The ASRock Z77 Extreme4 comes with two PCI Express 3.0/2.0 x16 slots, two PCI Express 2.0 x1 slots, and two standard PCI slots.
The two PCI Express 3.0/2.0 x16 slots are controlled by the CPU, with the first slot working at x16 when only one video card is installed, and with both working at x8 when two video cards are installed. Since these slots are controlled by the CPU, they will offer PCI Express 2.0 bandwidth (8 GB/s at x16) when a “Sandy Bridge” CPU is installed, but PCI Express 3.0 bandwidth (16 GB/s at x16) when an “Ivy Bridge” CPU is used.
The PCI Express x16 slots support both SLI and CrossFireX technologies.
Since the Z77 chipset doesn’t support standard PCI slots, these slots are connected to PCI Express x1 lanes using an ASMedia ASM1083 bridge chip.