XNU is the kernel underlying macOS and iOS systems. The repository contains multiple subsystems and libraries that comprise the kernel infrastructure. The DEVFS subsystem provides a device filesystem with a layered architecture using "back" nodes (kernel-internal blueprint) and "front" nodes (user-visible mounts), originally from FreeBSD 3.1. The libkdd library enables dynamic kernel data to be passed to userspace tools through a self-describing chunked data format (KCDATA) that remains compatible across kernel versions. Additional components include a C++ runtime system for kernel modules, the zlib 1.2.3 data compression library, and a kernel Power-On Self Test (POST) framework that runs subsystem verification tests at boot time using configurable boot arguments.