Microsoft has confirmed that which we all knew, but had hoped against: the ARM version of Windows 8 won't be able to run your existing Windows applications.
The news that Windows 8 would be developed for both the traditional x86 instruction set - as used by chip giants Intel, AMD, Via, and others - and the RISC-based ARM instruction set - as used by dozens of ARM licensees to make low-power mobile-centric processors - was welcomed by many as heralding a new dawn in portable computing.
The introduction of Windows 8 for ARM, it was argued, would bring about a revolution in netbooks and laptops, where low-power chips would power the full Windows 8 experience on cheap hardware with all-day battery life.
Those who know about how software operates were less convinced: a shift in architecture in the operation system requires the same shift in architecture for client applications. Put simply: the ARM version of Windows won't run your x86 software, including browsers, office suites, games, and security tools.
It was hoped that Microsoft, which has previously been silent on the issue of compatibility, was hiding an ace up its sleeve. When Apple made the move from the PowerPC architecture to x86, it had a tool made called Rosetta which offered a compatibility layer to legacy apps, allowing them to run on the new architecture. Although no such practical x86-to-ARM compatibility layer exists at present, the introduction of one in Windows 8 would solve the problem handily.
Sadly, that dream has been shattered. "We've been very clear since the very first CES demos and forward that the ARM product won't run any x86 applications," Windows Team head Steven Sinofsky told analysts at a meeting this week, with the very requirement for such a statement giving lie to his claims of clarity.
Claiming that the introduction of a Rosetta-style compatibility layer would have "challenges in some of the value propositions for [Windows 8 on] System on a Chip" such as battery life, Sinofsky confirmed that Windows 8 for x86 and Windows 8 for ARM will not feature any level of cross-compatibility. Developers will be expected to write for both architectures, providing two different executables for their applications.
The result is risky: the lack of compatibility with legacy applications was one of the biggest contributors to the failure of Linux on netbooks, leading to Microsoft's cut-down Windows versions receiving a massive majority market share despite launching significantly later. By keeping the familiar Windows interface but killing compatibility, Microsoft risks alienating customers and killing ARM's chances of breaking into the lucrative laptop and desktop market.