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Debian GNU/Linux System Administrator's Manual (Obsolete Documentation)
Chapter 2 - Overview of a Debian GNU/Linux System

author = Oliver Elphick

topics = Linux Kernel, Devices, Filesystems, Users

2.1 The main components of a system

A Debian system comprises a computer, with devices to enable it to communicate with the outside world, temporary and permanent storage, and the software to make it all work.

Linux is available for a number of different types of computer. The original Linux kernel was written for the Intel 80386, and descendants of this processor power the majority of Linux systems at present. However, there are also versions of Linux for DEC Alpha, Motorola, Power PC, ARM, Amiga, MIPS, Sun SPARC and others.

Debian has distributions for x86, DEC Alpha, ARM, Motorola 68k, MIPS, Motorola/IBM PowerPC, Sun SPARC, Sun UltraSPARC and GNU/Hurd. (The last-named is not a machine but a new operating system from the Free Software Foundation.)

2.1.1 The hardware

The minimum needed for any computer system is a processor (or CPU), some permanent storage (whether modifiable or not), some working memory, an input device (usually a keyboard) and an output device (usually a screen, but it could be a printer or even an array of lights). The usual configuration of a PC is a box containing a CPU, a small amount of Read Only Memory (ROM) and from 4 to 256 Megabytes of Random Access Memory (RAM). These are mounted on a motherboard, which also contains components such as a clock and interfaces to various kinds of input and output (I/O) devices. The motherboard also contains a BIOS chip which contains the program which starts the computer working when it is reset or powered up.

Inside the computer box, with the motherboard there are usually one or more disk drives, either removable or permanent, and there may well be a CD/ROM drive. Special "cards" are plugged into the motherboard to manage networking, video display, sound and so on. Some motherboards may include these functions and not need extra cards.

Outside the computer box will be peripheral devices of various kinds. These will almost always include a video monitor and a keyboard. There will probably also be a mouse. Systems with a soundcard will have speakers. There may be a modem, printers, graphics pad, music keyboard, scanner, external disk drives, and so on; the list is almost endless.

All these devices are controlled by software; first by the BIOS, then by the boot loader which is run by the BIOS and finally by the operating system. In Debian's case, that operating system is the Linux kernel.

2.1.2 The Linux kernel

The Linux kernel is a program that is running all the time from the moment that a Debian system is booted. It is responsible for servicing all running programs when they make `system calls', which are requests to run part of the kernel code - for instance, reading from a device. No program can operate without calling on the kernel for some services.

The kernel consists of the base kernel itself and a number of modules, which can be loaded on demand. Once loaded, these modules become part of the kernel itself, and can be called on in the same way. However, when they are no longer required they can be unloaded, thus saving memory.

Each Debian distribution comes with a pre-built kernel and modules that handle most devices you are likely to have. However, you can rebuild the kernel to match your own particular setup, either to accommodate unusual devices or to save space by removing support for devices you do not have.

Linux kernel development is continually in progress. Kernel version numbers indicate how up-to-date the kernel is. At the time of writing (September 1998), the current stable kernel is 2.0.35 and the development kernel is 2.1.119. Development kernels have an odd number in the second part of their version numbers. When 2.1 is thought sufficiently stable, it will be renamed 2.2 and will become the new stable release.

The current stable Linux kernel should always be available as a Debian package, and there is also a package called kernel-package which you can use to make your own customised version of the kernel without conflicting with Debian's packaging system.

2.1.3 Devices

Every computer has a number of devices, ranging from its own memory to external hardware. These devices are controlled by device drivers which are either built into the kernel or loaded from kernel modules. (It is possible, but unusual, for device drivers to be user programs rather than kernel code.)

Devices are accessed through device nodes, which are stored in the /dev directory. Most operations can be done through these device nodes. For device-specific actions, ioctl calls can be made to the device driver. Retensioning a tape, for example, is done through an ioctl call.

2.1.4 Filesystems

Disk devices support filesystems, in which all the information available to the computer is stored. Filesystems are organised as directory trees.

Every Unix computer has at least one filesystem available at all times: this is the root filesystem, which is under the directory `/'. All other filesystems that may be available are mounted under the root filesystem, so that all the filesystems appear as a unified whole. Filesystems on other machines can be mounted, and even these appear as if they were under the root.

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Debian GNU/Linux System Administrator's Manual (Obsolete Documentation)

This manual is OBSOLETE and DEPRECATED since 2006, 29 Dezember 2009. Instead see http://www.de.debian.org/doc/user-manuals#quick-reference.

Ardo van Rangelrooij ardo.van.rangelrooij@tip.nl
Tapio Lehtonen Tapio.Lehtonen@IKI.FI
Oliver Elphick - Previous maintainer