The deal with multi-GPU configs
One of the things that makes Skulltrail unique is its support for both Nvidia's SLI multi-GPU scheme and AMD's similar CrossFire technology. Both GPU makers tend to limit support for their multi-GPU capabilities on third-party chipsets in order to sell more of their own chipsets. ATI (now AMD's graphics division) has long made an exception for Intel's chipsets, but Nvidia has kept SLI largely exclusive to its own nForce lineup. Intel went to great lengths in order to incorporate SLI support into Skulltrail, and it affects the physical hardware on the board. Have a look at the block diagram below to see how.

The bit of the diagram we're concerned with is the top left portion, where the north bridge (or MCH) connects to the PCIe x16 slots. In between the MCH and each pair of PCIe x16 graphics slots sits an nForce 100 PCI Express switch chip from Nvidia. Each nForce 100 has 16 lanes routed to each PCIe x16 slot and 16 lanes back to the MCH. This is by no means the most optimal configuration, because Nvidia's switch chip only supports PCIe version 1.1, whereas the Seaburg MCH supports PCIe 2.0, with twice the bandwidth per lane. The more optimal configuration would be to route eight lanes of PCIe 2.0 connectivity directly from the MCH to each of the four PCIe x16 slots. The Seaburg MCH is designed to work this way, in fact, and such a configuration would yield fewer chips, fewer hops between chips, less complex trace routing, lower power consumption, lower costs, and twice the total bandwidth back to the MCH (via 32 lanes of PCIe 2.0 instead of 32 lanes of PCIe 1.1).

So why did Intel do things this way? Because by incorporating Nvidia's chips, they could win Nvidia's blessing for SLI configurations on this motherboard. You'd think that simply paying a license fee to Nvidia would be sufficient, but Nvidia apparently wanted to maintain the (technical) fig leaf covering its true (business) reasons for locking third-party chipsets out of SLI. Nvidia continues to claim intermittently that the PCI Express implementations on its nForce chipsets contain special sauce to make SLI work. I'm not buying it. CrossFire works quite well on Intel's chipsets, and it's a very, very similar thing.

Anyhow, Intel played along and put Nvidia's PCIe 1.1 switch chips on the Skulltrail board—two of them, in fact, to make three- and four-way SLI possible. After all, doing so only makes a certain kind of sense when you're setting out to build an "ultimate" motherboard like this one.

Here's where things get sticky. I was all set to try out three-way SLI on our Skulltrail board, just for kicks, but when I asked Intel about this possibility, they told me I'd need to check with Nvidia about drivers to make it happen. When I asked Nvidia about it, I got this response:

3-way SLI is not supported on Skulltrail because the MCP bridge chip only supports communication between two graphics cards. There is no driver workaround for this.

Uh, yeah. At this point, I reminded the folks at Nvidia that Skulltrail's dual nForce 100 configuration is, if anything, more elegant than the switch-chip-plus-south-bridge mish-mash on the nForce 780i. I told them I wasn't buying it.

Then, I went back to Intel and got this official statement from them:

Mechanically and electrically, with 4 PCIe slots, Skulltrail can support up to 4 graphics cards. Drivers and validation and are up to the graphics card vendors as always.

Somewhere in a back room at Intel, as the PR rep uttered these words, an engineer must have bit his tongue in two.

A little later, I got an interesting reply from Nvidia essentially admitting its earlier explanation didn't make any sense. I'm still awaiting a better rationale from Nvidia, but if I were betting, I'd say we'll never see support for three- or four-way SLI on Skulltrail.

Skulltrail is a niche product, and it's not one I care particularly about, but these shenanigans make me angrier than anything I've seen from this industry in a good while. This is the kind of crap that makes folks give up on PC gaming and go buy an Xbox. I don't believe for a minute that this is about anything other than vindictiveness. I've heard credible rumors that Nvidia has seeded some PC makers with three-way SLI drivers that work perfectly on Skulltrail. I've also heard whispers that Intel is paying as much as $100 per motherboard for those nForce 100 chips. Yet Nvidia is still locking them out. Sheesh.

On the bright side, Skulltrail ought to work fine with AMD's CrossFire X when it arrives—even though, heh, the D5400XS's PCIe x16 slots are driven by Nvidia silicon. Imagine that.