Microsoft has been heavily focused on low-end Windows Phone hardware over the past two years to grow market share, but its upcoming Windows 10 update won’t be finely tuned for these devices with low specifications. Microsoft’s Joe Belfiore revealed on Twitter yesterday that the software maker is working on Windows 10 for phones with 512MB of RAM, but that "features may vary."

It’s not clear what features won’t be available, but we’re hoping this isn’t a situation like the Windows Phone 7 to Windows Phone 8 upgrade, where some devices were left with a Windows Phone 7.8 update that only contained some minor Windows Phone 8 features. That’s not something Microsoft will want to repeat, so it’s likely that the company will be pushing hard to make sure Windows 10 runs on Windows Phone 8 devices with 512MB of RAM. Microsoft previously claimed all Lumia Windows Phone 8 devices will be upgraded to Windows 10, and Belfiore notes that the company is merely working on it right now and that it’s an "ambition" to get Windows 10 on low specification devices.

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 — joebelfiore (@joebelfiore) February 8, 2015

Microsoft’s most popular Windows Phone device is the Lumia 520 with 512MB of RAM, shortly followed by the Lumia 630, Lumia 625, and Lumia 530 — all with 512MB of RAM. It appears that the majority of popular Windows Phone devices won’t get all the new Windows 10 features once they’re available. Microsoft is planning to preview those features very soon, but not to all devices. Belfiore notes there will be a "subset of phones" that will be compatible with the Windows 10 preview, and The Verge understands that will focus on devices like the Lumia 730 and Lumia 830 that shipped recently. Microsoft is close to releasing its Windows 10 preview for phones, and it’s likely it will be made available this week, providing testing goes well internally.