Microsoft revealed its Windows Phone "Tango" engineering effort at MWC last week, but the company was fairly vague on the exact restrictions for its new low-memory devices. Nokia and ZTE will be the first device manufacturers to launch Windows Phones with just 256MB of RAM, but there will be restrictions for end users and developers. Microsoft updated its Windows Phone site this week with the full details:

• Windows Phone Marketplace app restrictions - Some processor-intensive apps have specific memory requirements, and will not work on devices with 256MB of RAM.
• Podcast subscriptions and video podcasts - Devices with 256MB of RAM will not be able to manage podcast subscriptions of watch video podcasts.
• Bing Local Scout - Microsoft is disabling Bing Local Scout on 256MB of RAM devices.
• Fast app switching - The fast application switching, introduced with Mango, will be disabled on 256 MB of RAM devices.
• SkyDrive automatic photo upload - Automatic upload of SkyDrive pictures will not be possible on 256MB of RAM devices, but users can still upload these manually.
• HD video playback - 256MB of RAM devices won't be able to play video compressed with certain codecs.
• Background agents - Microsoft has removed background agents functionality for devices with 256MB of memory.

In addition to the specific restrictions, Microsoft has also revealed some recommendations for Windows Phone developers. Optimizing start time and reducing memory usage appear to be the key messages from Microsoft to developers, focusing on the low specifications for 256MB of RAM devices. The software giant is also urging developers to handle feature reductions such as the removal of background agents and fast app switching, while suggesting that app developers may want to reduce or modify functionality on low-memory devices in order to reduce an app's memory footprint.

Although Microsoft is cutting some functionality for low-memory devices, the company has also added in a number of new features to Windows Phone, including MMS improvements, better SIM contact management, and a new location alert icon. Microsoft confirmed to us this week that these features will make their way to existing devices after new handsets (designed for 256MB of memory) are launched in Q2 2012.

Update: We've been informed that FAS and Task Switcher will be enabled on Windows Phones with 256MB of RAM, with one caveat: applications that use over 90MB will be killed immediately after deactivation, while those with smaller memory requirements will fast resume properly.