Operating Systems
Operating
Systems
The
operating system is a complex collection of many programs concerned with
keeping the hardware and software components of a computer system coordinated
and functioning. It is like a shop keeper who keeps a shop in order by
attending to customers, handling supplier deliveries, stocking the shelves,
doing the bookkeeping, and so on.
The operating system is
software; the same hardware can be used with many different operating systems
(although only one at a time.) Sometimes the operating system on a computer
becomes corrupted (perhaps because of a computer virus) and must be tediously
re-installed. Until it is up and running again, other programs will not be
available.
Starting
a Program
When
a computer is started up, the hardware will automatically load the operating
system and start it running. This process is called booting. The
reason for this odd term is that the operating system is itself involved in
getting itself running---a process that is like someone "pulling
themselves up by their bootstraps." Once the operating system is running,
it is used to start up any other program.
Here
is a (simplified) list of what happens when the user (you) starts up an
application. Assume that the operating system (OS) is already running.
1. The user asks to run an
application.
o
This
could be done by clicking on an icon, making a menu choice, or other means.
2. The OS determines the name
of the application.
3. The OS finds the section of
the hard disk where the application program and its data are stored.
4. The OS finds an unused
section of main memory that is large enough for the application.
5. The OS makes a copy of the
application and its data in that section of main memory.
o
The
software on the hard disk is unchanged; a copy of it has been made
in main memory
6. The OS sets up resources
for the application.
7. Finally, the OS starts the
application running.
As
the application runs, the OS is there in the background managing resources,
doing input and output for the application, and keeping everything else
running.
Networks
A
computer network consists of two or more computers connected
so that they can exchange data and programs. When a computer is a member of a
network, the programs it runs and the data it uses can be on the hard disk of
some other computer on the network. In business and industrial settings, most
computers are on a network. The operating system that runs on a networked
computer must manage its share of the network (along with managing all its
other responsibilities.) The operating system is able to find programs and data
that are stored on other network computers, and copy them into its own main
memory.
In
a local-area network only a few dozen computers are connected
together, usually all located within the same building. Each computer has
a network address that the other computers use to access it.
Usually the computers share a printer. There may be an especially powerful
computer called a server whose hard disk holds all the
application programs and data that the other computers are expected to need.
Each computer in a network
has a network interface card in its systems unit. This is an
input/output device that sends and receives data over cables. The network
interface cards of computers on a network are connected together with cables.
Wide-Area
Networks
Large
organizations need to connect many more computers than can be handled with a
local area network. A wide-area network can connect thousands
of computers together over great distances. The long distance connections are
made by using fiber optic lines, telephone lines, microwave communications, and
satellite communications. Each computer in the network has a network address
(as with local-area networks) to uniquely identify it.
Wide-area networks use a variety of special hardware to manage the flow of data. When two computers share data, this hardware makes it appear that the two computers are connected together directly. In reality, there may be dozens of network devices between the two computers.
All
these devices use the same method for dealing with data. Without a common
method of dealing with data, a large network would become a hopeless muddle. An
agreement about how to represent and transmit data over a network is called
a protocol. Usually large networks use a protocol called TCP/IP
(for transmission control protocol/internet protocol.)
The Internet consists
of many wide-area networks that have been connected together to form one huge
worldwide network. Even on this huge network, each computer must have a unique
network address, much like each telephone in the world has a unique telephone
number (if you include the country code and area code.)
World
Wide Web
Remember
that important idea (discussed several pages back):
Fundamental
Idea: Both
programs and data are saved in computer memory in the same way. The electronics
of computer memory (both main memory and secondary memory) make no distinction
between programs and data.
Communications equipment makes no
distinction between programs and data, either. It is all information as far as
it is concerned, and all information is transmitted the same way. The Internet
is like a worldwide package delivery service. It is concerned with moving
packages from one address to another, without concern about what is in the
packages.
The
Internet provides the hardware and the information transmission protocols for
the World-Wide Web. Data intended for the Web is transmitted over
the Internet just like any data. What makes Web data special is that it is
intended for Web browsers (such as the one you are probably looking at.) A
browser is a program that can read Web pages and display them in a nicely
formatted way.
A Web page is a package of
data that contains information on how it is to be displayed on a monitor. This
information is given using a language called Hypertext Markup Language
(HTML). If you want to see an example of what this looks like, left-click
on View in your browser, then left-click on Page Source. After you are done
viewing, click on the close button in the upper right corner (the button marked
with X
Hyperlinks
One
Web page is connected to another with a hyperlink. If you have been
reading these notes over the Web, you have been linking between Web pages by
using hyperlinks.
A
web browser (such as Netscape Navigator) usually displays a hyperlink in a distinguishing
color (usually blue.) When you click on it, the browser asks the operating
system of the computer to get a particular Web page from another computer
connected to the Internet. The Web page to get is specified with a uniform
resource locator URL. A URL specifies both the computer (among all the
Internet computers in the world) and the exact Web page on that computer.
End
of the Chapter
We
have ended this general overview of computer systems. At this point, you should
have an idea of what computer systems are all about. But you will likely be
uncertain about quite a few things. The following chapters will clarify some
topics, and others will remain mystifying until later courses. For now, you may
wish to review the following. Click on a blue subject that interests you to go
to where it was discussed.
·
Major
hardware components of a computer system
The
next chapter will discuss analog and binary signals and why
binary signals are important in computer syste
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