Windows Phone 7: A complete reset

Microsoft on Thursday confirmed that its new phone operating system is as different under the hood as it is to the eye.

In a blog post and at an event with a handful of journalists, the software maker said that those developing software for Windows Phone 7 Series devices will do so using either Silverlight or XNA, the toolset used to create Xbox games.

"Overnight those developers have become Windows Phone developers," said Charlie Kindel, the Microsoft executive in charge of the mobile developer strategy. "One of our principles is to build on the shoulders of giants."

Microsoft announced its plans for the new phones and showed off the user interface at last month's Mobile World Congress. However, it had been mum on what it would take to write software for the phones although speculation had centered on Silverlight and XNA.

While the move brings the phone closer to Microsoft's Web and gaming efforts, it also marks a pretty distinct break with past versions of Windows Mobile.

"In some cases, some work can be done to get those apps to run, but it's fundamentally a different platform," said Todd Brix, another member of the Windows Phone team.

The company also confirmed that it has no current plans to allow any current Windows Mobile phones to run the new OS, which is due to start shipping on new devices by this year's holiday season.

That the new OS is such a complete break with past versions is a relatively new development in the multi-year history of its development. Roughly 18 months ago, a largely new team of engineers and executives came in as part of what one employee termed "a complete reset" of the project.

What resulted was a product that resembles the Zune HD far more than any past version of Windows Mobile. Also gone was the notion of allowing both hardware makers and carriers a great deal of flexibility in customizing the phone.

In part by design and in part a nod to its weakening market position, recent Windows Mobile devices often hid the look of the operating system under a "skin" designed by the phone maker. With the new OS, Microsoft will insist its interface be preserved.

Microsoft is also far stricter on what hardware will go in the phones, mandating not just the screen size and type but also what buttons can be included and insisting the phone have things like an FM radio and Wi-Fi. Hardware makers will have some choice but mainly around external things such as whether the phone includes a physical keyboard or not.

One of the reasons for limiting choice, Kindel said, is to ensure that developers that write software for Windows Phones can do so without having to do separate testing for each phone on the market.

Kindel said that developers had grown frustrated with the experience of writing software for phones using Microsoft's OS. "To ship my app, I have to test on 35 devices or I will get nothing but support calls," Kindel said was the message he heard from developers.

Microsoft didn't offer the full details on what developers will have to do to write for Windows Phone Series 7 devices, but has said those details will come at the Mix 10 trade show in Las Vegas later this month. There it will offer details on the programming tools as well as the app store that will be part of the phone.

This article was first published as a blog post on CNET News.