Modern Operating Systems by Herbert Bos ...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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A modern computer consists of one or more processors, some main memory,
disks, printers, a keyboard, a mouse, a display, network interfaces, and various
other input/output devices. All in all, a complex system.oo If every application pro-
grammer had to understand how all these things work in detail, no code would ever
get written. Furthermore, managing all these components and using them optimally
is an exceedingly challenging job. For this reason, computers are equipped with a
layer of software called the
operating system
, whose job is to provide user pro-
grams with a better, simpler, cleaner, model of the computer and to handle manag-
ing all the resources just mentioned. Operating systems are the subject of this
Most readers will have had some experience with an operating system such as
Windows, Linux, FreeBSD, or OS X, but appearances can be deceiving. The pro-
gram that users interact with, usually called the
when it is text based and the
Graphical User Interface
)—which is pronounced ‘‘gooey’’—when it uses
icons, is actually not part of the operating system, although it uses the operating
system to get its work done.
A simple overview of the main components under discussion here is given in
Fig. 1-1.
Here we see the hardware at the bottom. The hardware consists of chips,
boards, disks, a keyboard, a monitor, and similar physical objects.
On top of the
hardware is the software. Most computers have two modes of operation: kernel
mode and user mode. The operating system, the most fundamental piece of soft-
ware, runs in
kernel mode
(also called
supervisor mode
). In this mode it has

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complete access to all the hardware and can execute any instruction the machine is
capable of executing. The rest of the software runs in
user mode
, in which only a
subset of the machine instructions is available. In particular, those instructions that
affect control of the machine or do
/Output" are forbidden to user-mode
programs. We will come back to the difference between kernel mode and user
mode repeatedly throughout this book. It plays a crucial role in how operating sys-
tems work.
User mode
Kernel mode
Operating system
User interface program
Figure 1-1.
Where the operating system fits in.
The user interface program, shell or GUI, is the lowest level of user-mode soft-
ware, and allows the user to start other programs, such as a Web browser, email
reader, or music player. These programs, too, make heavy use of the operating sys-
The placement of the operating system is shown in Fig. 1-1. It runs on the
bare hardware and provides the base for all the other software.
An important distinction between the operating system and normal (user-
mode) software is that if a user does not like a particular email reader, he† is free to
get a different one or write his own if he so chooses; he is not free to write his own
clock interrupt handler, which is part of the operating system and is protected by
hardware against attempts by users to modify it.
This distinction, however, is sometimes blurred in embedded systems (which
may not have kernel mode) or interpreted systems (such as Java-based systems that
use interpretation, not hardware, to separate the components).
Also, in many systems there are programs that run in user mode but help the
operating system or perform privileged functions. For example, there is often a
program that allows users to change their passwords. It is not part of the operating
system and does not run in kernel mode, but it clearly carries out a sensitive func-
tion and has to be protected in a special way.
In some systems, this idea is carried
to an extreme, and pieces of what is traditionally considered to be the operating
† ‘‘He’’ should be read as ‘‘he or she’’ throughout the book.

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