Seen most recently in our performance exploration of MSI's X58 Eclipse Plus motherboard, Nvidia's NF200 PCI Express bridge chip is necessary on a platform like this one, with its sixteen lanes of integrated PCI Express 2.0 connectivity.
Here's where the story gets a little crazy. X58 provides up to 36 lanes of PCIe 2.0, making it relatively easy to serve up four x8 links or, as we saw in the MSI story, mux lanes to deliver a trio of true x16 links (masking the fact that two x16 links actually share one x16 link from the host). So, with that many lanes available, building boards with six x16 slots makes sense. It's a niche proposition, sure, but for for the workstation users banking heavily on Nvidia's CUDA technology and running several Tesla cards in parallel, we see the allure.
So how does Asus do it with P55, which does sport eight of its own PCI Express 2.0 lanes, but connects to its Core i7 or Core i5 through a DMI interface? Granted, this generation of DMI could be as fast as 4 GB/s, since P55 includes PCIe 2.0 data rates. But even then, that's the equivalent of a x4 slot shared between peripherals attached to the PCH.
Enter NF200. The bridge takes the processor's 16 lanes and turns them into 32, enabling 16/8/8 configurations from a platform supposedly limited to one x16 slot or two x8s. Presumably, the other two slots support up to four lanes each and attach to the P55 PCH, but we're still waiting to hear back from Asus regarding the effect of populating all five x16 slots, and what that'd do to the two RTL8112L GigE controllers onboard.
Update: Asus just stopped by our Culver City office with a board diagram. Four PCIe x16 links actually attach to the NF200, switching to 8/8/8/8 when four cards are installed. The fifth card attaches to P55 and gets four lanes of PCIe 2.0 connectivity. Naturally, those aren't configurations you'd want for a four-way CrossFire setup or anything; according to Asus, this board is strictly for workstation-based applications.
Of course, there are performance implications to splitting 16 lanes between three high-end graphics cards, but we'll have to wait until after P55's official launch before we can start talking about them. For the time being, we're left impressed that Asus' engineers were able to use NF200 to enable a capability otherwise unavailable from P55. The bridge chip might not have been necessary for X58-based gaming machines, but it may very well prove useful to enthusiasts looking to build an inexpensive Core i5 box and put extra money into graphics. There's also the professional crowd, which can use the expansive PCIe connectivity with Nvidia's hardware/software technologies to accelerate engineering and science applications.