Question CPU PCIe lane allocation

[PuzzledToo]

Thought I had this figured out when I built my last PC but now I have questions.

It seems current Intel CPUs have 20 PCIe lanes. For example, for the i5-12400 Intel states 20 PCIe lanes and 8 DMI lanes. Since they’re listed separately I presume the DMI lanes are not part of the 20 PCIe lanes.

Intel® Core™ i5-12400 Processor (18M Cache, up to 4.40 GHz) - Product Specifications | Intel

Intel® Core™ i5-12400 Processor (18M Cache, up to 4.40 GHz) quick reference with specifications, features, and technologies.

ark.intel.com

This shows 16 PCIe lanes for the GPU, 4 PCIe lanes for video, and 8 DMI lanes.

Intel Z690 Chipset Block Diagram - ServeTheHome

Intel Z690 Chipset Block Diagram

www.servethehome.com

But here it says “the CPU typically controls one PCIe slot and one M.2”.
That means 16 GPU +4 Video +4 M.2 = 24 lanes which means someone is getting shorted. How are the 20 lanes really allocated?

Bottlenecks

Computer speed has always been an elusive target. Numerous components work together to create a seamless computer experience. If one of those components is dragging behind the other components, this creates a bottleneck

silentpc.com

The chart in the above link shows that some chipsets use 4 DMI lanes & some use 8 DMI lanes. What happens if you use a CPU with 8 DMI lanes & a chipset mobo with 4 DMI lanes? Or vice versa? Is the bandwidth cut in half in those scenarios?

 

[hotaru.hino]

In the block diagram, I'm presuming you're looking at the top left part. There's nothing that says what those PCIe lanes are used for. Just that you get 1x16 PCIe 5.0 lanes + 1x4 PCIe 4.0 lanes or 2x8 PCIe 5.0 lanes + 1x4 PCIe 4.0 lanes. See say the spec page for the i7-12700K under "PCI Express Configurations", it explicitly lists what configurations you can have.

Regarding DMI lanes, assuming they're the same DMI version, 4 lanes has half the bandwidth of 8 lanes. DMI is just Intel repurposing PCIe for chipset communication, but since they can't be used for generic PCIe devices, it's not counted in how many lanes the CPU has.

 

[PuzzledToo]

This block diagram of a 1700 socket CPU shows 16 PCIe lanes for the PCIe x16 slot and 4 PCIe lanes for the M.2. It shows the video is not using PCIe lanes but some other connection.

AsRock B760M Pro RS - Review - Einfoldtech

Today we have a new Intel B760 chipset-based motherboard for review, the ASRock B760M Pro RS socket LGA1700 motherboard. This is a new, budget-conscious,

einfoldtech.com

IOW the CPU only uses PCIe lanes for those 2 slots. Is that correct?
What happens if you don’t use a GPU or the M.2 slot? Do those PCIe lanes get used for something else?

 

[kanewolf]

This block diagram of a 1700 socket CPU shows 16 PCIe lanes for the PCIe x16 slot and 4 PCIe lanes for the M.2. It shows the video is not using PCIe lanes but some other connection.

AsRock B760M Pro RS - Review - Einfoldtech

Today we have a new Intel B760 chipset-based motherboard for review, the ASRock B760M Pro RS socket LGA1700 motherboard. This is a new, budget-conscious,

einfoldtech.com

IOW the CPU only uses PCIe lanes for those 2 slots. Is that correct?
What happens if you don’t use a GPU or the M.2 slot? Do those PCIe lanes get used for something else?

No. They are physically mapped to the slots. If you don't have anything plugged in, they are unused.

 

[PuzzledToo]

Thanks -

Is there any significant performance difference if an Intel i5-13500 CPU with 8 DMI lanes is used with a B760 mobo which has 4 DMI lanes vs. a Z790 mobo which has 8 DMI lanes? Does that difference in DMI lanes affect gaming, video editing, etc.?

 

[kanewolf]

Thanks -

Is there any significant performance difference if an Intel i5-13500 CPU with 8 DMI lanes is used with a B760 mobo which has 4 DMI lanes vs. a Z790 mobo which has 8 DMI lanes? Does that difference in DMI lanes affect gaming, video editing, etc.?

It would depend on what devices were attached to the chipset. With multiple USB 3.2 or something like that it could make a difference.

 

[hotaru.hino]

Thanks -

Is there any significant performance difference if an Intel i5-13500 CPU with 8 DMI lanes is used with a B760 mobo which has 4 DMI lanes vs. a Z790 mobo which has 8 DMI lanes? Does that difference in DMI lanes affect gaming, video editing, etc.?

No, unless you plan on plugging in a device in every available interface the chipset offers and have them blast data to RAM in some form or fashion.

 

[PuzzledToo]

Thanks everyone for the information.
I'm thinking about building a new PC and want to make sure I understand how lanes work & how many are needed.