According to the specifications of an AM4-compatible motherboard, there are two PCI Express x16 slots. If only one of the x16 slots is populated it will communicate using all 16 lanes. If both x16 slots are populated then they both communicate using 8 lanes.
I want to know which component, or piece of software decides when to use all 16 lanes, and when to use only 8 lanes? Do the slots themselves have some sort of a “device present” indicator, or is there a switch chip somewhere which dynamically “re-routes” the lanes if a device is connected?