Why does the Intel Core i9-10900K processor only support 16 PCI-e lanes?

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They are no motherboards, with 32 PCI-e slots.

you misunderstand the relationship between slots and lanes.  look up what your motherboard actually supports at the maker's website.  you'll need the exact model number, which you can get by running DxDiag from the command prompt.  -- grampa

i think you misunderstand something. the CPU itself doesn't have 16 lanes, it SUPPORTS up to 16 lanes at a time, which means it handles up to x16 PCI-Express slots on a motherboard.

many boards can handle multiple PCI-E slots, but NONE of them can be over x16 speed.

Usually when you use an NVMe drive it connects using the lanes provided by the chipset. But that depends on how you connect it. Most of the time the M.2 slots are linked to the chipset as with the PCI-E x1 slots.

The CPU actually has x20 lanes but x4 are reserved for the chipset. After it's connection with the chipset, The CPU is left with x16 lanes that can go to the GPU or other devices like add-on cards. This depend on which slots you use. If you're using a z-series board and you connect a double NVMe M.2 card into the 2nd PCI-E x16 slot then it will use 8x lanes from the CPU with the other x8 lanes going to the primary GPU.

Otherwise you can use the lanes from the chipset, in which case the z490 chipset has x24 PCI-E 3.0 lanes. That's why I have strongly discouraged anyone from buying the low end h110, h310, and h410 chipsets because they come with only x6 PCI-E 2.0 lanes. The b-series motherboards are usually $10 more and they come with more PCI-E lanes.

The motherboard chipset will also have its own PCI-E lanes. The i9-10900K is intended as a gamer / overclocker's chip, not for somebody who needs a lot of I/O. Yeah, if somebody was dumb and paired a 10900K with an H410, they might have a problem. But the Z490 and Q470 have 24 PCI-E by themselves, the H470 has 20, and the B460 has 16. Plenty of lanes for a couple NVMe drives.