XNU
Encyclopedia
XNU is the computer operating system
kernel that Apple Inc. acquired and developed for use in the Mac OS X
operating system
and released as free and open source software
as part of the Darwin
operating system. XNU is an acronym for X is Not Unix
.
Originally developed by NeXT
for the NeXTSTEP
operating system, XNU was a hybrid kernel
combining version 2.5 of the Mach kernel developed at Carnegie Mellon University
with components from 4.3BSD and an object-oriented API for writing drivers called Driver Kit.
After Apple acquired NeXT, the Mach component was upgraded to 3.0, the BSD components were upgraded with code from the FreeBSD
project and the Driver Kit was replaced with a C++
API for writing drivers called I/O Kit
.
and microkernel
s, attempting to make the best use of both technologies, such as the message passing
capability of microkernels enabling greater modularity and larger portions of the OS to benefit from protected memory
, as well as retaining the speed of monolithic kernels for certain critical tasks.
Currently, XNU runs on ARM
, IA-32
, x86-64
and PowerPC
based processors, both single processor and SMP
models.
, is a simple microkernel. As such, it is able to run the core of an operating system as separated processes, which allows a great flexibility (one 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 microkernel and that of the service daemons. With Mac OS X, the designers have attempted to streamline certain tasks and thus BSD functionalities were built into the core with Mach. The result is a combination of Mach and a classical BSD kernel, with some advantages and disadvantages of both.
Mach provides kernel threads
, processes, pre-emptive multitasking, message-passing (used in inter-process communication
), protected memory, virtual memory
management, soft real-time support, kernel debugging support, and console I/O
.
(BSD) portion of the kernel provides the POSIX
API
(BSD system calls), the Unix
process model atop Mach tasks, basic security policies, user and group ids, permissions, the network stack
, the virtual file system
code (including a filesystem independent journaling
layer), several local file systems such as HFS
/HFS+, the Network File System (NFS) client and server, cryptographic framework, UNIX System V
inter-process communication
(IPC), Audit subsystem, mandatory access control
, and some of the locking primitives. The BSD code present in XNU came from the FreeBSD
kernel. Although much of it has been significantly modified, code sharing still occurs between Apple and the FreeBSD Project.
(Darwin
version 10) comes in two varieties, a 32-bit
version called K32 and a 64-bit
version called K64. K32 can run 64-bit applications in userland. What's new in Mac OS X 10.6 is the ability to run XNU in 64-bit kernel space.
K32 is the default kernel for 10.6 Server when used on all machines except newer Mac Pro
and Xserve
models (2008 and later) and can run 64-bit applications. K64 has several benefits compared to K32:
Booting while holding down 6 and 4 will force 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.
is the device driver
framework, written in a subset of C++
based on Embedded C++
. Using its object-oriented
design, features common to any class of driver are provided within the framework itself, helping device drivers be written more quickly and using less code. The I/O Kit is multi-threaded, Symmetric multiprocessing
(SMP)-safe, and allows for hot pluggable devices and automatic, dynamic device configuration.
Many drivers can be written to run from user-space, 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 USB and FireWire host controllers, and drivers for virtual machine
software such as Parallels Desktop for Mac
and VMware Fusion
.
s, critical section
s, mutual exclusion
s ("mutexes"), and serializing tokens
are all possible methods that can be used to prevent concurrent access. Like recent versions of Linux
and FreeBSD
, XNU, as of Mac OS X 10.4 and Darwin 8.0, employs a fine-grained mutex model to achieve higher performance on multiprocessor
systems.
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
kernel that Apple Inc. acquired and developed for use in the Mac OS X
Mac OS X
Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
and released as free and open source software
Free and open source software
Free and open-source software or free/libre/open-source software is software that is liberally licensed to grant users the right to use, study, change, and improve its design through the availability of its source code...
as part of the Darwin
Darwin (operating system)
Darwin is an open source POSIX-compliant computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects....
operating system. XNU is an acronym for X is Not Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
.
Originally developed by NeXT
NeXT
Next, Inc. was an American computer company headquartered in Redwood City, California, that developed and manufactured a series of computer workstations intended for the higher education and business markets...
for the NeXTSTEP
NEXTSTEP
NeXTSTEP was the object-oriented, multitasking operating system developed by NeXT Computer to run on its range of proprietary workstation computers, such as the NeXTcube...
operating system, XNU was a hybrid kernel
Hybrid kernel
A hybrid kernel is a kernel architecture based on combining aspects of microkernel and monolithic kernel architectures used in computer operating systems. The category is controversial due to the similarity to monolithic kernel; the term has been dismissed by Linus Torvalds as simple marketing...
combining version 2.5 of the Mach kernel developed at Carnegie Mellon University
Carnegie Mellon University
Carnegie Mellon University is a private research university in Pittsburgh, Pennsylvania, United States....
with components from 4.3BSD and an object-oriented API for writing drivers called Driver Kit.
After Apple acquired NeXT, the Mach component was upgraded to 3.0, the BSD components were upgraded with code from the FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
project and the Driver Kit was replaced with a C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
API for writing drivers called I/O Kit
I/O Kit
The I/O Kit is an open-source framework in the XNU kernel that helps developers code device drivers for Apple's Mac OS X and iOS operating systems...
.
Kernel design
Like some other modern kernels, XNU is a hybrid, containing features of both monolithicMonolithic kernel
A monolithic kernel is an operating system architecture where the entire operating system is working in the kernel space and alone as supervisor mode...
and microkernel
Microkernel
In computer science, a microkernel is the near-minimum amount of software that can provide the mechanisms needed to implement an operating system . These mechanisms include low-level address space management, thread management, and inter-process communication...
s, attempting to make the best use of both technologies, such as the message passing
Message passing
Message passing in computer science is a form of communication used in parallel computing, object-oriented programming, and interprocess communication. In this model, processes or objects can send and receive messages to other processes...
capability of microkernels enabling greater modularity and larger portions of the OS to benefit from protected memory
Memory protection
Memory protection is a way to control memory access rights on a computer, and is a part of most modern operating systems. The main purpose of memory protection is to prevent a process from accessing memory that has not been allocated to it. This prevents a bug within a process from affecting...
, as well as retaining the speed of monolithic kernels for certain critical tasks.
Currently, XNU runs on ARM
ARM architecture
ARM is a 32-bit reduced instruction set computer instruction set architecture developed by ARM Holdings. It was named the Advanced RISC Machine, and before that, the Acorn RISC Machine. The ARM architecture is the most widely used 32-bit ISA in numbers produced...
, IA-32
IA-32
IA-32 , also known as x86-32, i386 or x86, is the CISC instruction-set architecture of Intel's most commercially successful microprocessors, and was first implemented in the Intel 80386 as a 32-bit extension of x86 architecture...
, x86-64
X86-64
x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
and PowerPC
PowerPC
PowerPC is a RISC architecture created by the 1991 Apple–IBM–Motorola alliance, known as AIM...
based processors, both single processor and SMP
Symmetric multiprocessing
In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...
models.
Mach
The basis of the XNU kernel, MachMach (kernel)
Mach is an operating system kernel developed at Carnegie Mellon University to support operating system research, primarily distributed and parallel computation. Although Mach is often mentioned as one of the earliest examples of a microkernel, not all versions of Mach are microkernels...
, is a simple microkernel. As such, it is able to run the core of an operating system as separated processes, which allows a great flexibility (one 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 microkernel and that of the service daemons. With Mac OS X, the designers have attempted to streamline certain tasks and thus BSD functionalities were built into the core with Mach. The result is a combination of Mach and a classical BSD kernel, with some advantages and disadvantages of both.
Mach provides kernel threads
Thread (computer science)
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...
, processes, pre-emptive multitasking, message-passing (used in inter-process communication
Inter-process communication
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...
), protected memory, virtual memory
Virtual memory
In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...
management, soft real-time support, kernel debugging support, and console I/O
I/O
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...
.
BSD
The Berkeley Software DistributionBerkeley Software Distribution
Berkeley Software Distribution is a Unix operating system derivative developed and distributed by the Computer Systems Research Group of the University of California, Berkeley, from 1977 to 1995...
(BSD) portion of the kernel provides the POSIX
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...
API
Application programming interface
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
(BSD system calls), the Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
process model atop Mach tasks, basic security policies, user and group ids, permissions, the network stack
Protocol stack
The protocol stack is an implementation of a computer networking protocol suite. The terms are often used interchangeably. Strictly speaking, the suite is the definition of the protocols, and the stack is the software implementation of them....
, the virtual file system
Virtual file system
A virtual file system or virtual filesystem switch is an abstraction layer on top of a more concrete file system. The purpose of a VFS is to allow client applications to access different types of concrete file systems in a uniform way...
code (including a filesystem independent journaling
Journaling file system
A journaling file system is a file system that keeps track of the changes that will be made in a journal before committing them to the main file system...
layer), several local file systems such as HFS
Hierarchical File System
Hierarchical File System is a file system developed by Apple Inc. for use in computer systems running Mac OS. Originally designed for use on floppy and hard disks, it can also be found on read-only media such as CD-ROMs...
/HFS+, the Network File System (NFS) client and server, cryptographic framework, UNIX System V
UNIX System V
Unix System V, commonly abbreviated SysV , is one of the first commercial versions of the Unix operating system. It was originally developed by American Telephone & Telegraph and first released in 1983. Four major versions of System V were released, termed Releases 1, 2, 3 and 4...
inter-process communication
Inter-process communication
In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...
(IPC), Audit subsystem, mandatory access control
Mandatory access control
In computer security, mandatory access control refers to a type of access control by which the operating system constrains the ability of a subject or initiator to access or generally perform some sort of operation on an object or target...
, and some of the locking primitives. The BSD code present in XNU came from the FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
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 10.6 Snow LeopardMac OS X v10.6
Mac OS X Snow Leopard is the seventh major release of Mac OS X, Apple's desktop and server operating system for Macintosh computers.Snow Leopard was publicly unveiled on June 8, 2009 at the Apple Worldwide Developers Conference...
(Darwin
Darwin (operating system)
Darwin is an open source POSIX-compliant computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP, BSD, and other free software projects....
version 10) comes in two varieties, a 32-bit
32-bit
The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....
version called K32 and a 64-bit
64-bit
64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...
version called K64. K32 can run 64-bit applications in userland. What's new in Mac OS X 10.6 is the ability to run XNU in 64-bit kernel space.
K32 is the default kernel for 10.6 Server when used on all machines except newer Mac Pro
Mac Pro
The Mac Pro is a workstation computer manufactured by Apple Inc. The machines are based on Xeon microprocessors, but are similar to the Power Mac G5 they replaced in terms of outward appearance and expansion capabilities...
and Xserve
Xserve
Xserve was a line of rack unit computers designed by Apple Inc. for use as servers. When the Xserve was introduced in 2002, it was Apple's first designated server hardware design since the Apple Network Server in 1996...
models (2008 and later) 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 GPUsGraphics processing unitA graphics processing unit or GPU is a specialized circuit designed to rapidly manipulate and alter memory in such a way so as to accelerate the building of images in a frame buffer intended for output to a display...
, as the kernel can map all of the devices in 64-bit space even if several have very large DMADirect memory accessDirect memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....
buffers.
Booting while holding down 6 and 4 will force 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
Loadable Kernel Module
In computing, a loadable kernel module is an object file that contains code to extend the running kernel, or so-called base kernel, of an operating system...
(KEXTs) so these must be ported to K64 to be able to load.
I/O Kit
I/O KitI/O Kit
The I/O Kit is an open-source framework in the XNU kernel that helps developers code device drivers for Apple's Mac OS X and iOS operating systems...
is the device driver
Device driver
In computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
framework, written in a subset of C++
C++
C++ is a statically typed, free-form, multi-paradigm, compiled, general-purpose programming language. It is regarded as an intermediate-level language, as it comprises a combination of both high-level and low-level language features. It was developed by Bjarne Stroustrup starting in 1979 at Bell...
based on Embedded C++
Embedded C++
Embedded C++ is a dialect of the C++ programming language for embedded systems. It was defined by an industry group led by major Japanese central processing unit manufacturers, including NEC, Hitachi, Fujitsu, and Toshiba, to address the shortcomings of C++ for embedded applications...
. Using its object-oriented
Object-oriented programming
Object-oriented programming is a programming paradigm using "objects" – data structures consisting of data fields and methods together with their interactions – to design applications and computer programs. Programming techniques may include features such as data abstraction,...
design, features common to any class of driver are provided within the framework itself, helping device drivers be written more quickly and using less code. The I/O Kit is multi-threaded, Symmetric multiprocessing
Symmetric multiprocessing
In computing, symmetric multiprocessing involves a multiprocessor computer hardware architecture where two or more identical processors are connected to a single shared main memory and are controlled by a single OS instance. Most common multiprocessor systems today use an SMP architecture...
(SMP)-safe, and allows for hot pluggable devices and automatic, dynamic device configuration.
Many drivers can be written to run from user-space, 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 USB and FireWire host controllers, and drivers for virtual machine
Virtual machine
A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...
software such as Parallels Desktop for Mac
Parallels Desktop for Mac
Parallels Desktop for Mac by Parallels, Inc., is software providing hardware virtualization for Macintosh computers with Intel processors.-Overview:Parallels, Inc...
and VMware Fusion
VMware Fusion
VMware Fusion is a virtual machine software product developed by VMware for Macintosh computers with Intel processors. Fusion allows Intel-based Macs to run x86 and x86-64 "guest" operating systems, such as Microsoft Windows, Linux, NetWare and Solaris as virtual machines simultaneously with Mac OS...
.
Protecting shared resources
In order to run safely on multiprocessor machines, access to shared resources (files, data structures etc.) must be serialized so that threads or processes do not attempt to modify the same resource at the same time. Atomic operations, spinlockSpinlock
In software engineering, a spinlock is a lock where the thread simply waits in a loop repeatedly checking until the lock becomes available. Since the thread remains active but isn't performing a useful task, the use of such a lock is a kind of busy waiting...
s, critical section
Critical section
In concurrent programming a critical section is a piece of code that accesses a shared resource that must not be concurrently accessed by more than one thread of execution. A critical section will usually terminate in fixed time, and a thread, task or process will have to wait a fixed time to...
s, mutual exclusion
Mutual exclusion
Mutual exclusion algorithms are used in concurrent programming to avoid the simultaneous use of a common resource, such as a global variable, by pieces of computer code called critical sections. A critical section is a piece of code in which a process or thread accesses a common resource...
s ("mutexes"), and serializing tokens
Serializing tokens
In computer science, serializing tokens are a concept in concurrency control arising from the ongoing development of DragonFly BSD. According to Matthew Dillon, they are most akin to SPLs, except a token works across multiple CPUs while SPLs only work within a single CPU's domain.Serializing...
are all possible methods that can be used to prevent concurrent access. Like recent versions of Linux
Linux
Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...
and FreeBSD
FreeBSD
FreeBSD is a free Unix-like operating system descended from AT&T UNIX via BSD UNIX. Although for legal reasons FreeBSD cannot be called “UNIX”, as the direct descendant of BSD UNIX , FreeBSD’s internals and system APIs are UNIX-compliant...
, XNU, as of Mac OS X 10.4 and Darwin 8.0, employs a fine-grained mutex model to achieve higher performance on multiprocessor
Multiprocessing
Multiprocessing is the use of two or more central processing units within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them...
systems.
External links
- XNU: The Kernel - 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"