Microsoft today announced an update for the Windows Phone software development kit (SDK) that adds support for Windows Phone 7.8. As you may recall, this interim release adds the Windows Phone 8 Start screen capabilities to Windows Phone 7.x handsets.

The update was announced via the Windows Phone Developer blog.

To be clear, this isn’t a new release of the Windows Phone SDK. Instead, Microsoft has released an optional update called SDK Update for Windows Phone 7.8 that you can apply to an existing install of the SDK. So you must have the Windows Phone SDK installed first.

The update adds the following features to the Windows Phone SDK:

Windows Phone 7.8 emulator. This OS image emulates your app running on a 512 MB device running Windows Phone 7.8 (build 8858).

Windows Phone 7.8 256MB emulator. This OS image emulates Windows Phone 7.8 (build 8858) running on a 256 MB device.

What the emulator doesn’t include are any new APIs. There are no new app capabilities in Windows Phone 7.8 beyond the new Start screen capabilities. These capabilities include three tiles sizes (small, medium, and wide), and three tile templates (Flip, Iconic, and Cycle), each of which can work with both primary and secondary tiles.

You can download the SDK Update for Windows Phone 7.8 and the Windows Phone SDK 8.0 from the Windows Phone web site.