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1 — Operating System
By Rachel and Robert Sartin
What is an Operating System?
Hardware Management, Part 1
Process Management
The UNIX Operating System
The History of UNIX
The Early Days
Berkeley Software Distributions
UNIX and Standards
UNIX for Mainframes and Workstations
UNIX for Intel Platforms
Source Versions of "UNIX"
Making Changes to UNIX
Introduction to the UNIX Philosophy
Simple, Orthogonal Commands
Commands Connected Through Pipes
A (Mostly) Common Option Interface Style
No File Types
Summary
1 — Operating System
By Rachel and Robert Sartin
What is an Operating System?
An operating system is an important part of a computer system. You can view a
computer system as being built from three general components: the hardware, the
operating system, and the applications. (See Figure 1.1.) The hardware includes
pieces such as a central processing unit (CPU), a keyboard, a hard drive, and a
printer. You can think of these as the parts you are able to touch physically.
Applications are why you use computers; they use the rest of the system to
perform the desired task (for example, play a game, edit a memo, send electronic
mail). The operating system is the component that on one side manages and
controls the hardware and on the other manages the applications.
Figure 1.1. Computer system components.
When you purchase a computer system, you must have at least hardware and an
operating system. The hardware you purchase is able to use (or run) one or more
different operating systems. You can purchase a bundled computer package, which
includes the hardware, the operating system, and possibly one or more
applications. The operating system is necessary in order to manage the hardware
and the applications.
When you turn on your computer, the operating system performs a series of tasks,
presented in chronological order in the next few sections.
Hardware Management, Part 1
One of the first things you do, after successfully plugging together a plethora
of cables and components, is turn on your computer. The operating system takes
care of all the starting functions that must occur to get your computer to a
usable state. Various pieces of hardware need to be initialized. After the
start-up procedure is complete, the operating system awaits further
instructions. If you shut down the computer, the operating system also has a
procedure that makes sure all the hardware is shut down correctly. Before
turning your computer off again, you might want to do something useful, which
means that one or more applications are executed. Most boot ROMs do some
hardware initialization but not much. Initialization of I/O devices is part of
the UNIX kernel.
Process Management
After the operating system completes hardware initialization, you can execute an
application. This executing application is called a process. (See Chapter 18,
"What Is a Process?") It is the operating system's job to manage execution of
the application. When you execute a program, the operating system creates a new
process. Many processes can exist simultaneously, but only one process can
actually be executing on a CPU at one time. The operating system switches
between your processes so quickly that it can appear that the processes are
executing simultaneously. This concept is referred to as time-sharing or
multitasking.
When you exit your program (or it finishes executing), the process terminates,
and the operating system manages the termination by reclaiming any resources
that were being used.
Most applications perform some tasks between the time that the process is
created and the time that it terminates. To perform these tasks, the program
makes requests to the operating system and the operating system responds to the
requests and allocates necessary resources to the program. When an executing
process needs to use some hardware, the operating system provides access for the
process.
Hardware Management, Part 2
To perform its task, a process may need to access hardware resources. The
process may need to read or write to a file, send data to a network card (to
communicate with another computer), or send data to a printer. The operating
system provides such services for the process. This is referred to as resource
allocation. A piece of hardware is a resource, and the operating system
allocates available resources to the different processes that are running.
See Table 1.1 for a summary of different actions and what the operating system
(OS) does to manage them.
Table 1.1. Operating system functions.
Action
OS Does This
You turn on the computerHardware management
You execute an applicationProcess management
Application reads a tapeHardware management
Application waits for dataProcess management
Process waits while other process runsProcess management
Process displays data on screenHardware management
Process writes data to tapeHardware management
You quit, the process terminatesProcess management
You turn off the computerHardware management
From the time you turn on your computer until you turn it off, the operating
system is coordinating the operations. As hardware is initialized, accessed, or
shut down, the operating system manages these resources. As applications
execute, request, and receive resources, or terminate, the operating system
takes care of these actions. Without an operating system, no application can run
and your computer is just an expensive paperweight.
The UNIX Operating System
The previous section looked at an operating system in general. This section
looks at a specific operating system: UNIX. UNIX is an increasingly popular
operating system. Traditionally used on minicomputers and workstations in the
academic community, UNIX is now available on personal computers, and the
business community has started to choose UNIX for its openness. Previous PC and
mainframe users are now looking to UNIX as their operating system solution. This
section looks at how UNIX fits into the operating system model.
UNIX, like other operating systems, is a layer between the hardware and the
applications that run on the computer. It has functions that manage the hardware
and functions that manage executing applications. So what's the difference
between UNIX and any other operating system? Basically two things: internal
implementation and the interface that is seen and used by users. For the most
part this book ignores the internal implementation. If you wish to know these
details, many texts exist that cover them. The interface is what this book
describes in detail. The majority of UNIX users need to be familiar with the
interface and need not understand the internal workings of UNIX.
The UNIX system is actually more than strictly an operating system. UNIX
includes the traditional operating system components. In addition, a standard
UNIX system includes a set of libraries and a set of applications. Figure 1.2
shows the components and layers of UNIX. Sitting above the hardware are two
components: the file system and process control. Next is the set of libraries.
On top are the applications. The user has access to the libraries and to the
applications. These two components are what many users think of as UNIX, because
together they constitute the UNIX interface.
Figure 1.2. The layers of UNIX.
The part of UNIX that manages the hardware and the executing processes is called
the kernel. In managing all hardware devices, the UNIX system views each device
as a file (called a device file). This allows the same simple method of reading
and writing files to be used to access each hardware device. The file system
(explained in more detail in Chapter 3, "The UNIX File System: Go Climb a Tree")
manages read and write access to user data and to devices, such as printers,
attached to the system. It implements security controls to protect the safety
and privacy of information. In executing processes (see Chapter 18), the UNIX
system allocates resources (including use of the CPU) and mediates accesses to
the hardware.
One important advantage that results from the UNIX standard interface is
application portability. Application portability is the ability of a single
application to be executed on various types of computer hardware without being
modified. This can be achieved if the application uses the UNIX interface to
manage its hardware needs. UNIX's layered design insulates the application from
the different types of hardware. This allows the software developer to support
the single application on multiple hardware types with minimal effort. The
application writer has lower development costs and a larger potential customer
base. Users not only have more applications available, but can rely on being
able to use the same applications on different computer hardware.
UNIX goes beyond the traditional operating system by providing a standard set of
libraries and applications that developers and users can use. This standard
interface allows application portability and facilitates user familiarity with
the interface.
The History of UNIX
How did a system such as UNIX ever come to exist? UNIX has a rather unusual
history that has greatly affected its current form.
The Early Days
In the mid-1960s, AT&T Bell Laboratories (among others) was participating in an
effort to develop a new operating system called Multics. Multics was intended to
supply large-scale computing services as a utility, much like electrical power.
Many people who worked on the Bell Labs contributions to Multics later worked on
UNIX.
In 1969, Bell Labs pulled out of the Multics effort, and the members of the
Computing Science Research center were left with no computing environment. Ken
Thompson, Dennis Ritchie, and others developed and simulated an initial design
for a file system that later evolved into the UNIX file system. An early version
of the system was developed to take advantage of a PDP-7 computer that was
available to the group.
An early project that helped lead to the success of UNIX was its deployment to
do text processing for the patent department at AT&T. This project moved UNIX to
the PDP-11 and resulted in a system known for its small size. Shortly afterward,
the now famous C programming language was developed on and for UNIX, and the
UNIX operating system itself was rewritten into C. This then radical
implementation decision is one of the factors that enabled UNIX to become the
open system it is today.
AT&T was not allowed to market computer systems, so it had no way to sell this
creative work from Bell Labs. Nonetheless, the popularity of UNIX grew through
internal use at AT&T and licensing to universities for educational use. By 1977
commercial licenses for UNIX were being granted, and the first UNIX vendor,
Interactive Systems Corporation, began selling UNIX systems for office
automation.
Later versions developed at AT&T (or its successor, Unix System Laboratories,
now owned by Novell) included System III and several releases of System V. The
two most recent releases of System V, Release 3 (SVR3.2) and Release 4 (SVR4;
the most recent version of SVR4 is SVR4.2) remain popular for computers ranging
from PCs to mainframes.
All versions of UNIX based on the AT&T work require a license from the current
owner, UNIX System Laboratories.
Berkeley Software Distributions
In 1978 the research group turned over distribution of UNIX to the UNIX Support
Group (USG), which had distributed an internal version called the Programmer's
Workbench. In 1982 USG introduced System III, which incorporated ideas from
several different internal versions of and modifications to UNIX, developed by
various groups. In 1983 USG released the original UNIX System V, and thanks to
the divestiture of AT&T, was able to market it aggressively. A series of
follow-on releases continued to introduce new features from other versions of
UNIX, including the internal versions from the research group and the Berkeley
Software Distribution.
While AT&T (through the research group and USG) developed UNIX, the universities
that had acquired educational licenses were far from inactive. Most notably, the
Computer Science Research Group at the University of California at Berkeley
(UCB) developed a series of releases known as the Berkeley Software
Distribution, or BSD. The original PDP-11 modifications were called 1BSD and
2BSD. Support for the Digital Equipment Corporation VAX computers was introduced
in 3BSD. VAX development continued with 4.0BSD, 4.1BSD, 4.2BSD, and 4.3BSD, all
of which (especially 4.2 and 4.3) had many features (and much source code)
adopted into commercial products. Various later releases from UCB have attempted
to create a publicly redistributable version of UNIX (prior releases had source
code available only to source licensees). Notably, the "Second Networking
Release" (Net2) was intended to make available all the parts of the Berkeley
Software Distribution that were not subject to license restrictions. UNIX System
Laboratories (USL) brought a lawsuit against the University and a company called
Berkeley Software Design, Incorporated (BSDI). USL claimed license infringements
by the BSD releases and BSDI's BSD/386 product, which was based in part on the
BSD code. Recently the lawsuit was settled; the result is that BSDI is shipping
BSD/386, and a new 4.4-Lite release of BSD, which requires no license from USL,
will be available from UCB.
UNIX and Standards
Because of the multiple versions of UNIX and frequent cross-pollination between
variants, many features have diverged in the different versions of UNIX. With
the increasing popularity of UNIX in the commercial and government sector came
the desire to standardize the features of UNIX so that a user or developer using
UNIX could depend on those features.
The Institute of Electrical and Electronic Engineers created a series of
standards committees to create standards for "An Industry-Recognized Operating
Systems Interface Standard based on the UNIX Operating System." The results of
two of the committees are important for the general user and developer. The
POSIX.1 committee standardizes the C library interface used to write programs
for UNIX. (See Chapter 17, "C Language.") The POSIX.2 committee standardizes the
commands that are available for the general user. (See especially Chapter 4,
"Listing Files," Chapter 5, "Popular Tools," Chapter 6, "Popular File Tools,"
Chapter 7, " Editing Text Files," Chapter 10, "What Is a Shell?" Chapter 11,
"Bourne Shell," Chapter 12, "Korn Shell," Chapter 13, "C Shell," Chapter 14,
"Which Shell Is Right for You? Shell Comparison," and Chapter 15, "Awk, Awk.")
In Europe, the X/Open Consortium brings together various UNIX-related standards,
including the current attempt at a Common Open System Environment (COSE)
specification. X/Open publishes a series of specifications called the X/Open
Portability Guide, currently at Version 4. XPG4 is a popular specification in
Europe, and many companies in the United States supply versions of UNIX that
meet XPG.
The United States government has specified a series of standards based on XPG
and POSIX. Currently FIPS 151-2 specifies the open systems requirements for
federal purchases.
UNIX for Mainframes and Workstations
Many mainframe and workstation vendors make a version of UNIX for their
machines. The best way to get information on these is directly from the
manufacturer.
UNIX for Intel Platforms
Thanks to the great popularity of personal computers, there are a great number
of UNIX versions available for Intel platforms. Choosing from the versions and
trying to find software for the version you have can be a tricky business
because the UNIX industry has not settled on a complete binary standard for the
Intel platform. There are two basic categories of UNIX systems on Intel
hardware, the SVR4-based systems and the older, more established SVR3.2 systems.
SVR4 vendors include NCR, IBM, Sequent, SunSoft (which sells Solaris for Intel),
and Novell (which sells UnixWare). The Santa Cruz Operation (SCO) is the main
vendor in the SVR3.2 camp. Vendors in the first camp are working toward cleaning
up the standards to gain full "shrink-wrap portability" between their versions
of UNIX. The goal is that this will make UNIX-on-Intel applications available,
shrink-wrapped for any version of UNIX, just as you can now buy applications for
MS-DOS or Microsoft Windows. SCO UNIX currently has a much larger base of
available applications and is working to achieve binary compatibility with
UnixWare.
Source Versions of "UNIX"
Several versions of UNIX and UNIX-like systems have been made that are free or
extremely cheap and include source code. These versions have become particularly
attractive to the modern-day hobbyist, who can now run a UNIX system at home for
little investment and with great opportunity to experiment with the operating
system or make changes to suit his or her needs.
An early UNIX-like system was MINIX, by Andrew Tanenbaum. His books Operating
Systems: Design and Implementations describes MINIX and includes a source
listing of the original version of MINIX. The latest version of MINIX is
available from the publisher. MINIX is available in binary form for several
machines (PC, Amiga, Atari, Macintosh, and SPARCStation).
In addition to the BSD386 product from BSDI, there is a free version of UNIX
also based on the BSD releases, and called, confusingly, 386BSD. This is an
effort by Bill and Lynne Jolitz to create a system for operating system research
and experimentation. The source is freely available, and 386BSD has been
described in a series of articles in Dr. Dobbs' Journal.
Another popular source version of UNIX is Linux. Linux was designed from the
ground up by Linus Torvalds to be a free replacement for UNIX, and it aims for
POSIX compliance. There are current efforts to make Linux reliably run both
SVR3.2 and SVR4 binaries. There is also a project called WINE to create
Microsoft Windows emulation capability for Linux.
Making Changes to UNIX
Many people considering making the transition to UNIX have a significant base of
PC-based MS-DOS and Microsoft Windows applications. There have been a number of
efforts to create programs or packages on UNIX that would ease the migration by
allowing users to run their existing DOS and Windows applications on the same
machine on which they run UNIX. Products in this arena include SoftPC and
SoftWindows from Insignia, WABI from SunSoft, and WINE for Linux and 386BSD.
Introduction to the UNIX Philosophy
As described in the section "The History of UNIX," UNIX has its roots in a
system that was intended to be small and supply orthogonal common pieces.
Although most UNIX systems have grown to be fairly large and monolithic
applications are not uncommon, the original philosophy still lives in the core
commands available on all UNIX systems. There are several common key items
throughout UNIX:
Simple, orthogonal commands
Commands connected through pipes
A (mostly) common option interface style
No file types
For detailed information on commands and connecting them together, see the
chapters on shells (Chapters 10—14) and common commands (Chapters 5—9 and
Chapter 15).
Simple, Orthogonal Commands
The original UNIX systems were very small, and the designers tried to take every
advantage of those small machines by writing small commands. Each command
attempted to do one thing well. The tools could then be combined (either with a
shell script or a C program) to do more complicated tasks. One command, called
wc, was written solely to count the lines, words, and characters in a file. To
count all the words in all the files, you would type wc * and get output like
that in Listing 1.1.
Listing 1.1. Using a simple command.
$ wc *
351 2514 17021 minix-faq
1011 5982 42139 minix-info
1362 8496 59160 total
$
Commands Connected Through Pipes
To turn the simple, orthogonal commands into a powerful toolset, UNIX enables
the user to use the output of one command as the input to another. This
connection is called a pipe, and a series of commands connected by pipes is
called a pipeline. For example, to count the number of lines that reference
MINIX in all the files, one would type grep MINIX * | wc and get output like
that in Listing 1.2.
Listing 1.2. Using a pipeline.
$ grep MINIX * | wc
105 982 6895
$
A (Mostly) Common Option Interface Style
Each command has actions that can be controlled with options, which are
specified by a hyphen followed by a single letter option (for example, -l). Some
options take option arguments, which are specified by a hyphen followed by a
single letter, followed by the argument (for example, -h Header). For example,
to print on pages with 16 lines each all the lines in the file minix-info that
mention Tanenbaum, you would enter wc minix-info | pr -l 16 and get output like
that in Listing 1.3.
Listing 1.3. Using options in a pipeline.
$ grep Tanenbaum minix-info | pr -l 16
Feb 14 16:02 1994 Page 1
[From Andy Tanenbaum <ast@cs.vu.nl> 28 August 1993]
The author of MINIX, Andrew S. Tanenbaum, has written a book describing
Author: Andrew S. Tanenbaum
subjects.ast (list of Andy Tanenbaum's
Andy Tanenbaum since 1987 (on tape)
Version 1.0 is the version in Tanenbaum's book, "Operating Systems: Design
$
The bad news is that some UNIX commands have some quirks in the way they handle
options. As more systems adopt the standards mentioned in the section "The
History of UNIX," you will find fewer examples of commands with quirks.
No File Types
UNIX pays no attention to the contents of a file (except when you try to run a
file as a command). It does not know the difference between a spreadsheet file
and a word processor file. The meaning of the characters in a file is entirely
supplied by the command(s) that uses the file. This concept is familiar to most
PC users, but was a significant difference between UNIX and other earlier
operating systems. The power of this concept is that any program can be used to
operate on any file. The downside is that only a program that understands the
file format can fully decode the information in the file.
Summary
UNIX has a long history as an open development environment. More recently, it
has become the system of choice for both commercial and some personal uses. UNIX
performs the typical operating system tasks, but also includes a standard set of
commands and library interfaces. The building-block approach of UNIX makes it an
ideal system for creating new applications.
--
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