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Apple talks about the M7 and M8 motion coprocessors for the iPhone pedometer and other operations.

My question is Why do we need a motion coprocessor? Surely this is just digital signal processing that the massively powerful primary processor can handle?

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You have implied the reason in " the massively powerful primary processor" It takes electrical power to produce power in computing. Thus if you need something to be on all the time you want a smaller processor than the main one to save battery power.

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    Just to give an idea, a fitbit zip (which is capable of recording step counts) can run for 6 months off a single watch battery. Using a low-power processor in these situation can lead to amazing battery life improvements. – Chris Jefferson Sep 23 '14 at 14:01
  • is this similar to a Galaxy phone having an 8-core processor, but making 4 high-power and 4 low-power? – Supuhstar Jul 12 '15 at 17:53
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    @Supuhstar No - this is a totally separate specialises processor and not more cores in a general purpose processor – Mark Jul 12 '15 at 18:29
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There are a few reasons why having a separate processor is appropriate for this.

The processor is for tasks that can be run all of the time, whether you're doing something with your device or not.

The tasks it's being asked to do are of critical priority, but likely require very little actual processing.

Device Usability:

By splitting this work to a separate processor, it means that developers don't have to concern themselves about how much of the main processor is being used. The main processor can be unused or maxed out, and it will have no impact on the processing of tasks that the motion coprocessor takes on.

In effect, it means that your normal device usage and the background motion processing will never interfere with each other.

Power Usage:

Since these tasks are being run continuously, putting them on a separate low-power processor also allows for the device to use less power. With battery life being a very large concern for a lot of users, Apple thought it was necessary to consider the power usage for always on functions.

These functions can also be implemented in a much more optimal way on a processor that has a very specific task. Consider this similar to how a GPU is able to process much more than a CPU (in most cases) because it's targeted for a specific task.

  • I suspect that this part: These functions can also be implemented in a much more optimal way on a processor that has a very specific task. Consider this similar to how a GPU is able to process much more than a CPU (in most cases) because it's targeted for a specific task. is likely the most critical consideration--that's probably the reason it can be low power (because it only has one job to do and can be dramatically simpler in design, smaller, and fully optimized to do that one thing). – msouth Sep 24 '14 at 5:35
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The first thing I noticed the first time I opened Nike+ Move on my iPhone 5s was that it immediately had data for the previous two weeks or so. I believe the motion coprocessor efficiently constantly tracks and records your movement sensor data and gives apps access to that data. This allows apps to collect this data even when not open without having to run constantly in the background. The processing difference between only collecting motion data (and doing so in a single unified place for all apps) and running an entire app in the background is enormous in terms of battery usage.

  • Afaik the A7/A8 could do the job without keeping an app running—it's that this would use more battery as the main processor would be active. – grg Sep 23 '14 at 18:35

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