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I'm currently on using a i3-4160 CPU and the total number of PCI-E lanes it supports is 16. I'm currently have a device taking up a 1x slot and have another 16x slot free. If I install a graphics card into the 16x slot, what will happen? Will:

  1. 1x slot device gets disabled to get the full 16x for the graphics card
  2. Only 8 lanes in 16x slot gets used such that both graphics card and other device work in conjunction
  3. 1x slot device still works but graphics card can only use 15 lanes

These 3 are the only answer that comes up to my mind currently. Which one is it? Or is it anything else other than these 3 answers? Thanks in advance.

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  • Intel has a great diagram that's explains how PCI-E is integrated into their CPUs and Chipsets go look at that. Your very first statement is partially incorrect – Ramhound Mar 31 '16 at 11:03
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How does PCI-e work with insufficient CPU lanes?

Let me answer that in three parts:

  1. It is possible to use a chip to multiplex PCI-e lanes. Sort of an electronic Y-splitter. This is done on a few expensive motherboards.
  2. Or they can switch the number of used lanes. E.g. from x16/x0 to x8/x8.
  3. Or they add other sources of PCI-e lanes. PCI-e direct from the CPU is relative new. Before all PCI-e lanes came from the chipset. These days you need to add both.
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  • This is a great point, "These days you need to add both." but it is also illustrated in Intel's chipset diagrams – Ramhound Mar 31 '16 at 13:46
  • Even before PCIe was integrated into the CPU you still had PCIe lanes from two seperate places. Lanes from the MCH/IOH/northbridge intended for graphics and lanes from the ICH/southbridge for slower perhiperals. Afaict there was only a very narrow window where the northbridges had PCIe but the southbridges did not. – plugwash Aug 21 '19 at 3:27
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I'm currently on using a i3-4160 CPU and the total number of PCI-E lanes it supports is 16.

In addition to the 16 PCIe lanes from the CPU, there is also an interface called "DMI", this connects the CPU to the PCH, the DMI interface on your generation of CPU is basically equivalent to PCI express 2.0 x4. The PCH in turn provides a number of PCI express lanes for slower prehiperals. If all of these lanes were maxed out at the same time then the DMI interface to the CPU would be a bottleneck, but in practice this is not typically a huge problem. Only a couple of devices are typically maxed out at any given time.

An x1 slot is almost certain to be connected to the PCH, so it will not have any impact on the graphics card, though in some cases it may not be able to be used at the same time as some onboard features.

An x8 (electrical) slot on the other hand is most-likely to be connected via signal-switches to the lanes from the CPU. So putting a card in it will reduce the number of PCIe lanes available to the graphics card from 16 to 8.

An x4 (electrical) slot may be connected either way, you would have to study the motherboard's documentation to find out.

On some high end boards you may also find PCI express bridge chips. Unlike the crude signal switches mentioned above these allow the bandwidth to be dynamically shared between different devices.

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The second one is the closest, but depends on the manufacturer of the motherboard. Third one is plain wrong. First could happen, also depends on manufacturer. You should take a look at the motherboard spec sheet to see if your x1 slot is from the chipset or CPU.

There are many configs possible; the x1 slots could be tied to SATA slots so that only one of them works; or to an M.2 slot(google for info on that), or anything else.

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Given that the x16 graphics card and the x1 card (in my experience, usually a serial/modem or multu-IO, though I've seen SSDs for x1) are used for different functions and the cards usually installed in x1 slots are typically for much less time-critical functions, both can work at full capacity (if the motherboard is well designed). The CPU will talk to the x16 card full time and, when necessary, skip a frame to talk to the x1 card.

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