multitaskingis used to describe essentially the same func-
tion as multiprogramming on larger machines.
Multithreading(thread is short for thread of execution) is
almost the same as multitasking except that the multiple
threads are different parts of the sameprogram that are
being executed near simultaneously, with the operating
system controlling the switching back and forth among
threads of the single program. All three terms—
multiprogramming,multitasking, and multithreading—
refer to the efforts of the operating system to maximize the
work done by the CPU.
Whereas multiprogramming or multitasking is pri-
marily concerned with the management of CPU time,
virtual memoryis concerned with the management of
main memory. Virtual memory makes it appear to the user
that an unlimited amount of main memory is available,
meaning that individual programs can be much larger than
the actual number of memory cells. The trick is the cre-
ative use of direct access storage devices (DASDs), with
the operating system switching portions of programs
(called pages) between main memory and DASDs. The
virtual memory concept recognizes that only one segment
of a large program is being executed at a time, while the
bulk of the program is inactive. Therefore, with virtual
memory, only a few pages of the program are kept in main
memory, with the rest relegated to a DASD. When we
combine the concepts of multiprogramming (switching
among pages of programs already in memory) with virtual
memory (requiring frequent page switches from DASDs to
memory), we begin to realize the incredible complexity of
tasks carried out by the operating system.
Finally, multiprocessingrefers to the processing, or
work, that takes place when two or more CPUs are
installed as part of the same computer system. Each CPU
works on its own job or set of jobs (often using multipro-
gramming), with all the CPUs under control of a single
operating system that keeps track of what the various
CPUs are doing. This is complexity piled on complexity!
It is easy to see that today’s computer systems would be
of much less use to us without the powerful operating
systems that exist and are continually being upgraded.
SOURCES OF OPERATING SYSTEMS For the most part,
operating systems are obtained from the manufacturer of
the hardware, although some other company might have
written the operating system. For example, when you buy a
new microcomputer from Dell or Hewlett-Packard, it like-
ly comes equipped with Windows 7, an operating system
from Microsoft. Many of the popular operating systems
areproprietary systemsthat were written expressly for a
particular computer system. Examples are PC-DOS and
MS-DOS, which are the same operating system written by
Microsoft for IBM microcomputers and IBM compatibles,
respectively; Windows Vista and Windows 7, which are
newer systems written by Microsoft for PCs; and z/OS and
z/VM, which are two alternative mainframe operating sys-
tems offered by IBM.
In contrast to these proprietary systems, the UNIX
operating system and the increasingly popular Linux oper-
ating system are open systems.^8 UNIX and Linux are not
tied to a particular computer system or hardware manufac-
turer. UNIX was originally developed by Bell Laboratories,
with subsequent versions created by the University of
California at Berkeley and a variety of hardware manufac-
turers. For example, Sun Microsystems and IBM have
developed their own versions of UNIX—Solaris for Sun
and AIX for IBM. UNIX is powerful and flexible, and it is
portable in that it will run on virtually any computer.
Linux is a cut-down version of UNIX originally
written by a young Finnish programmer, Linus Torvalds, in- Torvalds made his new operating system compact
and flexible, and he decided to share Linux freely. The
only stipulation to the free use of Linux is that if a pro-
grammer makes modifications or extensions to Linux, he
or she agrees to share them with the rest of the worldwide
Linux community. Torvalds then has the final say on
everything that goes into Linux. Although a knowledge-
able computer programmer can download Linux for free
and get it operating on his or her machine, most users
(including corporations) need a bit more help and buy a
Linux “distribution” from a vendor such as Red Hat and
Novell (with its SUSE distribution). This distribution
includes the free Linux system plus additional software,
documentation, and a way of installing the software. Linux
received a significant boost when many of the major players
in the IT field, including IBM, Hewlett-Packard, Intel, and
Dell, agreed to push the use of Linux. IBM, in fact, has
made Linux the centerpiece of its IT strategy and now
offers Linux on all of its varied computer platforms.
Many supercomputers and high-end midrange
computers run only UNIX or Linux. Many computer
professionals would like to see UNIX or Linux become the
standard operating system for all computer systems. That
appears unlikely to happen, but the use of Linux in partic-
ular will continue to spread, at least for servers and other
larger machines. Some organizations have adopted a
strategy of carrying out all new applications software
development in a Linux environment, and gradually
moving existing applications to Linux. Linux continues to
42 Part I • Information Technology
(^8) UNIX and Linux are part of the larger open source software movement,
which also includes the Firefox browser, the OpenOffice office suite, and
the Apache Web server software.