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The Linux kernel is a Unix-like operating system kernel. It is the namesake of the Linux, or GNU/Linux, family of operating systems. Released under the GNU General Public License (GPL) and developed by contributors worldwide, Linux is one of the most prominent examples of open source software..
Linux was created by Linus Torvalds in 1991. At the time, the GNU Project had created many of the components required for a free software operating system, but its own kernel, GNU Hurd, was incomplete and unavailable. The BSD operating system had not yet freed itself from legal encumbrances. This left a space for the Linux kernel to fill, and despite the limited functionality of the early versions it rapidly accumulated developers and users. Early on, the Minix community contributed code and ideas to the Linux kernel, and today it has received contributions from thousands of programmers.
In April 1991 - Linus Torvalds, then 21 years old, started working on some simple ideas for an operating system. He started with a task switcher in 386 assembly and a terminal driver. Then, on, 25 August 1991, Torvalds posted to comp.os.minix:
After that, many people contributed code to the project. By September 1991, Linux version 0.01 was released. It had 10,239 lines of code. In October 1991, Linux version 0.02 was released.
In December 1991, Linux 0.11 was released. This version was the first to be self-hosted - Linux 0.11 can be compiled under Linux 0.11. This was also around the time Linus adopted the GPL over his own do-not-sell-it license.
A newsgroup alt.os.linux was started, and on January 19, 1992, the first post to alt.os.linux was made.  On March 31, 1992 alt.os.linux became comp.os.linux 
The X Window System was soon ported to Linux. In March 1992, Linux version 0.95 was the first to be capable of running X. This large version number jump (from 0.1x to 0.9x) was due to a feeling that a version 1.0 with no major missing pieces was imminent. However, this proved to be somewhat overoptimisitc; during the whole of 1993, and early 1994, 15 development versions of version 0.99 appeared.
Finally, on March 14, 1994, Linux 1.0.0 was released, with 176,250 lines of code. In March 1995, Linux 1.2.0 was released (310,950 lines of code).
Linus decided, on May 9, 1996, to adopt Tux the penguin as mascot for Linux.
Version 2 of Linux, released on June 9, 1996, was a landmark. Strong development continued:
- January 25, 1999 - Linux 2.2.0 was released (1,800,847 lines of code).
- December 18, 1999 - IBM mainframe patches for 2.2.13 were published, allowing Linux to be used on enterprise-class machines.
- January 4, 2001 - Linux 2.4.0 was released (3,377,902 lines of code).
- December 17, 2003 - Linux 2.6.0 was released (5,929,913 lines of code).
In 1992, Torvalds explained how he pronounces the word Linux (IPA: /'lɪnʉks/)
An audio file of Torvalds saying "Hello, this is Linus Torvalds, and I pronounce Linux as Linux" can be found online  Note that in English, "Linux" and "Minix" are usually pronounced with a short i (IPA: /ɪ/) sound that is different from Torvalds' Finland Swedish pronunciation of these words.
Initially, Torvalds released Linux under a license which forbade any commercial exploitation. This was soon changed to the GNU General Public License (GPL). 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.
Torvalds has described licensing Linux under the GPL as the "best thing I ever did."
One point of licensing controversy is the Linux's use of firmware "binary blobs" to support some hardware devices. Richard Stallman claims that these blobs make Linux partially non-free, and that distributing Linux may even be violating the GPL (which requires "complete corresponding source code" to be available).
GPL version 3
Currently, Linux is licensed under version 2 of the GPL, and there is some controversy over how easily it could be changed to use later GPL versions such as the upcoming version 3 (and whether this is desirable). Torvalds himself indicated in version 2.4.0 that his own code is only under version 2. However, the terms of the GPL state that if no version is specified, then any version may be used, and Alan Cox pointed out that very few other Linux contributors have specified a particular version of the GPL.
Loadable Kernel Modules licensing
It is debated whether Loadable Kernel Modules (LKMs) should be considered derivative works under copyright law, and thereby fall under the terms of the GPL. Torvalds has stated his belief that LKMs using only a limited, "public" subset of the kernel interfaces can sometimes be non-derived works, thus allowing some binary-only drivers and other LKMs not obeying the GPL. Not all Linux contributors agree with this interpretation, however, and even Torvalds agrees that many LKMs are clearly derived works, and indeed he writes that "kernel modules ARE derivative 'by default'". On the other hand Torvalds has also said that "one gray area in particular is something like a driver that was originally written for another operating system (ie. clearly not a derived work of Linux in origin). [...] THAT is a gray area, and _that_ is the area where I personally believe that some modules may be considered to not be derived works simply because they weren't designed for Linux and don't depend on any special Linux behaviour." Especially proprietary graphics drivers are heavily discussed. Ultimately, such questions can only be resolved by a court.
Linux is a registered trademark of Linus Torvalds in the United States and some other countries. This is the result of an incident in which William Della Croce, Jr., who was not involved in the Linux project, trademarked the name and subsequently demanded royalties for its use. Several Linux backers retained legal counsel and filed suit against Della Croce, who agreed in 1998 to assign the trademark to Torvalds.
- For more details on this topic, see SCO-Linux controversies.
In March 2003, the SCO Group (SCO) filed a lawsuit against IBM claiming that IBM had contributed some portions of SCO's copyrighted Unix source code to Linux in violation of IBM's license to use that Unix source code. Additionally, SCO sent letters to a number of companies warning that their use of Linux without a license from SCO may be a violation of copyright law, and claimed in the press that they would be suing individual Linux users. This controversy has generated lawsuits by SCO against Novell, DaimlerChrysler (partially dismissed in July, 2004), and AutoZone, and retaliatory lawsuits by Red Hat and others against SCO.
Linux supports true preemptive multitasking (both in user mode and kernel mode), virtual memory, shared libraries, demand loading, shared copy-on-write executables, memory management, the Internet protocol suite, and threading.
Linux is a monolithic kernel. Device drivers and kernel extensions run in kernel space (ring 0), with full access to the hardware, although some exceptions run in user space. The GNU/Linux graphics subsystem (the X Window System) is not part of the kernel, is optional, and runs in user space, in contrast with Microsoft Windows.
Kernel mode preemption means device drivers can be preempted under certain conditions. This latter feature was added to handle hardware interrupts correctly, and to improve support for symmetric multiprocessing (SMP). Preemption also improves latency, increasing responsiveness and making Linux more suitable for real-time applications.
The fact that Linux is not a microkernel was the topic of the Tanenbaum-Torvalds debate which was started in 1992 by Andrew S. Tanenbaum with Linus Torvalds regarding Linux and kernel architecture in general on the Usenet discussion group comp.os.minix. Tanenbaum argues that microkernels are superior to monolithic kernels and that, for this reason, Linux is obsolete. This subject was revisited in 2006. 
Unlike traditional monolithic kernels, device drivers are easily configured as modules, and loaded or unloaded while running the system.
In Linux, a "panic" is an unrecoverable system error detected by the kernel as opposed to similar errors detected by user space code. It is possible for kernel code to indicate such a condition by calling the
panic function located in the header file sys/system.h. However, most panics are the result of unhandled processor exceptions in kernel code, such as references to invalid memory addresses. These are typically indicative of a bug somewhere in the call chain leading to the panic. They can also indicate a failure of hardware, such as a failed RAM cell or errors in arithmetic functions in the processor caused by a processor bug, overheating/damaged processor, or a soft error.
Linux is written in that version of the C programming language which is supported by GCC (which has introduced a number of extensions and changes to standard C), together with a number of short sections of code written in the assembly language (in GCC's "AT&T-style" syntax) of the target architecture. Because of the extensions to C it supports, GCC was for a long time the only compiler capable of correctly building Linux. Recently, Intel claims to have modified its C compiler so that it is also capable of correctly compiling it.
Many other languages are used in some way, primarily in connection with the kernel build process (the methods whereby the bootable image is created from the sources). These include Perl, Python, and various shell scripting languages. Some drivers may also be written in C++, Fortran, or other languages, but this is strongly discouraged. Linux's build system only officially supports GCC as a kernel and driver compiler.
While not originally designed to be portable, Linux is now one of the most widely ported operating system kernels, running on a diverse range of systems from the iPAQ (a handheld computer) to the IBM System z9 (a massive mainframe server that can run hundreds or even thousands of concurrent Linux instances). Linux runs as the main operating system on IBM's Blue Gene supercomputers. As of June 2006, Linux is the OS on 75% of systems on the Top 500 supercomputers list, including the top two on the list. Torvalds included, perhaps humorously, BogoMips into the kernel as a performance pseudo-comparison tool.
It is important to note that Torvalds' efforts were also directed successfully at a different sort of portability. Portability, according to Torvalds, 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 popular free software and other open source applications running.
Linux supports the following machine architectures:
- Argonaut RISC Core (ARC) from ARC International
- ARM architecture:
- Acorn Archimedes and Risc PC series
- DEC StrongARM
- Marvel (formerly Intel) XScale
- Sharp Zaurus
- Palm, Inc.'s Tungsten Handheld
- Gamepark Holdings' GP2X
- Nokia 770 Internet Tablet
- Nintendo DS via DSlinux
- Atmel AVR32
- Axis Communications' ETRAX CRIS
- Fujitsu FR-V
- Alpha architecture:
- DEC Alpha
- Samsung Alpha CPU.
- Hewlett-Packard's PA-RISC family
- H8 architecture from Renesas Technology, formerly Hitachi.
- Intel IA-64 Itanium, Itanium II.
- IBM's S/390 (31-bit)
- IBM's zSeries and System z9 mainframes (64-bit)
- x86 architecture:
- Intel 80386, 80486, and their AMD, Cyrix, Texas Instruments and IBM variants
- The entire Pentium series
- AMD 5x86, K5, K6, Athlon (all 32-bit versions), Duron, Sempron
- x86-64: AMD's 64-bit processor architecture now known as AMD64 or EM64T (Intel); supported by the Athlon 64, Opteron and Intel Core 2 processors, among others
- Cyrix 5x86, 6x86 (M1), 6x86MX and MediaGX (National/AMD Geode) series
- VIA Technologies Eden (Samuel II), VIA C3, and VIA C7 processors
- Microsoft's Xbox (Pentium III processor), through the Xbox Linux project
- SGI Visual Workstation (Pentium II/III processor(s) with SGI chipset)
- Sun Microsystem x86 Workstation (80386 and 80486).
- Support for 8086, 8088, 80186, 80188 and 80286 CPUs is under development (the ELKS fork)
- M32R from Mitsubishi.
- MIPS architecture:
- Cobalt Qube, Cobalt RaQ
- Godson (MIPS-like), Godson II, and Godson IIE from BLX IC Design Ltd (China).
- Some PlayStation 2 models, through the PS2 Linux project
- Freescale (formerly Motorola) 68K architecture (68020, 68030, 68040, 68060):
- Some Amigas: A1200, A2500, A3000, A4000
- Apple Macintosh II, LC, Quadra, Centris and early Performa series
- Sun Microsystems 3-series workstations (experimental, uses Sun-3 MMU) 
- NEC v850e
- OpenRISC open core processor series:
- Beyond Semiconductor OR1200
- Beyond Semiconductor OR1210
- Power Architecture:
- IBM Servers.
- PowerPC architecture:
- IBM's CELL
- Most pre-Intel Apple computers (all PCI-based Power Macintoshes, limited support for the older NuBus Power Macs)
- Clones of the PCI Power Mac marketed by Power Computing, UMAX and Motorola
- Amigas upgraded with a "Power-UP" card (such as the Blizzard or CyberStorm)
- AmigaOne motherboard from Eyetech Group Ltd (UK)
- Samantha from Soft3 (Italy).
- Amy'05 PowerPC motherboard from Troika.
- IBM RS/6000, iSeries and pSeries systems
- Pegasos I and II boards from Genesi.
- Nintendo GameCube, through Gamecube Linux
- Project BlackDog from Realm Systems, Inc.
- Sony Playstation 3
- V-Dragon CPU from Culturecom.
- Virtex II Pro Field Programmable Array (FPGA) from Xilinx with PowerPC cores.
- Sun-4 series
- SPARCstation/SPARCserver
- Sun Ultra series
- Sun Blade
- Sun Fire
- Clones made by the Tatung Company and others
- Sega Dreamcast
Supported binary formats
Until version 1.2, the a.out binary format was used. Now the default format is ELF, or Executable and Linkable Format. Linux supports many other binary formats.
Further developing his own code and integrating changes made by other programmers, Linus Torvalds keeps releasing new versions of the Linux kernel. These are called "vanilla" kernels, meaning they have not been modified by anyone. Many Linux operating system vendors modify the kernels of their product, mainly in order to add support for drivers or features which have not officially been released as stable, while some distributions, such as Slackware, rely on vanilla kernels.
The version number of the Linux kernel currently consists of four numbers, following a recent change in the long-standing policy of a three-number versioning scheme. For illustration, let it be assumed that the version number is composed thus: A.B.C[.D] (e.g. 2.2.1, 2.4.13 or 220.127.116.11).
- The A number denotes the kernel version. It is changed least frequently, and only when major changes in the code and the concept of the kernel occur. It has been changed twice in the history of the kernel: In 1994 (version 1.0) and in 1996 (version 2.0).
- The B number denotes the major revision of the kernel.
- Prior to the Linux 2.6.x series, even numbers indicate a stable release, i.e. one that is deemed fit for production use, such as 1.2, 2.4 or 2.6. Odd numbers have historically been development releases, such as 1.1 or 2.5. They were for testing new features and drivers until they became sufficiently stable to be included in a stable release.
- Starting with the Linux 2.6.x series, there is no significance to even or odd numbers, with new feature development going on in the same kernel series. Linus Torvalds has stated that this will be the model for the foreseeable future.
- The C number indicates the minor revision of the kernel. In the old three-number versioning scheme, this was changed when security patches, bugfixes, new features or drivers were implemented in the kernel. With the new policy, however, it is only changed when new drivers or features are introduced; minor fixes are handled by the D number.
- A D number first occurred when a grave error, which required immediate fixing, was encountered in 2.6.8's NFS code. However, there were not enough other changes to legitimize the release of a new minor revision (which would have been 2.6.9). So, 18.104.22.168 was released, with the only change being the fix of that error. With 2.6.11, this was adopted as the new official versioning policy. Bug-fixes and security patches are now managed by the fourth number, whereas bigger changes are only implemented in minor revision changes (the C number).
Also, sometimes after the version there will be some more letters such as 'rc1' or 'mm2'. The 'rc' refers to release candidate and indicates a non-official release. Other letters are usually (but not always) the initials of a person. This indicates a development branch of the kernel by that person. e.g. ck stands for Con Kolivas, ac stands for Alan Cox, whereas mm stands for Andrew Morton.
The development model for Linux 2.6 was a significant change from the development model for Linux 2.5. Previously there was a stable branch (2.4) where only relatively minor and safe changes were merged, and an unstable branch (2.5), where bigger changes and cleanups were allowed. This meant that users would always have a well-tested 2.4 version with the latest security and bug fixes to use, though they would have to wait for the features which went into the 2.5 branch. The 2.5 branch was then eventually declared stable and renamed to 2.6. But instead of opening an unstable 2.7 branch, the kernel developers elected to continue putting major changes into the 2.6 "stable" branch. This had the desirable effect of not having to maintain an old stable branch, making new features quickly available, and getting more testing of the latest code.
However, the new 2.6 development model also meant that there was no stable branch for people just wanting security and bug fixes, and not needing the latest features. Fixes were only put into the latest version, so if a user wanted a version with all known bugs fixed they would also get all the latest features, which had not been well tested, and risked breaking things which had previously worked. A partial fix for this was the previously mentioned fourth version number digit (y in 2.6.x.y), which are series of point releases created by the stable team (Greg Kroah-Hartman, Chris Wright, maybe others). The stable team only released updates for the most recent kernel however, so this did not solve the problem of the missing stable kernel series. Linux distribution vendors, such as Red Hat and Debian, maintain the kernels which ship with their releases, so a solution for some people is to just follow a vendor kernel.
As a response to the lack of a stable kernel tree where people could coordinate the collection of bugfixes, in December of 2005 Adrian Bunk announced that he would keep releasing 2.6.16.y kernels when the stable team moved on to 2.6.17 . He also plans to include driver updates, making the maintenance of the 2.6.16 series very similar to the old rules for maintenance of a stable series such as 2.4 .
While Linus Torvalds supervises code changes and releases to the latest kernel versions, he has delegated the maintenance of older versions to other programmers:
Other Linux kernel programmers include Robert Love and Ingo Molnar. (See the Linux MAINTAINERS file).
Stable version history
- Version 1.0 of March 1994 supported only single-processor i386 machines.
- Version 1.2 of March 1995 added support for Alpha, Sparc and MIPS.
- Version 2.0 of June 1996 added support for more processors and included SMP support.
- Version 2.2 of January 1999 (The Wonderful World of Linux 2.2).
- Version 2.4.0 of January 2001
- CPU support: Hewlett-Packard's PA-RISC processor, Axis Communications' ETRAX CRIS ("Code Reduced Instruction Set") processors
- added ISA Plug-and-Play
- added USB and PC Card support
- 2.4.6: added Bluetooth support
- Filesystem and data storage
- added Logical Volume Manager (LVM) version 1
- support for RAID devices
- 2.4.15: Support for InterMezzo filesystem were added.
- Version 2.6 - current (December 17, 2003 to the present)
- integrated uClinux (for microcontrollers) 
- CPU support: with support for Hitachi's H8/300 series, the NEC v850, and Motorola's embedded m68k processors, NUMA support, support for NCR's Voyager architecture, support for Intel's hyperthreading and Physical Address Extension (PAE)
- integrated the ALSA sound driver
- OS support:
- Improved APIC support.
- Increased the maximum number of users and groups each from 65,536 to over 4 billion.
- Increased the maximum number of process ids from 32,768 to over 1 billion.
- Increased the maximum number of device types (major device) from 255 to 4095 and the maximum number of devices of each type (minor device) from 255 to more than a million.
- Improved 64-bit support and filesystems of up to 16 terabytes on common hardware.
- Improvements to the "overall responsiveness" for interactive processes (the kernel became fully pre-emptible and the I/O scheduler was rewritten).
- Support for futexes, a rewrite of threading infrastructure to allow the Native POSIX Thread Library (NPTL) to be used.
- An improved module loader.
- User-mode Linux integration.
- 2.6.11 Infiniband support
- Storage Support:
- LVM version 2
- support for SGI's XFS filesystem.
- A new "system filesystem" called sysfs, destined to relieve procfs of its system related information.
- 2.6.12 (17 June 2005) iSCSI support
- 2.6.13 inotify support
- 2.6.14 9P support
- 2.6.14 FUSE support
- 2.6.17 Online reshaping of software raid5/6
- Torvalds, Linus; Diamond, David (2001). Just for Fun: The Story of an Accidental Revolutionary. HarperBusiness. ISBN 0-06-662072-4 (hardcover); HarperAudio ISBN 0-694-52539-1. (audio tape, abridged ed., read by David Diamond) - on the beginnings of the Linux kernel
- , Nikolai Bezroukov. Portraits of Open Source Pioneers. Ch 4: A benevolent dictator (Softpanorama e-book).
- LinkSys and binary modules. LWN.net Weekly Edition (October 16, 2003).
- FreeBSD/Linux kernel source code cross-reference. Browsable Linux (and other operating system) kernel source cross-reference.
- LXR: The Linux Cross-Reference project. Browsable Linux kernel source.
- KernelHQ - a browsable kernel source tree - with all versions present, and with browsable diffs
- ^ September 25, 2006 e-mail by Linus Torvalds: Linux is open source, not free software.
- ^ What would you like to see most in minix?.
- ^ Free minix-like kernel sources for 386-AT.
- ^ Troubles with Partitions.
- ^ It's here!.
- ^ http://hotwired.goo.ne.jp/matrix/9709/5_linus.html
- ^ http://www.gnu.org/philosophy/linux-gnu-freedom.html
- ^ http://lwn.net/Articles/169797/
- ^ http://www.uwsg.iu.edu/hypermail/linux/kernel/0009.1/0096.html
- ^ http://lwn.net/Articles/169831/
- ^ http://www.ussg.iu.edu/hypermail/linux/kernel/0312.0/0670.html
- ^ O'Reilly (1999). The Tanenbaum-Torvalds Debate. O'Reilly. Retrieved on November 22, 2006.
- ^ Tanenbaum, A. (1992). LINUX is obsolete. Google Groups. Retrieved on May 10, 2006.
- ^ "diegocgteleline.es" & "ScuttleMonkey" (2006). Torvalds on the Microkernel Debate. Slashdot. Retrieved on September 28, 2006.
- ^ Tanenbaum, A. (2006). Tanenbaum-Torvalds Debate: Part II. Tanenbaum, A.. Retrieved on November 22, 2006.
- ^ http://www.pyrillion.org/index.html?showframe=linuxkernelpatch.html
- ^ http://palmtelinux.sf.net
- ^ ELKS project
- ^ Wonderful World of Linux 2.4
- ^ The Wonderful World of Linux 2.6
- menuconfig — a tool for configuring Linux prior to building it from source
- Revolution OS — a documentary on the history of Linux featuring several interviews with prominent hackers, including Torvalds
- kernel.org - The Linux Kernel Archives, the official kernel repository
- Linux Kernel HowTo
- Linux kernel install mini-howto
- Kernel Traffic, a summary of the high-traffic Linux kernel mailing list up to 10-Nov-2005
- LWN.net kernel page
- Linux Kernel Janitor
- Linux Device Drivers, 3rd Edition
- Understanding the Linux Kernel, 3rd Edition (Book)
- Linux: The GPL And Binary Modules
The following are to be merged
- Motorola/Freescale 68000
- Upgrade to 2.6 kernel
- The Wonderful World of Linux 2.6; Joseph Pranevich; December 2003
- Understanding the Linux 22.214.171.124 CPU Scheduler; Josh Aas, 17 February 2005