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Many P55 and P67 motherboards include three x16 slots. Most of those slots are stuck at x8 or x4 bandwidth. Because the CPU itself is able to address a pair of devices from its sixteen lanes (Intel's desktop CPUs actually include three PCIe controllers), many motherboards add automatic switches that allow its first two x16 slots to share those lanes in x8 mode whenever a card is inserted into the second slot. The slower, third slot gets four lanes from the chipset's available PCIe lanes.
There are, of course, less expensive P55 and P67 motherboards that, lacking the somewhat-costly PCIe lane switches, only have a x16 and x4 slots to begin with. This configuration also covers enthusiast-grade H55 and H67 motherboards, since Intel artificially limits the CPU’s dual-card capability when used on its integrated graphics platforms.
While Nvidia prevents SLI from functioning on PCH-hosted lanes, x16/x4 configurations are completely possible in CrossFire. But should they be? We tested our motherboard in both x8/x8 and x16/x4 configurations to find out.
At 1680x1050, the best result is a 1% loss in Crysis, while the “worst” result is a 26% loss in F1 2010. The average difference between x8/x8 and x16/x4 is fairly bad at 10%. However, a worst-case scenario this bad will stick with us for a long time.
Our worst case is reduced to “only” a 24% performance loss at 1920x1080, with the average difference staying at 10%.
We’d expect to see GPU computational limits exceed bandwidth limits at 2560x1600, yet the worst-case scenario still shows 23% lower performance for the x16/x4 configuration. Again, the average difference is stuck at 10%.