Question 16 PCIe lanes CPU with 16x PCIe graphics card, 4x PCIe M2 SSD and 4x PCIe external network card?

vase7u

Commendable
Oct 7, 2018
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I'm thinking of upgrading my CPU to Intel I3-9100f and my motherboard to Gigabyte B365M DS3H and eventually adding an M2 NVMe SSD and probably an external 10 gigabit network card.
What confused me is that the specification for this CPU and every other Intel's 9th generation CPU that I checked, state that they support x16 PCIe lanes in configurations: 1x16, 2x8, 1x8+2x4, which seems like a bottleneck.

After some research I'm left with the impression that this 16 lanes are connected to the 1st PCIe slot and there are separate (undocumented) 4 lanes connected to the chipset, which itself is connected to all the rest of the PCIe peripherals (the rest of the PCIe slots, the M2 slot, USB and SATA interfaces). This sounds great, but why would there be different PCIe lanes configurations then (e.g. 1x8+2x4)?

I wanted to check if this is right and what is the deal with these PCIe lanes.
 

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