XNU kernel is part of the Darwin operating system for use in OS X and iOS operating systems. XNU is an acronym for XNU is Not Unix. XNU is a hybrid kernel combining the Mach kernel developed at Carnegie Mellon University with components from FreeBSD and C++ API for writing drivers called IOKit. XNU runs on I386, X86_64 for both single processor and multi-processor configurations.

XNU Source Tree config - configurations for exported apis for supported architecture and platform SETUP - Basic set of tools used for configuring the kernel, versioning and kextsymbol management. EXTERNAL_HEADERS - Headers sourced from other projects to avoid dependency cycles when building. These headers should be regularly synced when source is updated. libkern - C++ IOKit library code for handling of drivers and kexts. libsa - kernel bootstrap code for startup libsyscall - syscall library interface for userspace programs libkdd - source for user library for parsing kernel data like kernel chunked data. makedefs - top level rules and defines for kernel build. osfmk - Mach kernel based subsystems pexpert - Platform specific code like interrupt handling, atomics etc. security - Mandatory Access Check policy interfaces and related implementation. bsd - BSD subsystems code tools - A set of utilities for testing, debugging and profiling kernel.