PCIe lanes and Resizable BAR

In short: PCIe lanes are the data paths between CPU, chipset and expansion cards. How many each device gets decides its speed. Resizable BAR lets the CPU address all graphics memory at once.

In more detail: A lane carries data in both directions. Cards come with x1, x4, x8, x16 lanes. Each PCIe generation doubles the rate per lane.

In Depth

• Distribution: The CPU has a fixed number of lanes (for example 20 to 28 on desktop CPUs), the chipset hangs off a link. The graphics card (x16) and one NVMe SSD (x4) usually get direct CPU lanes. Further slots share lanes: an additional M.2 slot can disable a slot.
• Read the manual: The motherboard manual shows which slots share lanes.
• Effect: A graphics card loses usually only a few percent when run at x8 instead of x16. At x4 or an old generation more.
• Resizable BAR (ReBAR): Normally the CPU sees only 256 MB windows of VRAM. With ReBAR it can address everything, which gives a few percent in some games. Enable in BIOS (“Above 4G Decoding” on).
• Checking: GPU-Z or lspci -vv show link width and speed.
• See also: PCIe, SATA, PCIe and NVMe.

See also: VRAM