The Register does not specify which version this was, likely a recent but highly modified version, but Microsoft has demonstrated their Server OS running on ARM hardware.  This will give them another inroad to low cost server builds which don't necessarily have Intel or AMD inside, as well as hedging their bets against Linux.  Linux is already happily running on just about any hardware you could want, or will be soon and Microsoft is likely worried about losing share to the open source OS.  It will be interesting to see what Microsoft can offer the price conscious shopper to convince them to spend the money on an OS license when Linux is free.  The days when the older generations of techs who have grown up with large UNIX servers and through Microsoft replacing it are numbered and they have always been one of the obstacles for the growth of upstart young Linux.  The Register also points to the possibility of it being an in house solution to keep the costs of maintaining Microsoft's Cloud applications.

"That's not a stunning feat: having developed Windows RT – a version of Windows 8 running on ARM chippery – Microsoft clearly has the know-how to get the job done. And it's not an indication that Microsoft intends to make Windows Server on ARM a product. It's just a test."

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