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>3.1. Various parts of an operating system</H1
><P
>A UNIX operating system consists
of a <I
CLASS="glossterm"
>kernel</I
> and some
<I
CLASS="glossterm"
>system programs</I
>. There are also some
<I
CLASS="glossterm"
>application programs</I
> for doing work.
The kernel is the heart of the operating system.
<A
NAME="AEN114"
HREF="#FTN.AEN114"
><SPAN
CLASS="footnote"
>[1]</SPAN
></A
>
It keeps track of files on the disk, starts programs and runs them
concurrently, assigns memory and other resources to various
processes, receives packets from and sends packets to the network,
and so on. The kernel does very little by itself, but it provides
tools with which all services can be built. It also prevents anyone
from accessing the hardware directly, forcing everyone to use the
tools it provides.
<A
NAME="AEN116"
HREF="#FTN.AEN116"
><SPAN
CLASS="footnote"
>[2]</SPAN
></A
>
This way the kernel provides some protection for users from each
other. The tools provided by the kernel are used via
<I
CLASS="glossterm"
>system calls</I
>. See manual page section 2 for more
information on these. </P
><P
>The system programs use the tools provided by the kernel to
implement the various services required from an operating system.
System programs, and all other programs, run `on top of the
kernel', in what is called the <I
CLASS="glossterm"
>user mode</I
>.
The difference between system and application programs is
one of intent: applications are intended for getting useful
things done (or for playing, if it happens to be a game),
whereas system programs are needed to get the system working.
A word processor is an application; <B
CLASS="command"
>mount</B
>
is a system program. The difference is often somewhat blurry,
however, and is important only to compulsive categorizers.</P
><P
>An operating system can also contain compilers and their
corresponding libraries (GCC and the C library in particular under
Linux), although not all programming languages need be part of
the operating system. Documentation, and sometimes even games,
can also be part of it. Traditionally, the operating system has
been defined by the contents of the installation tape or disks;
with Linux it is not as clear since it is spread all over the
FTP sites of the world.</P
></DIV
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>Notes</H3
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><A
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><SPAN
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>[1]</SPAN
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><P
>In fact, it is often mistakenly considered
to be the operating system itself, but it is not.
An operating system provides many more services than a
plain kernel.</P
></TD
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><SPAN
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>[2]</SPAN
></A
></TD
><TD
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><P
>I always think of this as a form of encapsulation
which may help those of you with an object oriented programming
background to visualize it better.</P
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