Without an operating system, a computer would not be useful. Minimally, you need an operating
system to locate files and to start programs. The programs that you run need services
from the operating system to access devices and to interact with other programs. Operating
systems on large computers need to provide more services than those on personal computers.
Here are some typical services:
• Program loading. Every operating system provides some way of launching application
programs. The user indicates what program should be run, usually by typing in the name
of the program or by clicking on an icon. The operating system locates the program code,
loads it in memory, and starts it.
• Managing files. A storage device such as a hard disk is, electronically, simply a device
capable of storing a huge sequence of zeroes and ones. It is up to the operating system to
bring some structure to the storage layout and organize it into files, folders, and so on.
The operating system also needs to impose some amount of security and redundancy into
the file system so that a power outage does not jeopardize the contents of an entire hard
disk. Some operating systems do a better job in this regard than others.
• Virtual memory. Memory is expensive, and few computers have enough RAM to hold all
programs and their data that a user would like to run simultaneously. Most operating systems
extend the available memory by storing some data on the hard disk. The application
programs do not realize what is happening. When a program accesses a data item that is
currently not in memory, the processor senses this and notifies the operating system. The
operating system swaps the needed data from the hard disk into RAM, simultaneously
swapping out a memory block of equal size that has not been accessed for some time.
• Handling multiple users. The operating systems of large and powerful computers allow
simultaneous access by multiple users. Each user is connected to the computer through a
separate terminal. The operating system authenticates users by checking that they have a
valid account and password. It gives each user a small slice of processor time, then serves
the next user.
applications—for example, to read your e-mail in one window and run the C++ compiler
in another. The operating system is responsible for dividing processor time between the
applications you are running, so that each can make progress.
• Printing. The operating system queues up the print requests that are sent by multiple
applications. This is necessary to make sure that the printed pages do not contain a mixture
of words sent simultaneously from separate programs.
• Windows. Many operating systems present their users with a desktop made up of multiple
windows. The operating system manages the location and appearance of the window
frames; the applications are responsible for the interior.
• Fonts. To render text on the screen and the printer, the shapes of characters must be
defined. This is especially important for programs that can display multiple type styles
and sizes. Modern operating systems contain a central font repository.
• Communicating between programs. The operating system can facilitate the transfer of
information between programs. That transfer can happen through cut and paste or interprocess
communication. Cut and paste is a user-initiated data transfer in which the user
copies data from one application into a transfer buffer (often called a “clipboard”) managed
by the operating system and inserts the buffer’s contents into another application.
Interprocess communication is initiated by applications that transfer data without direct
user involvement.
• Networking. The operating system provides protocols and services for enabling applications
to reach information on other computers attached to the network.
Today, the most popular operating systems are Microsoft Windows, UNIX and its variants
(such as Linux), and the Macintosh OS.
