Difference between the M7 and M8 coprocessors? What's the difference between the M7 and M8 coprocessors? I've been unable to find a clear answer online. On the iPhone 6 webpage, Apple simply states:


  When you’re in motion, the M8 motion coprocessor continuously measures
  data from the accelerometer, compass, gyroscope, and a new barometer.
  This offloads work from the A8 chip for improved power efficiency. And
  now those sensors do even more, measuring your steps, distance, and
  elevation changes.


Is the only difference the ability to track data from the new barometer?
As you noted, the M8 coprocessor is able to collect data from the new barometer in the iPhone 6 series phones as well as the iPad Air 2, which provides a measure of relative elevation for floors climbed.

We may also speculate that it is more accurate and power-efficient, although whether accuracy improvements are up to the sensors or the M8 is hard to prove (software improvements would also come into play here, as CoreMotion attaches a confidence value to measurements, which could be tweaked in software).

However, Phil Schiller raised another point during his presentation that many people seem to have missed: the M8 chip is now also able to recognize cycling activities. This new capability is reflected in the updated CoreMotion API, as detailed here:

https://developer.apple.com/library/ios/documentation/CoreMotion/Reference/CMMotionActivity_class/index.html#//apple_ref/occ/cl/CMMotionActivity

The M7 only recognized stationary, walking, running, automotive, and unknown. The M8 adds the cycling property to the list.

It is worth mentioning that iOS 8 does not make use of this functionality, and that the iPhone 6 does not write cycling data to the Health app on its own. You will need a dedicated app for that.