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'''Linux''' is the most popular [[free software|free]] computer [[operating system]] [[kernel]].
It is a [[Unix]]-like system, and implements the [[POSIX]] standard.
The Linux kernel was first developed by [[Finland|Finnish]] hacker [[Linus Torvalds]] in an attempt to provide a free Unix-like operating system that ran on [[Intel 80386]] processors.
The project was launched in [[1991]] in a famous post to [[Usenet]].
From the early days, it received help from [[Minix]] [[hacker|hackers]], and today it has received contributions from thousands of programmers.
Technically speaking, Linux is a kernel.
The term "kernel" properly refers to the system software which provide a hardware abstraction layer,
disk and filesystem control, multi-tasking, load-balancing, networking and security enforcement.
A kernel is not a complete operating system. A complete system build around the Linux kernel is known as the [[Linux operating system]].
All Linux releases with an even sub-version (the second component) are part of a stable series, namely: 1.0.x, 1.2.x, 2.0.x, 2.2.x, and the current 2.4.x.
=== Architecture ===
Today Linux is a [[hybrid monolithic kernel]]. [[Device drivers]] and kernel extensions typically run in [[ring 0]], with full access to the hardware, although some run in [[user space]]. Unlike standard [[monolithic kernels]], [[device driver|device drivers]] are easily configured as [[module|modules]], and loaded or unloaded while running the system. Also unlike standard monolithic kernels, device drivers can be pre-empted under certain conditions. This latter feature was added to correctly handle [[hardware interrupt|hardware interrupts]], and to improve support for [[symmetric multiprocessing]].▼
▲Today Linux is a [[hybrid monolithic kernel]]. [[Device driver|Device drivers]] and kernel extensions typically run in [[ring 0]], with full access to the hardware, although some run in [[user space]]. Unlike standard [[monolithic kernels]], [[device driver|device drivers]] are easily configured as [[module|modules]], and loaded or unloaded while running the system. Also unlike standard monolithic kernels, device drivers can be pre-empted under certain conditions. This latter feature was added to correctly handle [[hardware interrupt|hardware interrupts]], and to improve support for [[symmetric multiprocessing]].
The fact that Linux is not a [[micro-kernel]] was the topic of a
famous flame war between Linus and Andy Tanenbaum, a summary
of which can be found at http://www.dina.dk/~abraham/Linus_vs_Tanenbaum.html
=== Portability ===
While Linus Torvalds didn't originally set out to make Linux a [[portability|portable]] operating system, it has evolved in that direction. Linux is now in fact one of the most widely ported operating system kernels, running on systems as diverse as the [[iPaq]] (a handheld computer) to the [[IBM S390|IBM S/390]] (a massive, hugely expensive [[mainframe]]).
However, it is important to note that Linus's efforts were also directed successfully at a different sort of portability. Portability, according to Linus, was the ability to easily compile applications from a variety of sources on his system; thus Linux originally became popular in part because it required the least effort to get everybody's favorite [[GNU General Public License|GPL'd]] and other open source applications running.
Linux currently runs on the following machine architectures:
*[[Intel]] [[80386]] and up: [[IBM PC]]s and compatibles with CPUs: [[80386]], [[80486]], and the entire [[Pentium]] series; [[AMD]] [[Athlon]], [[Duron]], [[Thunderbird]]
*[[IA64]]: PCs with Intel Itanium
*[[Compaq|Compaq's]] [[Alpha processor|Alpha]]
*[[MIPS architecture|Mips]]: [[Silicon Graphics, Inc.]] machines, ...
*[[Motorola]] 68020 and up: newer [[Amiga]]s, some [[Apple computer]]s
*[[PowerPC]]: most newer [[Apple computer]]s, ...
*[[SPARC]] and [[UltraSparc]]: [[Sun Microsystems|Sun]] workstations
*[[SuperH]]: [[Sony]] [[PlayStation 2]]
*[[IBM|IBM's]] [[IBM S390|S/390]]
=== Licensing Terms ===
Initially, Torvalds released Linux under a license which forbade any commercial exploitation. This was soon changed to the [[GNU General Public License]] (version 2 exclusively). This license allows distribution and even sale of possibly modified versions of Linux but requires that all those copies be released under the same license and be accompanied by source code.
== Resources ==
* http://www.kernel.org
* http://www.kernelnotes.org
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[[/Talk]]
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