It was originally developed for the Intel 80386 processor but has since been ported to many other platforms. It is written almost entirely in C with some GNU C language extensions, along with snippets of assembly language (in the GNU Assembler's "AT&T-style" syntax).
- "I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones..." 
At the time, the GNU project had created many of the components required for a free operating system, but its own kernel, the 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, Minix hackers contributed code and ideas to the Linux kernel, and today it has received contributions from thousands of programmers.
Originally, "Linux" was only the name of the kernel. The term "kernel" properly refers to the low-level system software that provides a hardware abstraction layer, disk and filesystem control, multi-tasking, load-balancing, networking and security enforcement. A kernel is not a complete operating system (as the term is usually understood). A complete system built around the Linux kernel is commonly known as the Linux operating system, although some prefer to call the system GNU/Linux and there is some controversy on the point. People often confuse the kernel with the operating system, leading to many mistaken notions, e.g. the idea that Torvalds wrote or coordinates other parts of the system than the kernel.
- Apr 1991 - Linus Torvalds, then 21, starts working on some simple ideas for an operating system. He starts with a task switcher in 386 assembly and a terminal driver.
- 25 Aug 1991 - Linus posts to comp.os.minix: 
- "I'm doing a (free) operating system (just a hobby, won't be big and professional like gnu) for 386(486) AT clones. This has been brewing since april, and is starting to get ready. I'd like any feedback on things people like/dislike in minix, as my OS resembles it somewhat (same physical layout of the file-system (due to practical reasons) among other things).
- I've currently ported bash(1.08) and gcc(1.40), and things seem to work. This implies that I'll get something practical within a few months [...] Yes - it's free of any minix code, and it has a multi-threaded fs. It is NOT portable (uses 386 task switching etc), and it probably never will support anything other than AT-harddisks, as that's all I have :-(.
- [...] It's mostly in C, but most people wouldn't call what I write C. It uses every conceivable feature of the 386 I could find, as it was also a project to teach me about the 386. As already mentioned, it uses a MMU, for both paging (not to disk yet) and segmentation. It's the segmentation that makes it REALLY 386 dependent (every task has a 64Mb segment for code & data - max 64 tasks in 4Gb. Anybody who needs more than 64Mb/task - tough cookies). [...] Some of my "C"-files (specifically mm.c) are almost as much assembler as C. [...] Unlike minix, I also happen to LIKE interrupts, so interrupts are handled without trying to hide the reason behind them"
- September 1991 - Linux version 0.01 is released. (10,239 lines of code.)
- October 1991 - Linux version 0.02 is released. 
- December 1991 - Linux 0.11 is released. This version is the first that is self-hosted. (that is: you can compile Linux 0.11 under Linux 0.11)
- 19 January 1992 - First post to alt.os.linux newsgroup. 
- 31 March 1992 - The newsgroup comp.os.linux is created. 
- March 1992 - Linux version 0.95 is the first to be capable of running the X Window System.
- During the whole of 1993, and early 1994 - 15 development versions 0.99.*, with 0.99.11 (July 1993) introducing BogoMips into the kernel
- 14 March 1994 - Linux 1.0.0 is released. (176,250 lines of code.)
- March 1995 - Linux 1.2.0 is released (310,950 lines of code.)
- 9 May 1996 - Tux the penguin is suggested as mascot for Linux
- 9 June 1996 - Linux 2.0.0 is released. (777,956 lines of code.)
- 25 January 1999 - Linux 2.2.0 is released, very buggy at first. (1,800,847 lines of code.)
- 18 December 1999 - IBM mainframe patches for 2.2.13 published, bringing Linux into the biggest enterprises.
- 4 January 2001 - Linux 2.4.0 is released. (3,377,902 lines of code.)
- 17 December 2003 - Linux 2.6.0 is released. (5,929,913 lines of code.)
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 providers of GNU/Linux operating systems 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 126.96.36.199).
- 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. 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 are development releases, such as 1.1 or 2.5. They are for testing new features and drivers until they become sufficiently stable to be included in a stable release.
- 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 legitimate the release of a new minor revision (which would have been 2.6.9). So, 188.8.131.52 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. Bugfixes and security patches are now managed by the fourth number, whereas bigger changes are only implemented in minor revision changes (the C number).
While Linus Torvalds supervises code changes and releases to the latest kernel versions, he has delegated the maintenance of older versions to other programmers:
|2.2||Marc-Christian Petersen (formerly Alan Cox)|
|2.6||Andrew Morton / Linus Torvalds|
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
- For more information see article: Wonderful World of Linux 2.4
- 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 PAE ("Physical Address Extension")
- integrated the ALSA sound driver
- OS support:
- Improved APIC support.
- Increased the maximum number of users and groups from 65,000 to over 4 billion.
- Increased the maximum number of process ids from 32,000 to 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 preemptible 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:
- (and more changes: The Wonderful World of Linux 2.6)
Today Linux is a module-loading 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 drivers are easily configured as 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 handle hardware interrupts correctly, and to improve support for symmetric multiprocessing. Preemption also improves latency, increasing responsiveness and making Linux more suitable for real-time applications.
The complete source code of various versions of the Linux kernel can be browsed at http://lxr.linux.no .
The Linux kernel is written in the version of the C programming language supported by the GNU GCC compiler (which supports a superset of standard C), together with relatively short sections of code written in the assembly language of the target architecture (and at last count Linux supported about 20 different processor families). Because of the extensions to C supported by the GCC compiler it was (in conjunction with its associated toolset) for long the only compiler capable of correctly building a Linux kernel. More recently Intel claims to have modified its C compiler so that it also is capable of compiling the kernel.
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 behaviour is strongly discouraged. The kernels build system only supports GCC as kernel and driver compiler.
While not originally intended as a portable operating system, Linux is now one of the most widely ported operating system kernels (although NetBSD has been ported to almost as many architectures), 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 is intended to run as the main operating system on IBM's new Blue Gene supercomputer architecture when it is finished. Linus included, perhaps humorously, BogoMips into the kernel as a performance pseudo-comparison tool.
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 popular GPLed and other open source applications running.
Linux currently runs on the following machine architectures:
- Axis Communications' CRIS
- Hewlett Packard's Alpha
- Hewlett Packard's PA-RISC
- Hitachi: SuperH (SEGA Dreamcast), H8/300
- IBM's S/390 (31-bit)
- IBM's zSeries and System z9 mainframes (64-bit)
- Intel 80386 and up: IBM PCs and compatibles with CPUs:
- 80386, 80486, and their AMD, Cyrix, TI and IBM variants
- the entire Pentium series;
- AMD 5x86, K5, K6, Athlon (all 32-bit versions), Duron;
- AMD64: AMD's 64-bit processor technology (formerly known as x86-64)
- Cyrix 5x86, 6x86 (M1), 6x86MX and MediaGX (National/AMD Geode) series.
- VIA Technologies VIA C3 and later processors
- Support for Intel 8086, 8088, 80186, 80188 and 80286 CPUs is under development (see ELKS project)
- Microsoft's Xbox (Pentium III processor) (see Xbox Linux)
- SGI Visual Workstation (Pentium II/III processor(s) with SGI chipset)
- Intel IA-64
- Motorola 68020 and up:
- NEC v850e
- Renesas M32R
- PowerPC and IBM POWER:
- most newer 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)
- IBM RS/6000, iSeries and pSeries systems
- Pegasos I and II boards
- several embedded PowerPC platforms
- SPARC and UltraSparc: Sun 4-series, SPARCstation/SPARCserver, Ultra, Blade and Fire series workstations and servers, as well as clones made by Tatung and others.
A complete list of ports is at Linux Ports.
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.
Torvalds has described licensing Linux under the GPL as the "best thing I ever did." 
One general question about the application of the GPL to Linux involves whether loadable kernel modules are considered "derived works" under copyright law, and thereby fall under the terms of the GPL. Torvalds has stated his belief that modules using only a limited, "public" subset of the kernel interfaces can sometimes be non-derived works, thus allowing some binary-only drivers and other modules not obeying the GPL. Not all kernel contributors agree with this interpretation, however, and even Torvalds agrees that many kernel modules are clearly derived works, and indeed he states that kernel modules ARE derivative "by default"; ultimately, such questions can only be resolved by a court.
Linux is a registered trademark of Linus Torvalds.
The mascot is a penguin because as Torvalds put it, "Linus likes penguins. That's it." The name Tux was suggested to represent (T)orvalds (U)ni(X), and it stuck. 
- Main article: Kernel panic
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.
- Torvalds, Linus; Diamond, David (2001). Just For Fun: The Story of an Accidental Revolutionary. HarperBusiness. ISBN 0066620724 (hardcover); HarperAudio ISBN 0694525391 (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.
- Revolution OS - a documentary on the history of Linux featuring several interviews with prominent hackers, including Torvalds
- www.kernel.org - The Linux Kernel Archives, the official kernel repository
- Linux Kernel HowTo
- Kernel Traffic, a summary of the high-traffic Linux kernel mailing list
- LWN.net kernel page
- Linux Kernel Source Code Viewer
- Linux Kernel Janitor
- Linux Device Drivers, 3rd Edition
- Understanding the Linux Kernel, 2nd Edition
- Linux: The GPL And Binary Modules
- The Linux kernel in colour
The following are to be merged
- Upgrade to 2.6 kernel
- The Wonderful World of Linux 2.6; Joseph Pranevich; December 2003
- Understanding the Linux 184.108.40.206 CPU Scheduler; Josh Aas, 17th February 2005
- Linux kernel @ dmoz.org
- Linux kernel @ google
- Kernel Newbies.org
- Linux Kernel Developers' email list
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