Explanation of Intel z170 chipset's PCI-Express lane counts?

Question

Officially z170 chipsets have total 20 PCI-E 3.0 lanes, and it is shown as 1x16 OR 2x8 or 1x8 + 2x4.

So it seems like if I insert one 16x PCI-E device (e.g. graphics card) I won't be able to use the rest of the lanes (one 4x speed PCI-E lanes). 20-16=4 PCI-E lanes wasted?

I have one 16x graphics card and one 1x speed sound card in my PC at this moment, so how does that work out? Is my graphics card working at 8x speed and sound card 1x speed or is it like 16x + 1x like it mathematically should be?

2nd Question is what happens to the speed of the PCI-E ports if the configuration now changes to one 16x graphics card+ one 4x m.2 ssd? Or what happens if one 16x graphics card+ one 4x ssd + one 1x sound card?

3rd Question and this is not important because I will probably never buy a PCI sound card, how do things play out if a PCI sound card or LAN card come into the equation?

I didn't actually find any proper answers to these questions in other sites.

Note: My hardware is Asus z170-p ddr4

Solution 1

The primary GPU is connected directly to the CPU. Its PCIe connection is not routed through the chipset.

The chipset’s lanes are used for the following:

• Secondary x16 slot (x4)
• Two x1 slots (x2)
• M.2 slot (x4)
• Gigabit LAN (x1)
• PCI bridge (x1)

That’s a total of 12 used lanes. I may have forgotten something, or there may be limitations why they couldn’t make the secondary x16 slot x8, who knows.

Solution 2

Your motherboard doesn't not seem to support any modes but 16+4: https://www.asus.com/Motherboards/Z170-P/specifications/

So if you install a graphic card on the gray x16 slot, you can still install some other card (graphic or not) on the black x16 slot and get x4 bandwidth (or lower depends on the card itself).

Each pci-e 3.0 lane is 8GT/s, i.e. 985MB/s: https://en.wikipedia.org/wiki/PCI_Express

I have NOT seen from the aforementioned spec or the user manual that the pci-e x1 slots / pci slots / m.2 slot shares with the pci-e x16 slots the 20 lanes.

Comments

[Daniel B]

The graphics card is usually directly connected to the CPU’s PCIe controller.

[Najeeb Taher]

What does that mean? I am sorry I am newbie at this.

[Burgi]

You should only have one question per question!

[Najeeb Taher]

The paragraph you referred to ('Expansion slots') indicates number of slots of each type not how fast/which mode they will operate when other slots are occupied. So according to your reply the answers of my questions are - 1. 16x+1x 2a. 16x gpu+4x m.2 2b. 16x gpu+4x m.2 + ? (1x sound card won't run?)

[Tom Yan]

Not really. Yes it does tell the number of types but it also tells the mode (x16 mode, gray) (max at x4 mode, black). If the x1 slots shares bandwidth with one of the x16 slots (x4 bandwidth), there should be a note about it on the spec and an option in UEFI setting, like my board: asus.com/Motherboards/H87PRO/specifications

[Tom Yan]

And here is one that has three x16 slots which support x16 / x8 + x8 / x8 + x4 + x4, with one extra x16 slot with independent x4 bandwidth: asus.com/Motherboards/MAXIMUS-VIII-EXTREME/specifications

[Tom Yan]

It might have something to with DMI: en.wikipedia.org/wiki/Direct_Media_Interface, which happen to have the same bandwidth of the "corresponding" version of pcie and it has always been 4-lane or less.