Linux kernel

Linux is the most popular free computer operating system kernel and forms the basis for the Linux operating system (also called GNU/Linux), which has begun to compete with proprietary operating systems such as Unix and Windows.

The Linux kernel, written mainly in C, was first developed by Finland-Swedish 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 with a famous post to the Usenet newsgroup comp.os.minix; the post can be found here (http://groups.google.com/groups?hl=en&selm=1991Aug25.205708.9541%40klaava.Helsinki.FI).

From the early days, it received help from Minix hackers, and today it has received contributions from thousands of programmers.

Technically speaking, Linux is a 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 build around the Linux kernel is commonly known as the Linux operating system, although some prefer to call the system GNU/Linux. People often confuse the kernel with the operating system, leading to many mistaken notions, e.g. the idea that Torvalds wrote/coordinates other parts of the system than the kernel.

Torvalds has continued to release new versions the kernel, consolidating contributions from other programmers and making changes of his own. 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, while releases with an odd sub-version, like the 2.5.x series, are development versions not intended for production use[?]. While Torvalds continues to release the latest development versions, the maintenance of the older "stable" versions has been delegated to others, including David Weinehall[?] (2.0), Alan Cox (2.2) and Marcelo Tosatti[?] (2.4). In addition to these "official" kernels, alternative "kernel trees" can be obtained from other sources. Distributors of complete operating systems typically maintain their own versions of the kernel, for example including drivers which have not yet been accepted into the official version.

The Linux mascot is a penguin named Tux, created by Larry Ewing[?].

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 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.

The fact that Linux is not a microkernel was the topic of a famous flame war between Linus Torvalds and Andy Tanenbaum, a summary of which can be found at http://www.dina.dk/~abraham/Linus_vs_Tanenbaum

The complete source code of various versions of the Linux kernel can be browsed at http://lxr.linux.no.

Portability

While Linus Torvalds didn't originally set out to make Linux a portable operating system, it has evolved in that direction. Linux is now in fact one of the most widely ported operating system kernels (although NetBSD has been ported to even more different systems), running on systems as diverse as the iPAQ (a handheld computer) to the IBM S/390[?] (a massive, hugely expensive mainframe). Linux is intended to run as the main operating system on IBM's new Blue Gene supercomputer architecture when it is finished.

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 GPLed and other open source applications running.

Linux currently runs on the following machine architectures:

• Acorn Archimedes, A5000[?] and RiscPC series (ARM, StrongARM, Intel XScale etc.)
• AMD64[?]: AMD's 64-bit processor technology (formerly known as x86-64)
• Axis Communications[?]' CRIS[?]
• Compaq's Alpha
• Hitachi[?] H8/300
• Hewlett Packard's PA-RISC
• IA-64: PCs with 64-bit Intel Itanium
• IBM's S/390[?]
• Intel 80386 and up: IBM PCs and compatibles with CPUs: 80386, 80486, and the entire Pentium series; AMD Athlon, Duron, Thunderbird; Cyrix series. Support for Intel 8086, 8088, 80186, 80188 and 80286 CPUs is under development (see ELKS (http://elks.sourceforge.net) project)
• MIPS: Silicon Graphics, Inc. machines, ...
• Motorola 68020 and up: newer Amigas, some Apple computers
• NEC[?] v850e[?]
• PowerPC: most newer Apple computers (all PCI-based Power Macintoshes, limited support for the older NuBus Power Macs), clones of the Power Mac marketed by Power Computing[?], UMAX[?] and Motorola, Amigas upgraded with a "Power-UP" card (such as the Blizzard or CyberStorm), both POWER and PowerPC-based IBM RS/6000 systems, several embedded PowerPC platforms
• SPARC and UltraSparc[?]: Sun workstations
• Hitachi SuperH: SEGA Dreamcast
• Sony PlayStation 2
• Microsoft's Xbox

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.

He has publicly referred to licensing Linux under the GPL as the "best thing I ever did". [1] (http://www.hotwired.co.jp/matrix/9709/5_linus)

See also

• Revolution OS - a documentary about the history of Linux with several interviews with prominent hackers, including Linus Torvalds.

Resources

• The Linux Kernel Archives (http://www.kernel.org/)
• Kernel Traffic, a summary of the high-traffic Linux kernel mailing list (http://kt.zork.net/)