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CPU PCIe Lanes vs. Chipset PCIe Lanes

I'm having difficulty finding info on this. Could someone who knows about this stuff let me know if my understanding is correct, and perhaps clarify this more for me?

 

My points of reference here are an Intel i7-8700k rated at 16 PCIe lanes, and an MSI MEG ACE Z390 motherboard with the Z390 chipset being rated at 24 PCIe lanes.

https://www.intel.com/content/www/us/en/products/processors/core/core-vpro/i7-8700k.html

https://www.intel.com/content/www/us/en/products/chipsets/desktop-chipsets/z390.html

 

  • My understanding is that I get the CPU's PCIe lanes PLUS the chipset's PCIe lanes to assign to stuff.
  • I can only assign CPU PCIe lanes at x4, x8, and x16 speeds.
  • I can only assign chipset PCIe lanes at x1, x2, and x4 speeds.
  • That means I'm probably not using any CPU PCIe lanes for stuff like my NVMe drive, ethernet, USB ports, etc., because they can all be assigned to the chipset's lanes.
  • And that also means that I probably can use all of my 16 CPU PCIe lanes for my graphics card (although I know that's not exactly necessary).

So am I understanding this right?

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Just a note, you do not assign PCIe lanes. The system and the connected device negotiate for the lanes. If slots/connectors share lanes, then there's going to be precedence over who gets them.

 

7 minutes ago, BecauseICanTBH said:

My understanding is that I get the CPU's PCIe lanes PLUS the chipset's PCIe lanes to assign to stuff.

Yes, but not all of the PCIe lanes are available for use. A lot of them go to the I/O included on the motherboard.

Quote
  • I can only assign CPU PCIe lanes at x4, x8, and x16 speeds.
  • I can only assign chipset PCIe lanes at x1, x2, and x4 speeds.

It can be any number of lanes in powers of 2, up to the maximum lane count the slot supports.

 

Quote
  • That means I'm probably not using any CPU PCIe lanes for stuff like my NVMe drive, ethernet, USB ports, etc., because they can all be assigned to the chipset's lanes.
  • And that also means that I probably can use all of my 16 CPU PCIe lanes for my graphics card (although I know that's not exactly necessary).

As I said before, you don't assign any lanes. Though any PCIe slot meant for the graphics card gets lanes from the CPU. Everything else gets it from the chipset.

Edited by Mira Yurizaki
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For better performance in NVMe I'd use the CPU lanes (check your manual; RTFM).

Ryzen 7 3700X / 16GB RAM / Optane SSD / GTX 1650 / Solus Linux

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