01
Foundations — the two kernels in one binary
Everything else assumes you understand that XNU is Mach + BSD glued together. Read these first; the vocabulary unlocks the rest.
Reading guide
Start here
There's a lot on this site. If you're new to macOS internals, follow these four stages in order. Each stage is one to three short articles plus the glossary entries you'll meet along the way. By the end you should be able to read XNU source on GitHub without getting lost.
02
How the kernel talks to itself
Now that you know what a Mach port is, see how it gets used everywhere — scheduling, signals, drivers, the VM system.
03
Storage and security
The two areas where macOS departs the most from a generic Unix. APFS is everywhere; the security stack is what makes the system trustworthy.
04
The modern hardware
Apple Silicon changed XNU in ways that aren't documented in the ARM spec. Once you've seen the rest of the kernel, you'll appreciate what's new here.
Once you've finished, every link on the site is fair game. Use the search bar (⌘K) to find any term as you encounter it in the wild.
Open the articles index