XNU
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(1)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.
Kernel design
Like some other modern
kernels, XNU is a hybrid, containing features of both
monolithic and
microkernels, 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 (2),
x86,
x86-64 and
PowerPC based processors, both single processor and
SMP models.
Mach
The core of the XNU kernel,
Mach, was originally conceived as 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 reduced 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, very
soft real-time support, kernel
debugging support, and console
I/O. The Mach component also allows the OS to host binaries for multiple distinct CPU architectures within a single file (such as
x86 and
PowerPC) due to its use of the
Mach-O binary format.
BSD
The
Berkeley Software Distribution (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
journalling layer), Network File System (NFS), 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 {{Fact|date=May 2007}} {{Dubious|date=March 2008}}.
I/O Kit
I/O Kit is the
device driver framework, written in a subset of
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 include Parallels, EyeTV and the Apple USB driver.
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,
spinlocks, critical sections,
Mutual exclusions ("mutexes"), and serializing tokens are all possible methods that can be used to prevent concurrent access. Like both
Linux and
FreeBSD 5, 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.{{Fact|date=July 2008}}
References
-
[WEB, 2005,weblink Porting UNIX/Linux Applications to Mac OS X: Glossary, Apple Computer, 2005-12-13, ]
-
[iPhone processor found: 620MHz ARM CPU (2007-07-01 {{accessdate|2008-01-06}}]
External links
{{Mac OS X}}
XNUXNUXNUXNUXNUXNU
(...as imported from WP)
article has not been saved locally
