XNU

XNU is the kernel at the core of Apple’s Darwin operating system and serves as the foundation for macOS, iOS, iPadOS, watchOS, and tvOS. It is a hybrid kernel, combining a Mach microkernel with BSD-derived components and Apple’s I/O Kit driver framework. The name XNU is commonly explained as “X is Not Unix.”

Architecturally, XNU integrates a Mach-origin core with BSD subsystems and IOKit. Mach provides low-level primitives such as inter-process communication, thread scheduling, and virtual memory management. BSD components supply the user-visible system call interface, POSIX compatibility, file systems, and networking. IOKit offers a scalable, object-oriented framework for device drivers, with kernel extensions (kexts) that add hardware support. The kernel is designed for 64-bit systems and supports both x86-64 and ARM64 (Apple Silicon) hardware, with legacy PowerPC support phased out in later releases.

History and open source: XNU evolved from NeXTSTEP’s kernel lineage and the Mach kernel developed at Carnegie Mellon University, later merged into Apple’s Darwin project. Apple released Darwin and the XNU kernel under the Apple Public Source License, making the core components publicly available and guiding ongoing development alongside proprietary updates.

Platform scope and maintenance: As the kernel for Apple’s operating systems, XNU implements preemptive multitasking, virtual memory, and inter-process communication, while emphasizing performance, security, and UNIX-like compatibility. Ongoing development integrates new hardware support, energy efficiency improvements, and security features, with Apple maintaining the kernel in close alignment with its platform goals.