How many PCI-e Lanes do I have?

[SaintsEnd]

So this motherboard here has a Z170 Chipset that supports up to 20 pcie lanes, and this processor supports up to 16 pcie lanes. So does this mean that the motherboard has a total of 36 pcie lanes combined, or does it only have 16/20?

I have questions based on the answer

 

[Geekwad]

16 directly to the CPU (through PCIe lanes), and then 'at least' four dedicated to the CPU via the PCH (for SATA devices, USB, etc).

The short answer is: you have 16 to use.

 

[faalin]

your top PCI-e slot runs at 16x and the lower one runs at 4x

Your cpu only has 16 lanes so if you only put a gpu on your board it will run at 16x. If you run a gpu and something else in the 4x slot your cpu will split its lanes and run 8x on the top slot and 4x on the bottom.

So no you do not have 36 lanes, you are limited by the cpu.

 

[SaintsEnd]

Why can't I find a CPU for the LGA1151 Socket that supports 20 pci lanes?

 

[giantbucket]

the top slot's PCIe lanes are directly connected to the CPU, and the bottom set of slots are usually channeled through the chipset (Z107, H81, A75, whatever)

the answer to your inevitable question is "it won't matter right now" (just guessing)

 

[Geekwad]

It's worded weirdly, but it does. It has to do with the way the CPU, motherboard, and chipset are designed, but what you are finding is how many 'dedicated' PCIe lanes it has. 16 is the answer.....which is what you can assign yourself. 4 of them minimum are used with the chipset, so is why you see a 20 figure floating out there. It 'can' make use of the other 16 dedicated lanes if you have not used them, but as you subtract lane by adding devices that use up to 16 lanes, the 20 figure goes down to a minimum of four.

http://images.anandtech.com/doci/9485/Z170%20Platform.jpg

Make sense?

 

[SaintsEnd]

I believe that does make sense, so although I cannot assign to 4 pcie lanes, the CPU physically has them? It has 20 but 4 are dedicated to the chip set for sata, usb ect? Let me know if Im getting it, either way Ill read up more about it online.

 

[giantbucket]

typically yeah. 16 are dedicated to the PCIe-16 slot, and the others (4 in your case) are dedicated and hard-wired to the chipset which then distributes and manages the traffic amongst the other slots, the SATA ports, the USB headers, etc.

 

[Geekwad]

You've got it. The chipset has 4, you have 16, for a total of 20. The chipset can have what you don't use, but you can't assign the chipset's minimum allocation of 4.

How you use your 16 is up to you and the particular motherboard though. It could be all 16 in one GPU, or 8x8 with two GPUs. Could be 8x4x4 for one GPU and two x4 devices....each motherboard will offer a breakdown of how the slots can be used. If you go to the manufacturer's site and check out the mobo manual, it will tell you what your possibilities are.

 

[SaintsEnd]

Do you know of any good reads that go further in depth for this topic?

 

[giantbucket]

intel probably has a designer's guide or some such thing. probably more white papers than you could digest before you die.

 

[SaintsEnd]

Thanks for your help giantbucket, Geekwad and faalin.

 

[Geekwad]

Do you know of any good reads that go further in depth for this topic?

This article may help:

http://www.anandtech.com/show/9485/intel-skylake-z170-motherboards-asrock-asus-gigabyte-msi-ecs-evga-supermicro

Or make it worse

It starts to (I think) conflate the PCIe 3.0 connections from resources/devices to the PCH, which can easily be confused with PCIe 3.0 connections to the CPU. They are different.