It is six years since Steve Ballmer issued a wake-up call to Microsoft's developers, warning of the threat posed by the Linux operating system and other open source projects.
The danger turned out not to be imminent: Microsoft's Windows is still shipped on 97 per cent of all new PCs.
However, Mr Ballmer would do well to dust off that old memo. Thanks to the emergence of new classes of portable, internet-connected devices, a potentially disruptive sea-change is now underway in the fastest-growing areas of personal computing.
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