Microsoft's Operating Systems Group team has added code to the next test build of Windows 10 Mobile that will allow that release to work on the majority of Windows Phones.
When Microsoft released the first test build of Windows 10 Mobile in February 2015, it was built to run only on a handful of Lumia devices, specifically, Lumia 630, 635, 636, 638, 730 and 830 phones. Microsoft execs attributed the small number of initially supported devices to the "very tight OS partitions" that left insufficient room for the installation process to update the OS in place.
Head of Microsoft's Windows Insider program Gabe Aul said that the required partition stitching feature code required to support these phones came into Microsoft's main code branch late last week. Last week, Microsoft tested that code on individual devices inside the company. Aul also said the next test build will include support for phones that didn't have China Mobile Operator support in the first test build.
There's no mention in Aul's post about when Microsoft will support small ARM- and Intel-based tablets with a Windows 10 Mobile test build. Windows 10 Mobile is the version of Windows 10 that is built to run on both phones and small (under eight-inch) tablets. One Windows tester noted there's currently no Visual Studio emulator for Windows 10 Mobile for Intel tablets. I'm not sure why it's not there (change in Microsoft plans or just not ready yet, maybe?).