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Microsoft is busy prepping its upcoming Window 10 Technical Preview for smartphones. But which phones will get the preview, and which ones will receive all the features of the new OS, is a key question.

In a series of tweets posted Sunday, Joe Belfiore, vice president for Microsoft's Operating Systems Group, laid out the plans for the rollout of the Windows 10 Technical Preview for Windows handsets.

The first tweet aimed to allay some anxieties: "Hey all: we ARE working on Win10 for 512MB, including 520. Our ambition is to make #Win10 available for these devices but features may vary, including 520."

Several Lumia phones, which Microsoft now sells courtesy of its buyout of Nokia's handset division, come with only half a gigabyte of memory. Those phones include the Lumia 520, Lumia 630, Lumia 625 and Lumia 530. Many owners of these and other phones with only 512MB of RAM have complained in the past that certain Windows Phone apps and features require more memory and therefore won't run on their devices. Belfiore's tweet implies that the same limitation may hold true for Windows 10, which may fuel more complaints from users of affected devices.

Windows 10 is a next leap forward for a marquee operating system that has long been a mainstay of desktop computing, but that seemed to lose its way with the last big version, Windows 8. At the same time, Microsoft's efforts to attract consumers to its mobile OS have largely gone unheeded -- in 2014, Windows Phone hovered at about 3 percent of the worldwide market for smartphones, far behind Apple's iOS and market leader Android.

Now, with the upcoming Windows 10, Microsoft will drop the Windows Phone distinction altogether, aiming to make clear that all Windows-powered devices, from phones and tablets to desktops and laptops, will run on a single platform. At its big coming-out party for Windows 10 on January 21, Microsoft said it plans to have a version of the operating system tailored for devices with screen sizes below 8 inches.

What has yet to be fully set out is when exactly everything will fall into place.

On Sunday, a second tweet from Belfiore said: "Timing for various phones may be variable as well. When we first launch insider program, it'll be a subset of phones with more coming latr." He didn't indicate which subset of phones would be first on the list. But considering the issues with 512MB phones, the first batch that qualify could be those equipped with more RAM.

In a third tweet, Belfiore promised that the first build of Windows 10 for phones is still slated for this month: "Also, we're on track for Insider build coming 'in Feb' as Terry said at our 1/21 event. We need to complete internal testing 1st, hang in!"

Microsoft naturally wants to eliminate as many bugs as possible before it rolls out the technical preview. Many Microsoft insiders who were supposed to test Windows 10 build 12492 on their phones last week weren't even able to upgrade their devices, a source at Microsoft revealed to Neowin. Those kinds of bugs are obviously best discovered by Microsoft employees rather than the large number of users who'll be playing with the Technical Preview.

The Windows 10 Technical Preview has already been available for PCs and tablets, giving people the ability to check out the new OS. Microsoft's goal with Windows 10 is to provide more consistency across PCs, tablets and mobile phones as one way to increase customer interest and demand for all three types of devices.

(Via NeoWin and The Verge)