XNU is the computer

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

(OS)

kernel Kernel may refer to: Computing * Kernel (operating system), the central component of most operating systems * Kernel (image processing), a matrix used for image convolution * Compute kernel, in GPGPU programming * Kernel method, in machine learn ...

developed at Apple Inc. since December 1996 for use in the Mac OS X (now

macOS macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and la ...

) operating system and released as

free and open-source software Free and open-source software (FOSS) is a term used to refer to groups of software consisting of both free software and open-source software where anyone is freely licensed to use, copy, study, and change the software in any way, and the source ...

as part of the Darwin OS, which in addition to macOS is also the basis for the

Apple TV Software An apple is an edible fruit produced by an apple tree (''Malus domestica''). Apple trees are cultivated worldwide and are the most widely grown species in the genus ''Malus''. The tree originated in Central Asia, where its wild ancest ...

, iOS,

iPadOS iPadOS is a mobile operating system developed by Apple Inc. for its iPad line of tablet computers. It is a rebranded variant of iOS, the operating system used by Apple's iPhones, renamed to reflect the diverging features of the two product ...

watchOS watchOS is the operating system of the Apple Watch, developed by Apple Inc. It is based on iOS, the operating system used by the iPhone, and has many similar features. It was released on April 24, 2015, along with the Apple Watch, the only d ...

, and

tvOS tvOS (formerly known as Apple TV Software) is an operating system developed by Apple Inc. for the Apple TV, a digital media player. In the first-generation Apple TV, Apple TV Software was based on Mac OS X. Starting with the second-generation, ...

OSes. ''XNU'' is an abbreviation of ''X is Not

Unix Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...

''. Originally developed by

NeXT Next may refer to: Arts and entertainment Film * ''Next'' (1990 film), an animated short about William Shakespeare * ''Next'' (2007 film), a sci-fi film starring Nicolas Cage * '' Next: A Primer on Urban Painting'', a 2005 documentary film Lit ...

for the

NeXTSTEP NeXTSTEP is a discontinued object-oriented, multitasking operating system based on the Mach kernel and the UNIX-derived BSD. It was developed by NeXT Computer in the late 1980s and early 1990s and was initially used for its range of propri ...

operating system, XNU was a

hybrid kernel A hybrid kernel is an operating system kernel architecture that attempts to combine aspects and benefits of microkernel and monolithic kernel architectures used in computer operating systems. Overview The traditional kernel categories are mono ...

derived from version 2.5 of the

Mach kernel Mach () is a kernel developed at Carnegie Mellon University by Richard Rashid and Avie Tevanian to support operating system research, primarily distributed and parallel computing. Mach is often considered one of the earliest examples of a microk ...

developed at

Carnegie Mellon University Carnegie Mellon University (CMU) is a private research university in Pittsburgh, Pennsylvania. One of its predecessors was established in 1900 by Andrew Carnegie as the Carnegie Technical Schools; it became the Carnegie Institute of Technology ...

, which incorporated the bulk of the 4.3BSD kernel modified to run atop Mach primitives, along with an

application programming interface An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

(API) in

Objective-C Objective-C is a general-purpose, object-oriented programming language that adds Smalltalk-style messaging to the C programming language. Originally developed by Brad Cox and Tom Love in the early 1980s, it was selected by NeXT for its NeXT ...

for writing drivers named Driver Kit. After Apple acquired NeXT, the kernel was updated with code derived from OSFMK 7.3 from OSF, and the

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular ...

project, and the Driver Kit was replaced with a

C++ C++ (pronounced "C plus plus") is a high-level general-purpose programming language created by Danish computer scientist Bjarne Stroustrup as an extension of the C programming language, or "C with Classes". The language has expanded significan ...

API for writing drivers named I/O Kit.

Kernel design

XNU is a

, containing features of both

monolithic kernel A monolithic kernel is an operating system architecture where the entire operating system is working in kernel space. The monolithic model differs from other operating system architectures (such as the microkernel architecture) in that it alone ...

s and

microkernel In computer science, a microkernel (often abbreviated as μ-kernel) is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system (OS). These mechanisms include low-level address space management, ...

s, attempting to make the best use of both technologies, such as the

message passing In computer science, message passing is a technique for invoking behavior (i.e., running a program) on a computer. The invoking program sends a message to a process (which may be an actor or object) and relies on that process and its supporting ...

ability of microkernels enabling greater modularity and larger portions of the OS to benefit from

memory protection Memory protection is a way to control memory access rights on a computer, and is a part of most modern instruction set architectures and operating systems. The main purpose of memory protection is to prevent a process from accessing memory that h ...

, and retaining the speed of monolithic kernels for some critical tasks. , XNU runs on

ARM64 AArch64 or ARM64 is the 64-bit extension of the ARM architecture family. It was first introduced with the Armv8-A architecture. Arm releases a new extension every year. ARMv8.x and ARMv9.x extensions and features Announced in October 2011, AR ...

and

x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging ...

processors, both one processor and

symmetric multiprocessing Symmetric multiprocessing or shared-memory multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single, shared main memory, have full access to all ...

(SMP) models.

PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple– IBM– ...

support was removed as of the version in Mac OS X 10.6. Support for

IA-32 IA-32 (short for "Intel Architecture, 32-bit", commonly called i386) is the 32-bit version of the x86 instruction set architecture, designed by Intel and first implemented in the 80386 microprocessor in 1985. IA-32 is the first incarnatio ...

was removed as of the version in

Mac OS X 10.7 OS X Lion, also known as Mac OS X Lion, (version 10.7) is the eighth major release of macOS, Apple's desktop and server operating system for Mac computers. A preview of OS X 10.7 Lion was publicly shown at the "Back to the Mac" Apple Speci ...

; support for 32-bit ARM was removed as of the version in .

Mach

The basis of the XNU kernel is a heavily modified (hybrid)

Open Software Foundation The Open Software Foundation (OSF) was a not-for-profit industry consortium for creating an open standard for an implementation of the operating system Unix. It was formed in 1988 and merged with X/Open in 1996, to become The Open Group. Despit ...

Mach kernel ( OSFMK) 7.3. As such, it is able to run the core of an operating system as separated processes, which allows a great flexibility (it could run several operating systems in parallel above the Mach core), but this often reduces performance because of time-consuming kernel/user mode context switches and overhead stemming from mapping or copying messages between the address spaces of the kernel and that of the service daemons. With

, the designers have attempted to streamline some tasks and thus BSD functions were built into the core with Mach. The result is a heavily modified (hybrid) OSFMK 7.3 kernel, Apple licensed OSFMK 7.3, which is a microkernel, from the OSF. OSFMK 7.3 includes applicable code from the

University of Utah The University of Utah (U of U, UofU, or simply The U) is a public research university in Salt Lake City, Utah. It is the flagship institution of the Utah System of Higher Education. The university was established in 1850 as the University of De ...

Mach 4 kernel and from the many Mach 3.0 variants forked from the original

Mach 3.0 microkernel.

BSD

The

Berkeley Software Distribution The Berkeley Software Distribution or Berkeley Standard Distribution (BSD) is a discontinued operating system based on Research Unix, developed and distributed by the Computer Systems Research Group (CSRG) at the University of California, Be ...

(BSD) part of the kernel provides the Portable Operating System Interface (

POSIX The Portable Operating System Interface (POSIX) is a family of standards specified by the IEEE Computer Society for maintaining compatibility between operating systems. POSIX defines both the system- and user-level application programming in ...

)

(API, BSD system calls), the

process model atop Mach tasks, basic security policies, user and group ids, permissions, the network

protocol stack The protocol stack or network stack is an implementation of a computer networking protocol suite or protocol family. Some of these terms are used interchangeably but strictly speaking, the ''suite'' is the definition of the communication protoco ...

(protocols), the virtual file system code (including a

file system In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...

independent journaling layer), several local file systems such as Hierarchical File System (HFS,

HFS Plus HFS Plus or HFS+ (also known as Mac OS Extended or HFS Extended) is a journaling file system developed by Apple Inc. It replaced the Hierarchical File System (HFS) as the primary file system of Apple computers with the 1998 release of Mac OS ...

(HFS+)) and

Apple File System Apple File System (APFS) is a proprietary file system developed and deployed by Apple Inc. for macOS Sierra (10.12.4) and later, iOS 10.3 and later, tvOS 10.2 and later, watchOS 3.2 and later, and all versions of iPadOS. It aims to fix co ...

(APFS), the

Network File System Network File System (NFS) is a distributed file system protocol originally developed by Sun Microsystems (Sun) in 1984, allowing a user on a client computer to access files over a computer network much like local storage is accessed. NFS, lik ...

(NFS) client and server, cryptographic framework,

UNIX System V Unix System V (pronounced: "System Five") is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, an ...

inter-process communication In computer science, inter-process communication or interprocess communication (IPC) refers specifically to the mechanisms an operating system provides to allow the processes to manage shared data. Typically, applications can use IPC, categoriz ...

(IPC), audit subsystem,

mandatory access control In computer security, mandatory access control (MAC) refers to a type of access control by which the operating system or database constrains the ability of a ''subject'' or ''initiator'' to access or generally perform some sort of operation on a ...

, and some of the locking primitives. The BSD code present in XNU has been most recently synchronised with that from the

kernel. Although much of it has been significantly modified, code sharing still occurs between Apple and the FreeBSD Project .

K32/K64

XNU in

Mac OS X Snow Leopard Mac OS X Snow Leopard (version 10.6) is the seventh major release of macOS, Apple's desktop and server operating system for Macintosh computers. Snow Leopard was publicly unveiled on June 8, 2009 at Apple’s Worldwide Developers Conference. ...

, v10.6, ( Darwin version 10) comes in two varieties, a

32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calculati ...

version called ''K32'' and a

64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit CPUs and ALUs are those that are based on processor registers, address buses, or data buses of that size. A ...

version called ''K64''. K32 can run 64-bit applications in userland. What was new in Mac OS X 10.6 was the ability to run XNU in 64-bit

kernel space A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

. K32 was the default kernel for 10.6 ''Server'' when used on all machines except

Mac Pro Mac Pro is a series of workstations and servers for professionals that are designed, developed and marketed by Apple Inc. since 2006. The Mac Pro, by some performance benchmarks, is the most powerful computer that Apple offers. It is one of ...

and

Xserve Xserve is a line of rack unit computers designed by Apple Inc. for use as servers. Introduced in 2002, it was Apple's first designated server hardware design since the Apple Network Server in 1996. In the meantime, ordinary Power Macintosh G3 a ...

models from 2008 onwards and can run 64-bit applications. K64 has several benefits compared to K32: * Can manage more than 32 GB RAM, as the memory map would consume a disproportionately large area of the 32-bit kernel space. * Cache buffer sizes can be larger than what the 32-bit kernel space allows, potentially increasing I/O performance. * Performance is increased when using high-performance networking devices or multiple

graphics processing unit A graphics processing unit (GPU) is a specialized electronic circuit designed to manipulate and alter memory to accelerate the creation of images in a frame buffer intended for output to a display device. GPUs are used in embedded systems, m ...

s (GPUs), as the kernel can map all of the devices in 64-bit space even if several have very large

direct memory access Direct memory access (DMA) is a feature of computer systems and allows certain hardware subsystems to access main system memory independently of the central processing unit (CPU). Without DMA, when the CPU is using programmed input/output, it is ...

(DMA) buffers. Booting while holding down ''6'' and ''4'' forces the machine to boot K64 on machines supporting 64-bit kernels. K64 will run 32-bit applications but it will not run 32-bit kernel extensions (KEXTs), so these must be ported to K64 to be able to load. XNU in OS X Mountain Lion, v10.8, and later only provides a 64-bit kernel.

I/O Kit

I/O Kit is the

device driver In computing, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabling operating systems and o ...

framework, written in a subset of

based on Embedded C++.I/O Kit uses a restricted subset of C++
/ref> Using its

object-oriented Object-oriented programming (OOP) is a programming paradigm based on the concept of "objects", which can contain data and code. The data is in the form of fields (often known as attributes or ''properties''), and the code is in the form of ...

design, features common to any class of driver are provided within the framework, helping device drivers be written in less time and code. The I/O Kit is multi-threaded,

(SMP)-safe, and allows for hot-pluggable devices and automatic, dynamic device configuration. Many drivers can be written to run from

user space A modern computer operating system usually segregates virtual memory into user space and kernel space. Primarily, this separation serves to provide memory protection and hardware protection from malicious or errant software behaviour. Kernel ...

, which further enhances the stability of the system. If a user-space driver crashes, it will not crash the kernel. However, if a kernel-space driver crashes it will crash the kernel. Examples of kernel-space drivers include disk adapter and network adapter drivers, graphics drivers, drivers for Universal Serial Bus ( USB) and

FireWire IEEE 1394 is an interface standard for a serial bus for high-speed communications and isochronous real-time data transfer. It was developed in the late 1980s and early 1990s by Apple in cooperation with a number of companies, primarily Sony an ...

host controllers, and drivers for

virtual machine In computing, a virtual machine (VM) is the virtualization/ emulation of a computer system. Virtual machines are based on computer architectures and provide functionality of a physical computer. Their implementations may involve specialized h ...

software such as

VirtualBox Oracle VM VirtualBox (formerly Sun VirtualBox, Sun xVM VirtualBox and Innotek VirtualBox) is a type-2 hypervisor for x86 virtualization developed by Oracle Corporation. VirtualBox was originally created by Innotek GmbH, which was acquired by S ...

Parallels Desktop for Mac Parallels Desktop for Mac is software providing hardware virtualization for Macintosh computers with Intel processors, and since version 16.5 also for Apple silicon-based Macintosh computers. It is developed by Parallels, since 2018 a subsidiary ...

, and VMware Fusion.

References

External links

* , official repository * – an overview of the components of XNU, written by Amit Singh in December 2003
Inside the Mac OS X Kernel
– "This talk intends to clear up the confusion by presenting details of the Mac OS X kernel" {{DEFAULTSORT:Xnu iOS Mach (kernel) Monolithic kernels MacOS Software using the Apple Public Source License