Microsoft has given its users an unprecedented view into the development of Windows 8 but nothing matches the scope of the latest unveil. Steven Sinofksy himself has descended from his lofty throne and taken up pen and paper to describe Windows on ARM (WOA) in great detail. The full article is available at the Building Windows 8 Blog, though calling the 8,000+ word treatise a “post” stretches the definition of the word like Rosie O’Donnell slipping in to one of Kate Moss’s bikinis.

Redmond intends to ship WOA and Windows 8 x86/64 simultaneously, with strong channel support for both products. One significant difference between the two platforms is the degree of control MS is wielding. WOA systems, for example, will ship with desktop versions of Microsoft Word, Excel, PowerPoint, and OneNote. Microsoft has ad-supported versions of Excel and Word that it distributes to OEMs in the PC space, but you’ll never see the company say “All Dell and HP systems will ship with Office.”

Contrary to prior reports, WOA users will have access to the conventional Windows desktop — but there won’t be much for them to do with it. Sinofsky writes: “WOA does not support running, emulating, or porting existing x86/64 desktop apps. Code that uses only system or OS services from WinRT can be used within an app and distributed through the Windows Store for both WOA and x86/64. Consumers obtain all [WOA] software, including device drivers, through the Windows Store and Microsoft Update or Windows Update. (Emphasis added)

It’s hard to underemphasize just how huge a change that is. It’s one thing to say that ARM CPUs won’t support x86 emulation; something else entirely to split software delivery and installation. Up until now, one of the biggest differences between desktop and mobile operating systems has been the ability to install software. It’s true that Microsoft’s decision to wall off unapproved software installation is similar to the approach of Android and iOS — but iOS isn’t the same thing as OS X. Combining both of these decisions under the “Windows” brand could be disastrous, not because Microsoft is evil, but because it creates two entirely different user experiences on the basis of which ISA your CPU supports. What’s even more schizophrenic is that the entire point of WinRT was supposedly to eliminate the need to support multiple architectures and recompile code. This change doesn’t impact the programming model, but it undercuts the idea of a unified Windows.

Using Windows means being able to download a program from SourceForge, Download.com, MajorGeeks, or Microsoft’s own web page. Now, x86 developers who might want to recompile software to target ARM won’t be able to do so without going through the Microsoft store. The simple portability that’s defined Windows to such a degree that no one even thinks about it is gone. Sure, if you rely exclusively on things like Microsoft cloud services, or Office, you won’t notice. What about everyone else?

Even if we don’t see them straight away, there are going to be x86 tablets and ARM notebooks. If you own an x86 chip, you’ll be able to do all the things you do now, plus take advantage of the new UI. If you buy an ARM product, you get something that looks very much like a conventional tablet experience. There’s nothing necessarily wrong with that, but it’s not what people think of when they think “Windows.” Microsoft claims that users won’t confuse x86 and ARM devices, but it’s not clear how the company plans to address the problem.

Up until now, Windows 8 has straddled the gaps between its disparate platforms remarkably well, but this could seriously harm the operating system’s uptake. It’s not that the App Store model is morally evil, but that the Windows brand is going to be split between two families of devices. Microsoft might well have been better off calling its ARM effort “Windows Tablet” or even moving it to Windows Phone — WOA devices aren’t going to be Windows products in some of the ways the term has traditionally been applied. That’s not good for the brand, or for users, and it blurs one of the most important distinction between a device that’s essentially a toy and one that people can use for both work and creation. It’s possible that Microsoft will include the ability to load unapproved apps, similar to the way that unsigned drivers can be loaded in a 64-bit OS, but the company’s previous decision to force UEFI Secure Boot on ARM devices make this seem unlikely.

Now read about how Microsoft is rearchitecting Windows for ARM hardware