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A question about PCIe lanes and how they are divided

Most intel desktop processors have 16 PCIe lanes, which is just enough for an x16 GPU. Obviously, you can't use the GPU without an OS to run on, thus you will need an SSD. So now you have an x4 NVMe SSD. Because the SSD needs to have 4 lanes to operate, does the GPU go down to x8 because it doesn't have enough lanes to operate at x16?

I have noticed that many of intel's desktop chipsets for Sky and KabyLake have anywhere from 6-24 "Max # of PCI Express Lanes" when all of the processors have only 16 lanes. If you put a 16 lane i7-7700 on a Z270 motherboard with a "Max # of PCI Express Lanes" of 24, does the motherboard have its own PCIe lanes that stack on top of the processor's?

@danj I was hoping you could explain this, please.

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what motherboard are you looking at?

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@captainsnowball I've built several PC's before. I understand them perhaps better than you. I'm not looking for any specific motherboard, I'm looking for a deeper understanding of how PCIe lanes work, which @danj hopefully can provide.

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OK! I'll keep an eye on this topic, maybe I can learn something from it.

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when a mobo maker talks about having 24 PCIE lanes they are actually referring to HSIO. a HSIO is the same (sort of but not really) to a pcie lane. a mobo connects everything together. so a mobo maker will use 16 for the 16x pcie slot. they'll use 4 for ram. and 4 for all your sata ports, m.2 slots,usb connectivity and ethernet. thats a rough outline and not gospel as all mobo makers can budget the HSIO lanes a CPU has however they want for a particular mobo product.

thats the reason why connecting an M.2 slot ssd can disable one or more sata ports. they are using the same HSIO lanes to the cpu

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There is no one answer here:

  • The CPU has more than one means of interconnection depending on the CPU series.
  • The CPU's connections can be directed to a PCH chip which can manage a greater number of PCI lanes.

So between the logic board and the CPU and how they work together will set the total number of lanes and then the lanes are distributed to the different slots as well as any dedicated interfaces on the logic board.

Update (01/04/2018)

Lets work off of a real CPU, chip set & logic board. Review these and focus your questions from these documents.

Update (01/05/2018)

George, it’s not that cut & dry! Look at this diagram

Block Image

As you can see the CPU’s PCI lanes are dedicated for graphics boards in this design.

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Hmm, okay. Your guess is more educated than mine, do you think that the 24 on the "Max # of PCI Express Lanes" means a total of 24, or simply that the chipset does not support any more than 24 lanes coming from the processor? Do macs have the same chipsets as the ones intel makes for motherboard manufacturers, or are they all custom? Also, do Sata and USB and RAM use PCIe lanes?

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Okay. I am familiar with all these components, and where what I had in mind.

The motherboard, has two PCIe x16 slots, four PCIe x1 slots, and one x4 slot for an NVMe SSD. That doesn't include USB, SATA, Ethernet.

Do all these devices have to work with only 16 lanes? Or, are you suggesting that the motherboard's lanes really do stack on top of the processor's, for a total of 40 lanes?

I noticed that the Z370 does have a PCH chip, right next to the PCIe slots. I thought that a PCH chip may have been something you would only find in Macs and expensive workstations.

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Depending on the generation of the system you'll have a North & South bridge chip or just the combined PCH. Here's a good reference: Intel® 7 Series / C216 Chipset Family Platform Controller Hub (PCH) Review the PCI hub and the I/O sections

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Oh, PCH is the abbreviated form of platform controller hub! I know what that is. Alright, thank you. Just one last question, the PCH's lanes plus the processor's do add up together, correct? For a total of 40 PCIe lanes.

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I can't answer it that way! How I make bread is not the same way you would!

Different designers will do different things depending on what they are aiming for.

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So, is there a way I can check how many PCIe lanes I have left on the chipset?

The GPU is obviously using some lanes? And I’m using 4 lanes for my new M.2 SSD.

Is there a way to check how many I have left because I want to add a capture card later and also another SATA adapter as well to get more SATA ports.

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You'll need to review your systems PCH logic to see how many PCIe lanes it supports. You might want to talk to the system or logic board vendor if you built your own system to see what they say. And, lastly look at your BIOS settings it often offers information on whats being used.

The GPU slots use lanes that go directly to the CPU and don't count for PCIe lanes served by the PCH.

Don't forget your system might have a PCIe bridge which then allows additional lanes to be available. But you'll then need to figure out which devices don't need direct access and can then use the bridged connection to off load lanes on the primary PCH.

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