Apple has silently released ARM-ready source code for the XNU kernels found in iOS and macOS, marking a first for iOS, and possibly showing off some of the groundwork for Macs with ARM-based processors.
The code is available through a GitHub repository. While exposing some of the operating systems' internal architecture, many proprietary elements — such as apps and frameworks — are of course absent, and Apple's kernels are being offered under a more restrictive license than some other open-source projects, such as most Linux distributions.
All of the Apple A-series processors in iPhones, iPads, iPods, and the Apple TV use ARM-based designs. Macs have depend primarily on Intel processors since 2006.
Recent rumors have claimed that Apple could be designing its own ARM-based CPU for future Macs in an attempt to reduce its dependence on Intel. While Intel chips are generally regarded as powerful, the company puts out CPUs on an independent schedule and doesn't specifically optimize them for Mac hardware.
ARM-compatible kernels aren't necessarily a sign that a transition is imminent, or even planned. MacBook Pros already use one Apple ARM chip, the T1, to run the Touch Bar and Touch ID. The company has also made it a tradition of having code ready for alternate architectures — that policy helped the company make its original switch to Intel faster than expected.
There could be several potential complications with a wider ARM transition, such as Boot Camp, which lets Macs run Windows through a separate partition. Microsoft is however working on ARM compatibility for Windows, which might smooth out some issues.