The Gigabyte X79-UD3 comes with four PCI Express x16 slots, two PCI Express x1 slots, and one standard PCI slot. Socket LGA2011 processors have a total of 40 PCI Express 3.0 lanes for video cards. This allows a very high-end configuration for the PCI Express x16 slots, but the exact configuration used will depend on the motherboard model.
On the Gigabyte X79-UD3, the first PCI Express x16 slot (PCIEX16_1) always run at x16 speed. The third x16 slot (PCIEX16_2) works at x16 if two or three video cards are installed, but drops to x8 when four video cards are present. The second and fourth PCI Express slots (PCIEX8_1 and PCIEX8_2) always work at x8 speed. Please note that if you have two video cards, for better performance the second one must be installed in the third slot, not in the second slot.
The first three PCI Express x16 slots support the installation of dual-slot video cards, but the fourth slot only supports dual-slot video cards if you have a computer case with at least eight expansion slots (cases usually have seven). When installing dual-slot video cards, you “kill” the slot immediately to the left (looking at the motherboard with its rear connectors facing up) of the slot being used.
All PCI Express x16 slots support both SLI and CrossFireX technologies.
The PCI Express x1 and standard PCI slots are controlled by the chipset.