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Add section about kernels from the HTML document.

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Robert Strandh 2016-11-24 09:10:45 +01:00
parent 6332a49b1a
commit f73d7768dc
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34
Documentation/chap-intro.tex
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@ -200,6 +200,40 @@ addresses to be resolved by the dynamic linker at program start-up
(which also complicates the code, but also slows down program start-up
because of additional work that the linker must do).
\subsection{The concept of a kernel}
The kernel of an operating system is a fairly large, monolithic
program that is started when the computer is powered on. The kernel is
not an ordinary program of the computer. It executes in a privileged
state so that it has direct access to devices and to data structures
that must be protected from direct use by user-level programs.
The very existence of a kernel is problematic because the computer
needs to be restarted whenever the kernel is updated, and then all
existing state is lost, including open files and data structures that
reside in volatile memory. Some programs, such as web browsers,
compensate somewhat for this problem by remembering the open windows
and the links that were associated with each window.
The fact that the kernel is monolithic poses a problem because when
code needs to be added to the kernel in the form of a kernel module,
such code has full access to the entire computer system. This
universal access represents a security risk, of course, but more
commonly, it can be defective and then it will fail often by crashing
the entire computer.
We have had solutions to this problem for many decades. The Multics
system, for example, did not have a kernel at all. An interrupt or a
system call was executed by the user-level process that issued the
system call or that happened to be executing when the interrupt
arrived. The code that executed then was not part of a monolithic
kernel, but existed as independent programs that could be added or
replaced without restarting the system. The system could still crash,
of course, if some essential system-wide data structure was corrupted,
but most of the time, only the user-level process that issued the
request would crash.
\section{Objectives for a Lisp operating system}
The three main objectives of a Lisp operating system correspond to