Third Long PCIe Slot On High-End Z77 Boards Wired to CPU, Works Only with Ivy Bridge

It turns out that the "third" long (physical x16, electrical x4) PCI-Express slot on most higher-end Z77 chipset-based motherboards, across vendors, are wired to the CPU, and not the Z77 PCH, as the media assumed. Early buyers of these motherboards were greeted by an informative sticker stuck to the third slot, which tells them that to use the third slot, a 3rd Generation Core "Ivy Bridge" processor must be installed, although the motherboard very much supports 2nd Generation Core "Sandy Bridge" processors.

This can be explained by taking a close look at the block diagram of Intel Z77 Express system. Z77, in combination with "Ivy Bridge" processors, allows the CPU root complex to drive three devices. Its single PCI-Express x16 link can be arranged in three ways: x16/NC/NC; x8/x8/NC; and x8/x4/x4. As you can see, the third long slot is taken into the configuration. Intel figured out that since PCI-Express 3.0 x4 offers bandwidth comparable to PCI-Express 1.0 x16 to gen 3-compliant graphics cards, it's wise if the third electrical x4 slot is also wired to the CPU's PCIe root complex. This renders most high-end LGA1155 motherboards with such CPU-driven third x16 (x4) slots 3-way SLI/CrossFireX-capable. Sweet.