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Intel Z370 Motherboard Buyer's Guide (Page 6)

By Steven Bassiri from Jan 17, 2018 @ 18:00 CST

PCI-E Considerations

All 8th Generation Core processors offer x16 CPU lanes, and can fan that out to x8/x8 or x8/x4/x4. However, not all vendors allow the CPU's lanes to go to x8/x4/x4, and instead provide the last x16 full sized slot as x4 from the PCH, so they can do x8/x8/x4, but if you run three GPUs in it like that you are limited because the last GPU's bandwidth has to go through the PCH and DMI.

If you have one GPU and want to use another PCI-E slot and all are routed to the CPU, then your GPU will be downgraded to x8 mode, even if your other device only uses x4. However, if latency is an issue, then it makes sense to route lanes to the CPU, especially if you want to run 3-way CrossFireX.


The implementation above has the first and second x16 slots routed to the CPU and the last x4 slot routed to the PCH. The CPU's lanes are distributed x16/x0 or x8/x8. It's the simplest implementation, and we can see how the PCI-E switches are routing lanes, so that each of the ASM1480 PCI-E 3.0 2-lane quick switches can move lanes between the first and second slots. If a motherboard only has four PCI-E quick switches, then it only can move x8 PCI-E between two slots.


Here we have an implementation that provides the CPU the ability to have its lanes put out as x16/x0/x0, x8/x0/x8, or x8/x4/x8. The last slot is routed to the PCH's PCI-E lanes. If a motherboard has six PCI-E quick switches, then it can switch x16 PCI-E lanes to three slots for a maximum fan out of x8/x4/x4. We can even see how two of the PCI-E quick switches route directly to another set of two switches to move that other x4 around. However, not all quick switches are used for PCI-E slot routing; many are used for SATA/M.2 switching.


Here we see another x16/x0/x0, x8/x0/x8, or x8/x4/x8 implementation, but you are probably wondering why there are only four PCI-E quick switches. The answer is because the other two switches are hidden on the back of the motherboard to make things look cleaner.


The best way to know how lanes are distributed is to look at the manual or specifications, but some people take shortcuts. One shortcut is to count PCI-E quick switches, but quick switches are also used outside of the PCI-E lane area to switch M.2 and SATA bandwidth between slots, ports, and connectors, so it's not a foolproof method unless you know what you are looking at.

However, as a PSA, I wanted to show the two motherboards above. Many people can recognize that many vendors have put metal shields around slots connected to the CPU and not around those connected to the PCH. However, that is not always true, as both of the motherboards shown above have the same PCI-E allocation where only the first two slots are routed to the CPU while the last x16 slot is routed directly to the PCH.

Many times, if that last x16 slot is routed to the PCH, it also shares all of its bandwidth with one of the M.2 slots, and that is true on the boards above. You need to look at your manual to figure out how that routing is done. The best advice we can give is to read the manual, as vendors still put a lot of time and effort into them. In conclusion, we hope this guide helped you better understand motherboards and our reviews. If you have any questions, please let us know in the comment section, or you can email the author.

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