Microsoft’s Windows chief Steven Sinofsky has written a long post describing Windows on ARM (WOA), which he says is a:

new member of the Windows family, much like Windows Server, Windows Embedded, or Windows Phone

There are many point of interest in the post, but the one which stands out for me is that while the traditional Windows desktop exists in WOA, third party applications will not be allowed there:

Developers with existing code, whether in C, C++, C#, Visual Basic, or JavaScript, are free to incorporate that code into their apps, so long as it targets the WinRT API set for Windows services. The Windows Store can carry, distribute, and service both the ARM and x86/64 implementations of apps (should there be native code in the app requiring two distributions).

says Sinofsky. He writes with extreme care on this issue, since the position for which he argues is finely nuanced. Why have the Windows desktop on WOA at all?

Some have suggested we might remove the desktop from WOA in an effort to be pure, to break from the past, or to be more simplistic or expeditious in our approach. To us, giving up something useful that has little cost to customers was a compromise that we didn’t want to see in the evolution of PCs

he says, while also saying:

WOA (as with Windows 8 ) is designed so that customers focused on Metro style apps don’t need to spend time in the desktop.

From a developer perspective, the desktop is more than just a different Windows shell. Apps that run on the Windows Runtime (WinRT) are isolated from each other and can call only a limited set of “safe” Windows APIs, protecting users from malware and instability, but also constraining their capabilities. The desktop by contrast is the old Windows, an open operating system. On Windows 8 Intel, most things that run on Windows 7 today will still work. On WOA though, even recompilation to target the ARM architecture will not help you, since Microsoft will not let desktops apps install:

Consumers obtain all software, including device drivers, through the Windows Store and Microsoft Update or Windows Update.

What if you really want to use WOA, but have some essential desktop application without which you cannot do your work, and which cannot quickly and easily be ported to WinRT? Microsoft’s answer is that you must use Windows on Intel.

That said, Microsoft itself has this problem in the form of Office, its productivity suite. Microsoft’s answer to itself is to run it on the desktop:

Within the Windows desktop, WOA includes desktop versions of the new Microsoft Word, Excel, PowerPoint, and OneNote, codenamed “Office 15”.

No Outlook, which I take to imply that a new WinRT-based Exchange client and PIM (Personal Information Manager) is on the way – a good thing.

Microsoft’s aim is to give customers the security and stability of a locked-down machine, while still offering a full version of Office. If you think of this as something like an Apple iPad but with no-compromise document editing and creation, then it sounds compelling.

At the same time, some users may be annoyed that the solution Microsoft has adopted for its legacy desktop application suite is not also available to them.

The caveat: it is not clear in Sinofsky’s post whether there may be some exceptions, for example for corporate deployments, or for hardware vendors or mobile operators. It will also be intriguing to see how Office 15 on ARM handles extensibility, for example with Office add-ins or Visual Basic macros. I suspect they will not be supported, but if they are, then that would be a route to a kind of desktop programming on WOA.

It will be interesting to see how Microsoft locks down Explorer, which Sinofksy says is present:

You can use Windows Explorer, for example, to connect to external storage devices, transfer and manage files from a network share, or use multiple displays, and do all of this with or without an attached keyboard and mouse—your choice.

By the way, this is a picture of the Windows ARM desktop as it looked at the BUILD conference last September. The SoC (System on a Chip) on this machine is from NVIDIA.