Microsoft is busy putting its Windows 8 operating system and Office 15 suite on the sort of ARM chips that are used in most smartphones and tablets. So, you might think, ARM-based PCs will be just like Intel-based PCs with a different processor inside. Well, they will look the same, but looks can deceive. Conceptually, what Microsoft calls WOA (Windows On Arm) is more like an upgrade to Windows Phone than anything else. And it’s even more locked down.

WOA will be launched later this year on tablets, but it could also run on smartphones, laptops, all-in-one touch-screen desktops, games consoles and other devices yet to be invented. They will all have a tile-based Metro user interface similar to Windows Phones, but the underlying operating system is Windows 8, not Windows CE.

Steven Sinofsky, who is in charge of Windows, outlined Microsoft’s plans on the Building Windows blog last week. He says he’s “re-imagining” Windows, but it’s more of a revolution for users, software developers, and PC manufacturers. They will all have to find new ways of doing things.

* Windows and Windows Live president Steven Sinofsky outlines developer opportunities with the upcoming "Windows 8" in his keynote address at the BUILD conference in Anaheim, Calif. on Sept. 13, 2011.

WOA machines are designed to be secure, to have touch-first user interfaces (though they will also work with a mouse or keystrokes), and to live in a continuous state that Microsoft calls “Connected Standby”. In other words, they’ll work like smartphones. This has some important repercussions.

• WOA machines use a new start-up chip, certified code, and a TPM (Trusted Platform Module) to ensure that they can only run Windows 8. This “trusted boot” system means you can’t run a different version of Windows, or an alternative operating system, even if one becomes available. Also, you will not be able to buy a copy of WOA and run it on a different system. WOA devices are consumer electronics appliances, rather than personal computers.

• WOA machines will only run Metro apps and drivers that have been pre-installed by the manufacturer or downloaded from Microsoft’s app store. They won’t run your old Windows software, even if you have the source code and the skills needed to port it from Intel to ARM. Metro apps are also “sandboxed” to block malware and viruses.

• The traditional mouse-and-windows desktop still exists underneath WOA’s Metro interface, but only Microsoft can write applications for it. Currently, that means some Office 15 programs — Word, Excel, PowerPoint, and OneNote — which will appeal to businesses. Most WOA users will do most things using apps, but they can also go to a Windows 7-style desktop and use new versions of familiar utilities, such as Windows Explorer and Task Manager.

One drawback is that WOA will start with relatively few Metro apps, including adapted versions of Windows Phone 7 apps. Old Windows programs will be hard to convert, because Metro programs can only use the new applications programming interface (API) called Windows Runtime (WinRT), not the old Win32 system. Further, programmers can’t use “background processes, polling loops, timers, system hooks, startup programs, registry changes, kernel mode code, admin rights, unsigned drivers, add-ins, or a host of other common techniques,” says Microsoft.

On the other hand, Metro apps written for Intel-based Windows 8 PCs will also run on WOA, so there could soon be millions of them.

Will people buy WOA devices? That remains to be seen, but the success of today’s smartphones and tablets shows that many customers are happy to buy into the locked-down appliance-style approach. Metro can match rivals on that score, while offering some of the familiar benefits of Windows and Office.

But I expect many of you are throwing up your hands in horror. That’s fine. Windows 8 will still be available on PCs with Intel processors. They will work just like Windows 7 PCs, except that you also get a shiny new Metro interface instead of a Start button, and the option to run tablet-style Metro apps as well as Windows programs.

Nobody likes change, of course, but WOA does have a point. If it succeeds, you will be able to have compatible apps and shared data on every type of device from a smartphone to a supercomputer, instead of three or more different operating systems.

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