Even though crucial pieces of the puzzle won’t be in place until next year, there is a heck of a lot of hype flying around over PCI-Express 3. Asus has now announced a handful of Z68 based motherboards with support for the new technology, joining MSI, ASRock and Gigabyte with products. But the more we hear about the technology, the more we realise that it isn’t something that anyone should really care about.
The nitty gritty of how PCI-Express 3 works is complex and somewhat muddied by the fact that there won’t even be controllers until Intel launches its Ivy Bridge CPUs next year. We have spent a few weeks digging into the technicalities of the technology, and you can find a deep explanation on our sister site, Atomic. But crucially, these PCI-Express 3 announcements have no bearing whatsoever on how these motherboards work today.
Switching and signalling
Instead the touted support revolves around the use of PCI-Express switches to enable multiple graphics cards, and the shift to a new generation of them. At a basic level in modern systems the CPU controls 16 lanes of PCI-Express, a design that enables fast access to graphics hardware. In order to enable multiple graphics cards, where each runs over a x8 PCI-E connection, half of these lanes are routed directly from the CPU to the PCI-E slot. The other half get routed through switches, small ICs designed to route the lanes to the second graphics card slot when needed. When there is only one graphics card installed, the switches route the lanes to the first slot, enabling a full x16 connection.
This switching is nothing new, but the current ICs cannot handle the increased signalling that will be needed for PCI-Express 3. While this is not an issue with current CPUs, it means that the maximum PCI-Express 3 speed would be x8, effective only on the lanes running from the CPU to the graphics slot. This is ample bandwidth for a modern graphics card, and PCI-Express 3 won’t even be enabled until you have a supporting CPU and graphics card at each end of the wires.
So the announcements about PCI-Express 3 support refer to the future compatibility of the motherboards, which for various marketing-based reasons is being given grossly inflated importance. Neither Nvidia nor AMD has made any PCI-Express 3 announcements, and even then the technology is more focused on ultra-high bandwidth devices like SSDs rather than graphics cards.
Even more than that, we have plenty of AMD’s next generation 990X motherboards in our labs at the moment. These are designed for AMD’s upcoming ‘Zambezi’ high end CPU, and feature no PCI-Express 3 support. Similarly the early samples of Socket 2011 motherboards, designed for Sandy-Bridge E, that were shown at Computex had PCI-Express 3 switches, but it now looks like these new CPUs won't be PCI-E 3 ready. When the next enthusiast platforms lack a technology it is a pretty good sign that mainstream penetration isn’t happening anytime soon.
The bottom line
Normally we wouldn’t even bother talking about PCI-Express 3 this early in its life. But the increased focus upon the marketing and advertising of the tech means that clarification is needed.
The bottom line is that PCI-Express 3 support on Z68 motherboards is nice, but ultimately irrelevant. If and when it actually becomes a requirement there will be a need to consider it in your purchasing decisions, but for now it is about as relevant to system performance as the colour of your heatsink.
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