Apple has published ARM-optimized source code of the iOS kernel, but don't get all excited about the prospect of being able to download and run a homebrew version of iSO. It's just not going to happen.
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Apple has always published the source code for the macOS kernel, and since macOS and iOS share the same Unix-based XNU core called Darwin (where XNU is an abbreviation for "X is Not Unix"), technically this was also the iOS kernel.
However, because it wasn't optimized for ARM chips, it was next to useless because the code wouldn't run on any smartphone or tablet platform.
But now Apple has quietly published ARM-optimized source code, we - developers and security folks especially - can finally get a deeper look into how iOS ticks at a low level.
However, beyond that, it doesn't mean that much. You're unlikely to see any homebrew iOS builds being made because the source code includes only the kernel, and not the user interface or developer framework or the apps that lay on top of that.
And even if you had the time and effort to put all that in place, Apple's source code is offered with a pretty restrictive license that wouldn't allow it to be used in the same way as, say, Linux.
I suppose the most interesting angle of this is that now Apple has published ARM-based code that could be integrated into macOS, and possibly pave the way either for MacBooks powered by ARM chips, or devices that contain ARM chips in addition to Intel CPUs to handle specific tasks.
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