Microsoft has made available for download its software development kit (SDK) for Windows Phone 7.8 .

This is an update to the existing Windows Phone SDK. Officials shared download links and more information on January 22.

Microsoft officials still are not saying when the actual Windows Phone 7.8 operating system will be made available to those with existing Windows Phones. The only official guidance was that it would be in early 2013 . The 7.8 operating system already is shipping on new Windows Phones.

The Windows Phone 7.8 operating system brings the resizable live tiles that are part of the Windows Phone 8 operating system to some, but not all, existing Windows Phone devices. It also adds a couple of other more minor feature s, like new themes/accent colors and accidental-device-wipe prevention.

Because Windows Phone updates are still gated by mobile operators, it's hard to say exactly when users of the various Windows Phone devices will get the 7.8 operating system. There are reports the 7.8 operating system could begin to be pushed out starting January 31. But that doesn't mean every existing Windows Phone user will get the 7.8 release this month.

Typically, if a device has Tango (the most recent pre-Windows Phone-8 version of the Windows Phone operating system), users can expect 7.8 to arrive sooner rather than later, once of my contacts said. But users who are on Mango (Windows Phone OS 7.5) may have to wait longer, the contact added.

Back to the Windows Phone 7.8 SDK. It includes two Windows Phone emulators (one for a 256 MB device and one for a 512 MB device). But, as Microsoft's Janaury 22 post on the Windows Phone Developer blog points out, "There are no new APIs (application programming interfaces) available to Windows Phone apps in Windows Phone 7.8."

Microsoft is providing cloud-compilation for existing Windows Phone 7.x applications to help make them run on Windows Phone 8, which includes a new core architecture (based on the Windows NT core).