the board, X570 will bring more of that

I know, but the itx model had them only connected to the CPU and the backside one shared lanes with the GPU.

[GoldenLag]

1 minute ago, Sharpman85 said:

So the 8/16 lanes will run at the speed of the device connected? That cleares a lot of confusion from my side, thank you.

yes. it will run at the slowest of the connected ends. 

 

2 minutes ago, Sharpman85 said:

I know, but the itx model had them only connected to the CPU and the backside one shared lanes with the GPU.

highly dependant on which model we are talking about. there is usually a CPU NVMe, unless the mobo vendor redirected it. 

[Sharpman85]

1 hour ago, GoldenLag said:

highly dependant on which model we are talking about. there is usually a CPU NVMe, unless the mobo vendor redirected it. 

I understand, in the case of the ASUS ROG STRIX X470-I there were two CPU NVMe drives, but one shared it with the GPU. I'll check when the specification is out. Thank you for your answer regarding the PCIe speed.