Linux kernel

Tux, the Linux penguin

The Linux kernel is a free computer operating system kernel initiated by Linus Torvalds in 1991. It is the kernel of the Linux operating system (also called GNU/Linux) and the Linux distributions, which are compatible in many ways with Unix. It was developed initially 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 and assembly language.

Table of contents
1 History
2 Architecture
3 Portability
4 Licensing terms
5 References
6 Resources

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
  386(486) AT clones..."
The complete post can be found here.

At the time, the GNU project had created many of the components required for a free operating system, but its own kernel, the Hurd, was conspicuously incomplete and unavailable. The BSD operating system had not yet freed itself from legal encumberances. This left a space for the Linux kernel to fill, and dispite 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. 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 of 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, 2.4.x and the current 2.6.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), Marcelo Tosatti (2.4) and Andrew Morton (2.6). 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.

Stable kernel releases were:

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.

Version 2.4 of January 2001 added ISA Plug-and-Play, USB and PC Card support. Support for Axis Communications' ETRAX CRIS ("Code Reduced Instruction Set") processors and the InterMezzo filesystem were added later.

Version 2.6 of December 17, 2003 integrated uClinux (for microcontrollers, 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 and NEC's PC-9800, support for Intel's hyperthreading and PAE ("Physical Address Extension"), 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, a new "system filesystem" called sysfs, and User Mode Linux integration (and more changes: see Joseph Pranevich's article in external link.)

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.html

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:

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]

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.

References

Resources







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