Reading the undocumented MEMS accelerometer on Apple Silicon MacBooks via iokit | Hacker News Reader

Reading the undocumented MEMS accelerometer on Apple Silicon MacBooks via iokit

github.com

154 pointstodsacerdoti61 comments

Way back in ~2008 I wrote the Newton Virus https://www.everita.com/how-the-newton-virus-was-made + https://www.youtube.com/watch?v=eh75j6OHhRc (sorry for the broken images, need to update that site). Between that and using a hidden API to take screenshots of each individual element on your desktop (from icons, to taskbar, to windows) the effect was pretty believable. One of the most fun (and frustrating) projects I ever worked on.

Troika! Hello from a friend in London :)

Offtopic, but I have nostalgic feelings for the era of MacBook in the video

I've tried to find this for so long. I remember seeing it at the time as a teenager and thinking it was SO COOL. Basically made me discover Apple and want a Mac. :)

When I saw the post the Newton Virus was the first thing I thought of. Thanks for making it. I remember showing my family the video and remarking about how cool it was.

>have a hard to find mems accelerometer managed by the sensor processing unit

How did OP even know that an accelerometer exists in the first place?

The presence of the sensor is well documented as part of Apples Sudden Motion Sensor hard drive protection system.

How to access it is undocumented.

Aaackshually, the Sudden Motion Sensor was introduced on 2005 in the PowerBook G4, and continued through the intel MacBooks with hard drives.

While officially undocumented, people figured out how to access it back then, with novel uses like smacking your MacBook to change spaces (virtual desktops) or swinging the Mac around to make lightsaber noises.

- https://www.youtube.com/watch?v=6uvQTTPr9Rw

- https://osxdaily.com/2006/12/06/macsaber-turn-your-mac-into-...

(I should know, I was in university back then and swung my Mac around like an idiot, lol.)

On the first Retina MacBook Pro 15" in 2012, and moving forward with all MacBooks that were SSD-only, they removed the SMS as it was not needed.

To my knowledge, this is the first time we're hearing that Apple Silicon machines have an accelerometer on the SoC, officially or otherwise. It's also certainly not branded or marketed as the SMS was. (https://support.apple.com/en-us/100871)

Happy to be corrected on this!

I think there's some sort of motion sickness reducing feature in MacOS Tahoe which would require an accelerometer.

In 2022 "a little birdy pointed out an accelerometer" to iFixit on their teardown of an M2 MBA: https://www.ifixit.com/News/62674/m2-macbook-air-teardown-ap...

They could not figure out what it was for.

Ars Technica commenters at the time believed it was to record drops so Apple repair teams could rebuff requests :)

Given that current drives don't have moving parts, what function is this serving today?

> the sensor lives under AppleSPUHIDDevice in the iokit registry, on vendor usage page 0xFF00, usage 3. the driver is AppleSPUHIDDriver which is part of the sensor processing unit.

Apple has a motion sickness mitigation feature that displays dots on your screen that move based on physical motion, so it’s fairly well known that the accelerometer exists.

That's for iOS devices though

It’s also for macOS

No.

https://support.apple.com/en-om/guide/mac-help/mchla3c4f1da/...:

“Vehicle Motion Cues

Vehicle Motion Cues may help reduce vehicle motion sickness while using a Mac and riding as a passenger in a car or other on-road vehicle.

To customize Vehicle Motion cues, click Customize Appearance, then set any of the following options:

- Pattern: Select Regular for a stable and predictable pattern of onscreen dots, or Dynamic for a more engaging visual experience.

- Color: Select a color of onscreen dots. Color saturation will automatically adjust to maintain contrast with the content behind each dot.

- Larger dots: Turn on Large dots to increase the size of the dots that appear onscreen.

- More dots: Turn on More dots to increase the number of dots that appear onscreen.

Note: This option is available on Mac laptop computers. It’s not available on MacBook Air (M1) or 13-inch MacBook Pro (M1) or earlier.”

undocumented

The one thought that comes to mind is this: "Your warranty claim was denied because we determined that the laptop was subjected to a sudden shock."

idk you can just use simple liquid-container or sticker?

maybe apple was preparing for "carrying-around laptop experience"?

That's an entirely different product build path compared to the electronics production line though.

If a pick and place machine can drop it on and reflow it, that's what you want.

well it would be hardened when contact with air or something

see "Shipping Damage Indicators"

Did it park the drive heads?

Back in the days this was to lock up the hard disk read/write head. Maybe a relic from those times instead?

Apple is not known for backwards-compatibility, and they were already using SSDs in their laptops long before switching to ARM.

From elsewhere in this discussion, it's likely for the motion sickness functionality they added in recent years: https://support.apple.com/en-om/guide/mac-help/mchla3c4f1da/...

If it can read your heartbeat from your wrists resting next to the trackpad, maybe it can use that as a user satisfaction signal for gratuitous UI changes.

If it's sensitive enough to read a heart beat, then surely it can be used as a covert microphone?

depends on the bandwidth

Depends on the bandwidth.

From testing, it seems to require me to press my wrists quite hard against the macbook to get a somewhat accurate reading on the heartbeat. Non the less a cool project and I wasn't even aware my macbook has an accelerometer.

The laptop also contains a normal microphone. You can't access this without root; if you have that you have permissions to access the real microphone.

... but think how much "engagement" a "security expert" would get out of this!

I can access the IMU without root on my M4 mac running Sequoia

The sample rate is only ~800hz (probably very difficult/impossible to increase)

The fundamental frequencies of voice are below half that, I think that means you can use it to determine what is said writing it's sensitivity range.

https://osxdaily.com/2006/12/06/macsaber-turn-your-mac-into-...

https://techcrunch.com/2008/06/01/the-macbook-air-thin-enoug...

On my M4 14-inch MacBook Pro, it looks like there are two accelerometers: One with {"DeviceUsagePage"=0xff00,"DeviceUsage"=3}, and one with {"DeviceUsagePage"=0xff00,"DeviceUsage"=9} - They both identify as Bosch BMI286

Ah, after some testing, it looks like these both refer to the same IMU, DeviceUsage=3 is for the accelerometer and DeviceUsage=9 is for the gyroscope. The serial number is also the same for both.

"hidden", checks API: https://developer.apple.com/documentation/coremotion/cmdevic...

and there is Mac Catalyst (iOS on Mac) and Mac support.

what am I missing ? that this uses a raw stream ?

I would like an app to lock my screen on sudden movement; optionally disable TouchID for next login.

I remember that existed way back then.

And MacSaber... MacSaber!!!

author here, quick update: the sensor code is now a standalone pip package

    pip install macimu

    from macimu import IMU
    with IMU() as imu:
        print(imu.latest_accel())  # Sample(x, y, z) in g
        print(imu.latest_gyro())   # Sample(x, y, z) in deg/s

requires sudo. details: https://github.com/olvvier/apple-silicon-accelerometer

Here’s a tilt sim for Mac - https://github.com/NSEvent/tilt-sim-experiment

I've been wondering about this for a while, glad someone's finally managed to access it.

Could this be used as "shake your mac for highly random seed" bits?

Probably not as random as you want it to be.

well wouldn't it add up?

someWhatRandom1 xor someWhatRandom2 xor notRandom3 xor ...

should be more 'random' than just 'someWhatRandom1'

i'm not sure it would necessarily be more random if you're mixing in a lower-entropy source which the accelerometer probably is

XOR is the worst hashing function imaginable, especially if the data isn't truly random, because it can cancel out to become even less random!

It's better to use a KDF or something. Even a block cipher should work far better than XOR.

Shake your Mac to undo would be consistent with iPhone, iPad, Vision Pro. Yes, you shake your head to undo when wearing AVP.

This is a milder version of “Shake to Reboot” which is standard for all Etch-a-Sketch models

There's lots of noise in the accelerometer readings, even without shaking

This is so cool thank you!

I wonder if this sensor is used for Vision Pro display mirroring.

I think it's used for the motion sickness reducing feature in MacOS Tahoe that puts a bunch of dots on your screen that react to motion.

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