Linux kernel |
The Linux kernel is a free software Unix-like operating system Kernel (computer science) that was created by Linus Torvalds in 1991 and subsequently improved with the assistance of developers around the world.
It was originally developed for the Intel 80386 Microprocessor but has since been ported to many other platforms. It is written almost entirely in C programming language with some GNU Compiler Collection language extensions, along with snippets of assembly language (in the GNU Assembler s AT&T-style syntax).
Developed under the GNU General Public License, the source code for Linux is free software.
The kernel is best known as the core of GNU/Linux operating systems. Distributions of Software based on this kernel are called GNU/Linux distributions.
=History=
The project was launched in 1991 with a famous post to the Usenet newsgroup comp.os.minix that includes this sentence:
: I m doing a (free) operating system (just a hobby, won t be big and professional like gnu) for Intel_80386(Intel_80486) AT clones... [http://groups.google.com/groupshl=en&selm=1991Aug25.205708.9541%40klaava.Helsinki.FI]
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 Berkeley Software Distribution 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 File system control, Computer multitasking, Load balancing, Computer network and Computer 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 GNU/Linux naming controversy. 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.
== Timeline ==
::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
=Versions=
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.
==Version numbering==
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 2.6.12.3).
==Maintenance==
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 Ingo Molnar . (See the [http://lxr.linux.no/source/MAINTAINERS 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 Symmetric multiprocessing support. *Version 2.2 of January 1999 ([http://kniggit.net/wwol22.html The Wonderful World of Linux 2.2]).
*Version 2.4.0 of January 2001
*Version 2.6 of December 17, 2003 **integrated UClinux (for microcontrollers) **CPU support: with support for Hitachi (company) s H8/300 series, the NEC Corporation v850, and Motorola s embedded m68k processors, Non-Uniform Memory Access support, support for NCR Corporation s NCR Voyager architecture, support for Intel s hyperthreading and PAE ( Physical Address Extension ) **integrated the ALSA sound driver **OS support: ***Improved IO-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 preemption (computing) 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.
= Architecture =
The Linux kernel includes true multitasking, virtual memory, shared library, demand loading, shared copy-on-write executables, proper memory management, and TCP/IP networking.
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 module (Linux), 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 interrupt correctly, and to improve support for symmetric multiprocessing. 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 a famous flame war between Linus Torvalds and Andrew S. Tanenbaum on comp.os.minix in 1992. [http://www.dina.dk/~abraham/Linus_vs_Tanenbaum.html] [http://www.google.com/groupsthreadm=12595%40star.cs.vu.nl]
The complete source code of various versions of the Linux kernel can be browsed at http://lxr.linux.no .
=Programming languages=
The Linux kernel is written in the version of the C programming language supported by the GNU GNU_Compiler_Collection 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 programming language, and various shell scripting languages. Some drivers may also be written in C plus plus, Fortran, or other languages, but this behaviour is strongly discouraged. The kernels build system only supports GCC as kernel and driver compiler.
= Portability =
While not originally intended as a portability 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 GNU General Public License and other open source applications running.
Linux currently runs on the following machine architectures: *Advanced RISC Machines
A complete list of ports is at [http://www.cyut.edu.tw/~ckhung/l/linux_ports.html Linux Ports].
= 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.
Torvalds has described licensing Linux under the GPL as the best thing I ever did. [http://hotwired.goo.ne.jp/matrix/9709/5_linus.html]
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.
=Mascot=
The Linux mascot is a penguin named Tux, created by Larry Ewing.
Many variants of the Tux penguin exist, with some in 3D computer graphics.
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. [http://www.sjbaker.org/tux/]
= Kernel panic =
: 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 exception handling in kernel code, such as references to invalid memory addresses. These are typically indicative of a computer bug somewhere in the call chain leading to the panic.
= References =
= External links =
The following are to be merged *http://web.archive.org/web/19990117070510/www.forbes.com/forbes/98/0810/6203094a.htm *http://old.lwn.net/2001/features/Timeline/ *http://www.nd.edu/~ljordan/linux/tuxhistory.html *http://www.linuxjournal.com/article.phpsid=6000 *https://netfiles.uiuc.edu/rhasan/linux
== Linux 2.6 ==
== Pages with links to kernel resources ==
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