First USB3 Host Controller Now Available
The host controller supports up to two USB 3.0 devices, and connects to the northbridge via a PCI-E Gen 2 link, as shown in the diagram below.
And hey—if diagrams are really your thing, we've got this:
What we've got here is a USB controller with two external USB 3.0 ports. If a USB device is plugged in that doesn't support USB 3, there's an entire USB 2.0 implementation already on-chip. On a happy note, the presence of a separate USB 3 controller means two different devices can hook into the same port without both defaulting down to USB 2. Both controllers hook into Intel's xHCI (Extensible Host Controller Interface), which in turn attaches to a PCIe Gen 2 x1 interface. At that point, you drop the card in an actual PCIe slot on your motherboard, and everything is good to go.
In theory, this is actually a less-than-ideal situation. A first-generation PCIe x1 slot has a maximum theoretical transfer rate of 200MB/s, Gen 2 improves this to 400MB/s. The single-lane connection should deliver more than enough bandwidth, regardless of whether or not its Gen 2, but the USB 3 bus's maximum speed of 4.8Gb/s means we'll have an external interface that's potentially faster than the internal port it attaches to. If you want to saturate USB 3, even on an x1 electrical connection, you're going to need some serious mojo.
Now all we need are devices that use it.