With every new version of Android comes a new version of the Android Compatibility Definition Document (CDD). If OEMs want to license the Google Play Store and other Google apps, they must be declared by Google to be "compatible" with Android, and the requirements for this compatibility are laid out in the CDD. The 74-page document details how to implement APIs, parts of the user interface, media codecs, and hardware compatibility. Companies like Samsung and LG use this document to create different hardware that will run the same operating system and apps.

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Android Auto finally gets Waze integration—in beta, at least View more stories One of the biggest changes is probably the mandatory encryption for any device fast enough to handle it, which we've already covered in a separate article. While combing over the new the document, though, we noticed something: there is an awful lot of car stuff in here.

Requirements for a car operating system have no reason to be in the CDD, because Google doesn't make a car operating system. Right now there are three versions of the Android operating system: vanilla "Android" for phones and tablets, Android TV, and Android Wear. "Android Auto" is a fourth form factor, but it isn't an operating system—Android Auto is an app. It's a "casted" interface, just like Apple's Car Play. The app runs on your phone hardware, which is plugged into the car with a USB cable. The phone takes over the in-dash display and uses it the same way a desktop computer would use an external touchscreen.

Since Android Auto isn't an operating system, it has no need for the CDD and all of the operating system requirements it details—and yet things like this are in the CDD:

Android Automotive implementation refers to a vehicle head unit running Android as an operating system for part or all of the system and/or infotainment functionality. Android Automotive implementations: MUST declare the feature android.hardware.type.automotive.

MUST support uiMode = UI_MODE_TYPE_CAR [Resources, 5].

This is not for the Android Auto app. This is for a Google-certified car operating system that would run directly on the car's hardware—something that Toyota, GM, and other car manufacturers would ship from the factory. Since the whole reason for the CDD is for app compatibility and the Google Play ecosystem, we'd imagine that these cars would also ship with a specialized version of the Google Play Store build just for cars.

We've heard talk of Google building an Android-based infotainment OS before. Reuters reported that "Android M" (now Marshmallow) will support use as an in-dash infotainment OS, and sure enough, the Marshmallow CDD contains updated specs for car use. We've also heard that Harman International, a top GM supplier, is working with Google to build an Android-based OS for cars. Both reports said that Google is working on hardening the OS for car use, with things like startup time being a big focus.

Some car manufacturers are shipping Android as the in-dash car infotainment OS now, but these are all AOSP-derived versions that the manufacturer (or a component vendor) created itself. These are usually extremely old (and insecure) versions of Android. A car running Android isn't something you'll find in the spec sheet, but from firsthand experience, we know the 2016 Honda Accord runs Android 4.2 and the 2016 Hyundai Sonata runs Android 2.3, two ancient OSes from 2011 and 2012.

Most mentions of "Android Automotive" in the CDD take the form of exceptions from many of the requirements. For instance, the section about browser compatibility is prefaced with a statement that browsers are optional on "Android Television, Watch, and Android Automotive implementations."

One of the interesting requirements says, "Android Automotive implementations MUST provide the Home function and MAY provide Back and Recent functions." Home is a software button today in the Android Auto app, but there is no top-level Back or Recent button. Android Auto omits a recent apps interface because there are only three "apps"—Music, Phone, and Maps—which makes a Recent apps button a needless complication.

A Recent button would signal a much more complex interface than what Google currently has in the Android Auto app. An in-car version of Android would need to handle air conditioning, the radio, system settings, Bluetooth and Internet access, and many other additional features.

Mentions of "Android Automotive" in the CDD actually date back to Android 5.1, but it seems no one noticed. Google is now on its second revision of the Automotive requirements, which gives us a peek into the ongoing work on bringing the Android OS to yet another form factor.

We really liked the Android Auto app when we reviewed it. It had a professionally designed, smartphone-caliber car interface—something that is nearly impossible to get from the baked-in car OS. Our biggest complaint was how limited it was, which forced us to frequently switch between the ugly, old, built-in OS and the limited-but-beautiful Android Auto interface for some features. A full Android car OS would take the Android Auto interface and expand it to cover everything the car computer needs to do, which is what we've wanted from the beginning.