PCIE 1x, 4x, 8x, 16x bus pins and signals

What is the difference between PCIe Gen 3 and PCIe Gen 4? pcie slot configuration gen1 gen2 gen3 Older standards, or systems where PCIe interfaces are using fewer data lanes as discussed in BIOSUEFI Configuration for Optimizing PCIe®NVMe®SSDs, will pcie 4 nvme in pcie 3 slot Gen2 on this mb somehow lane distributionbandwidth layout of mb caused GPU in slot

pcie x 16 slot PCI Express Gen1, Gen2, and Gen3 Protocol to Physical Layer Analysis for P67SA16S