FREE ESSAY ON CONTRAST DOS TO UNIX

CONTRAST DOS TO UNIX

Compare and Contrast Microsoft DOS with UNIX
Arthur Bennis
Microcomputer Operating Systems
CGS 1560
INTRODUCTION
As is suggestive of its name, an operating system (OS) is a collection of programs that
operate the personal computer (PC). Its primary purpose is to support programs that
actually do the work one is interested in, and to allow competing programs to share the
resources of the computer. However, the OS also controls the inner workings of the
computer, acting as a traffic manager which controls the flow of data through the system
and initiates the starting and stopping processes, and as a means through which software
can access the hardware and system software. In addition, it provides routines for device
control, provides for the management, scheduling and interaction of tasks, and maintains
system integrity. It also provides a facility called the user interface which issues
commands to the system software. Utilities are provided for managing files and documents
created by users, development of programs and software, communicating between users with
other computer systems and managing user requirements for programs, storage space and
priority.
There are a number of different types of operating systems with varying degrees of
complexity. A system such as DOS can be relatively simple and minimalistic, while others,
like UNIX, can be somewhat more complicated. Some systems run only a single process at a
time (DOS), while other systems run multiple processes at once (UNIX). In reality, it is
not possible for a single processor to run multiple processes simultaneously. The
processor of the computer runs one process for a short period of time, then is switched
to the next process and so on. As the processor executes millions of instructions per
second, this gives the appearance of many processes running at once.
User programs are usually stored on a hard disk and need to be loaded into memory before
being executed. This presents the need for memory management, as the memory of the
computer would need to be searched for a free area in which to load a users program. When
the user was finished running the program, the memory consumed by it would need to be
freed up and made available for another user when required (CIT).
Process scheduling and management is also necessary, so that all programs can be executed
and run without conflict. Some programs might need to be executed more frequently than
others, for example, printing. Conversely, some programs may need to be temporarily
halted, then restarted again, so this introduces the need for inter-program
communication. In modern operating systems, we speak more of a process (a portion of a
program in some stage of execution (CIT, 3)) than a program. This is because only a
portion of the program is loaded at any one time. The rest of the program sits waiting on
the disk until it is needed, thereby saving memory space.
UNIX users speak of the operating system as having three main parts: the kernel, the
shell and the file system. While DOS users tend not to use the term kernel and only
sometimes use the term shell, the terms remain relevant. The kernel, also known as the
"Real Time Executive", is the low-level core of the OS and is loaded into memory right
after the loading of the BIOS whenever the system is started. The kernel handles the
transfer of data among the various parts of the system, such as from hard disk to RAM to
CPU. It also assigns memory to the various system-level processes that occur whenever the
computer does anything. The kernel is also responsible for scheduling the CPU's
operations and for letting the shell access the CPU (PC Mag, 1).
The shell is the visible user interface to the OS and is a program that loads on top of
the operating system and offers users commands that lets them access the OS. Strictly
speaking, the shell is an input utility that offers access to the operating system.
Technically speaking, the shell, being a separate program, is not a part of the OS at
all. In the UNIX world a number of shells are available, among them the Korn shell, the
C-shell, the Bourne shell and the Bourne Again shell (yes, really). In DOS, the standard
shell is COMMAND.COM, again nothing more than a program. As different versions of
command.com came with different versions of DOS, each added new commands and new things
that could be done by the user. For example, DOS 4's COMMAND.COM added the /P switch to
DEL to verify each deletion, and DOS 5's COMMAND.COM provided the ability to sort the
output of the DIR command.
HISTORY
An acronym for disk operating system, the term DOS can refer to any operating system, but
is most often used as shorthand for MS-DOS. Originally developed by Microsoft for IBM,
MS-DOS was the standard operating system for IBM-compatible computers. The initial
version of DOS was somewhat uncomplicated and resembled another operating system called
CP/M. Subsequent versions have become increasingly sophisticated, however DOS remains a
16-bit operating system without support for multiple users or multitasking.
The earliest forms of DOS were crude and utilized only a few commands, but as computers
became more advanced, so did DOS. By keeping up with technology, DOS was implemented into
more "user friendly" operating systems. However, as more sophisticated operating systems
were released, DOS became less important. "Today, cyberpunks involved with the latest OS
trends joke that DOS stands for 'Dad's Operating System'" (Comerford, 23).
In 1980, IBM asked the Microsoft Corporation to produce the operating system for its
first personal computer, the IBM PC. Prior to this, a company called Seattle Computer
Products had sold an operating system called 86-DOS to Microsoft. Microsoft hired the
author of 86-DOS, Tim Paterson, in April of 1981 to modify the system, and renaming it
MS-DOS (Microsoft Disk Operating System), it was released with the IBM PC. Thereafter,
most manufacturers of personal computers licensed MS-DOS as their operating system
(Brittanica, 1). Limitations of the early PC's hardware were a big influence on MS-DOS.
Although the 8088 model computer had a 1Mb address space, IBM decided to allocate the
first 640K of this to RAM, and the rest to ROMs, video boards and other things.
Consequently, MS-DOS was set up to support programs whose maximum size was 640K.
Version 1.0 of DOS was released along with the IBM PC in August 1981. It occupied 12K of
the systems 640K of memory, was somewhat compatible with CP/M and, much like CP/M,
supported only a single directory. By contrast, even the first version of UNIX had a full
hierarchical file system. In addition, Version 1.0 supported only a 160K single sided
51/4-inch floppy diskette. Version 1.1 was released by Microsoft in October 1982 and
supported double sided 320K diskettes. Aside from fixing some bugs, this release was
similar to Version 1.0. Releases such as 1.1, in which the number to the left of the
decimal point is the same as the previous version depict relatively minor changes from
the previous release. By contrast, Version 2.0 was largely a new system.
In March 1983, IBM introduced the PC/XT, its first personal computer with a hard disk. It
came with a new variant of MS-DOS, Version 2.0. In this version, Microsoft incorporated
many ideas from the UNIX system for which it was also a vendor. For example,
incorporating minor changes, the MS-DOS file system was taken largely from UNIX. In
addition, the shell was improved, and Version 2.0 supported a new floppy diskette format,
the 360K as well as user installable device drivers, print spooling, system configuration
and memory management. At this point, MS-DOS was established as the dominant operating
system in PC market.
In August 1984, IBM released its first 286 chip based PC, the PC/AT. The PC/AT supported
memory up to 16 Mb and had the ability to run multiple programs at once. However, the
version of MS-DOS that shipped with the PC/AT was 3.0, which supported neither of these.
Rather, it ran the PC/AT in a mode that simulated the 8088, only faster. Since the PC/AT
came with a 1.2Mb disk drive, battery backup clock, and configuration information in the
CMOS, support for these devices was added. What's more, hard disks larger that 10Mb were
now supported. In addition, the command processor (shell) was removed from the operating
system and made into a separate program. In November 1984, 3.0 was replace by 3.1 which
provided the first support for networking. In 1987, IBM came out with the PS/2 line of PC
which shipped with MS-DOS 3.3, providing support for both 720K and 1.44Mb 31/3 floppy
disk drives.
With Version 4.0, Microsoft added the DOS shell, a menu driven shell rather than the
previous keyboard driven ones. In addition, it now provided support for hard drives
larger than 32 Mb. A major new release, MS-DOS Version 5.0 was shipped in April 1991.
Although this was the first version that made any serious use of the extended memory, it
still had the restrictions that programs could not exceed 640K. However, it had the
ability to locate most of MS-DOS itself in extended memory, so about 600K of the lower
640K was now available for user programs. Version 5.0 also came with a useful HELP
utility, to aid new users. For the first time, MS-DOS was sold in stores to the public
(previous versions were only sold to computer vendors who delivered them with their
machines) (CIT, 1-3).
The MS-DOS 6 family provided more memory management for applications such as Microsoft
Windows. In addition, newer utilities were provided for disk-defragmentation, file
compression, file backups and anti-virus checking. Other variations of MS-DOS exist, such
as PC-DOS by IBM, DOS-V, Dr. DOS and others. There is even a FREE DOS available on the
Internet as an MS-DOS clone. Although it can still be found on many computers, MS-DOS is
technically an obsolete operating system, being replaced by Microsoft Windows.
For personal computers, MS-DOS is a single user, single tasking operating system. Single
user means only one person uses the computer at a time. Single tasking means that it
essentially runs one application program at a time, and has no inherent support for
running more than one application program simultaneously (CIT, 2).
If we want to look at the basic DOS operating system itself, there is no need to look
further than three system files, command.com, Io.sys and (in DOS6.x and earlier)
Msdos.sys. These files are crucial in DOS versions up to 6.22. Io.sys represents the
lowest level of the interface and contains the routines necessary for interfacing the OS
with the system's BIOS. It implements MS-DOS as seen by the hardware and has default
drivers for console display and keyboard, printer, serial communications, clock, and a
boot disk drive.
Msdos.sys handles the higher-level routines such as converting commands from applications
into instructions for Io.sys. It implements MS-DOS as seen by application programs. It
supports file and record management, memory management, character device input and
output, execution of other programs, and access to a real-time clock (CIT, 3).
Both of these files are in the root directory, and both are hidden from view by default.
The idea is that you are not suppose to see them, so that you don't do anything
destructive to them (such as deleting them). They are also read-only so that they can't
be deleted accidentally. Command.com is the shell program which interprets user commands,
presents the shell prompt, and contains a set of internal commands. The rest of MS-DOS
consists of a number of utility programs.
Although DOS had cornered the PC market, UNIX was still dominant on the larger
workstations. The birth of UNIX in 1969 provided the world with its first modern
operating system. An interactive multi-user operating system, UNIX was initially
developed by programmers for their own use. Working for Bell Laboratories, Ken Thompson
and Dennis Ritchie created UNIX as an operating system for the PDP-7 computer. Designed
as a simplification of an operating system named Multics, UNIX was developed in Assembly
language, a primitive computer language specific to one type of machine (Osiris, 1).
However, Thompson developed a new programming language "B" which Ritchie enhanced to "C",
and in 1973 this was used to rewrite UNIX which lended the OS portability (Linux Intl.,
1). The original design philosophy for UNIX was to distribute functionality into small
parts, the programs (Theochem, 1). In this way, functionality could be achieved by
combining the small parts (programs) in new ways. Moreover, if a new program were to
appear, it could be integrated into the system.
UNIX was slow to catch on outside of academic institutions but soon was popular with
businesses as well. The first five versions were part of an internal research effort of
Bell Labs, and it was not until the sixth version, called UNIX Timesharing Sixth Edition
V, that UNIX was widely distributed (Osiris, 1). Relatively recent developments are
graphical interfaces (GUI) such as MOTIF, X Windows and Open View. UNIX has two major
versions. One, jointly developed by UNIX Systems Laboratories (USL) and by AT&T
researchers together with Bell Labs, generically known as System V, is the commercial
version and is the most widely distributed by major manufacturers. The second, developed
by the University of Berkley and Berkley Software Distribution (BSD), is the educational
version and is completely focused on research. The USL version is now on its fourth
release, or SVR4, while BSD's latest version is 4.4. However, there are many different
versions of UNIX besides these two. The operating system has been licensed to several
manufacturers who in turn developed their own versions of UNIX, based on System V or BSD,
but adding new characteristics. Most versions of UNIX developed by software companies are
derived from one of the two groupings and, recent versions of UNIX actually incorporate
features from both of them. However, UNIX has had an unregulated history with over 200
versions (Berson, 16) existing today.
The UNIX system is made up of three primary components, the kernel, the shell, and the
utilities (which includes the file system). The central part of the OS, the kernel is the
first program to start when the system is turned on and the last program to do anything
when the system is halted. In addition to scheduling tasks, it manages data/file access
and storage, enforces security mechanisms and performs all hardware access. The name
"KERNEL" represents the fact that it is a program designed as a central nucleus, around
which other functions of the system were added. The heart of the operating system, it not
only interacts directly with the system's hardware, but presents each user with a prompt,
interprets commands typed by a user, executes user commands and supports a custom
environment for each user. The two most common shells are the Bourne shell, default for
the System V, and the C-shell used mainly with the BSD version (Osiris, 1). The utilities
consist of file management (rm, cat, ls, rmdir, mkdir), user management (passwd, chmod,
chgrp), process management (kill, ps) and printing (lp, troff, pr).
In order to obtain a basic understanding of the UNIX operating system, it is necessary to
touch upon several of the principal characteristics that have permitted it to remain
competitive through the years.
1. Advanced Administration of Processes
UNIX has a process manager known as Process Scheduler, which handles the allotment of
time to each of the processes according to the priority it was assigned.
2. Multiprocessing
Many UNIX variants allow the use of various processors to execute user tasks. This means
that UNIX has support for symmetric processing, with which it can take advantage of the
fact that there are two or more CPUs in the machine.
3. File Management
The hierarchical files system that UNIX runs, as well as file access control and
directory control have served as models for the majority of modern operating systems such
as MS-DOS, OS/2 and even Windows NT.
4. Utilities Access
For the UNIX operating system, each of the machines devices, whether it be a hard drive,
printer, modem, etc. is seen as a file. Thus, access to any device is carried out as
access to a file. This is possible through the fact that UNIX differentiates between
kinds of files. In fact, the processes themselves are seen as files, which permits the
establishment of another important UNIX characteristic, interprocess communication.
5. Virtual Memory
The fact that UNIX has virtual memory allows the number of processes being executed to
require more memory than exists in the machine.
6. Graphic Interface
Although not exactly a novel characteristic of UNIX, most versions now have a graphic
interface.
7. Interplatform Support
This is another characteristic that was added to UNIX which lends the capability to
execute programs from other platforms (DOS and Windows), within the UNIX environment.
8. Networks
The usual UNIX communications protocol is TCP/IP. This allows variants of UNIX based
operating systems to communicate between themselves or with other platforms (Osiris,
1-2).
CONTRAST
Both DOS and UNIX present a number of similarities, several of which shall be addressed
here. First, both systems are interactive, meaning that the shell presents a prompt and
waits for the user to enter a command. After the return or enter key is pressed, the
shell processes the command and when the command is finished, the shell re-displays the
prompt. Second, DOS batch files and UNIX script files can be used which can store
commonly used commands in a file, which when executed, runs each command as though it has
been typed from the command line. A sequence of commands can be executed by executing the
file which contains the command(s). Third, the handling of files in both DOS and UNIX is
simplified by using wild-card characters to match files which match particular patterns.
Also, with both operating systems, users can customize and control the behavior of the
shell by using special variables that the shell supports, such as the prompt (20,1). In
addition, both systems make use of "pipes" whose symbol is a vertical bar ( | ). With
this convention, the output from one command becomes the input for another command.
Several dissimilarities are worth noting. As was previously mentioned, DOS is a single
user, single task operating system. Its user interface is not case sensitive, which means
that commands may be typed in either upper case, lower case or a combination of the two.
UNIX however, is a multi-user, multi-task OS. Non-interactive tasks which do not require
keyboard input can be run in the background as a separate task while the user continues
working with other interactive programs (20,1). Differing from DOS, its user interface is
case sensitive, meaning that only upper or lower case commands must be used.
APPLICATIONS & COMPUTER TYPES
Whereas DOS has been used primarily on PCs and standalone computers, UNIX can be run on
single- or multi-user computers of all sizes with a wide range of microprocessors (Flynn
& McHoes 319). UNIX is the widely supported operating system in the field of computer
science, used extensively in business as well as educational institutions. Conversely,
DOS is used mainly in businesses with older computer systems.
BENEFITS & DISADVANTAGES
The major advantage that DOS has over UNIX is its basic simplicity. Between this and the
uncomplicated commands presented by the user interface, it is a relatively simple OS to
learn. DOS also has the advantage of allowing the user to create an environment tailored
strictly for the particular task they wish to accomplish. In addition, one can customize
DOS to suit the current hardware. This can be accomplished with commands such as date,
time, prompt, path, set, assign and subst. Unlike DOS, UNIX's main feature is that it is
a multi-user system, meaning more than one user can use the machine at a time when
supported via terminals provided by a serial or network connection. Offering true
preemptive multi-tasking, UNIX can run more than one program at a time with a CPU that
services all applications equally. In addition, it has a hierarchical directory structure
which supports the organization and maintenance of files. Other advantages are that it
has been in the market for a number of years, and is therefore considered a stable
product. Also, due to the fact that the kernel is in "C", UNIX works in just about every
machine in the market, once again, making it a portable system with a collection of very
powerful utilities. Also, there are many applications developed for DOS and UNIX which
fall into the category of "shareware" available via the Internet (8,1).
As with advantages, both DOS and UNIX have their share of disadvantages. It can be said
that DOS has two main drawbacks. Since MS-DOS was originally written for a particular
family of microprocessors, it displays an incredible lack of flexibility and limited
ability to meet the needs of programmers and experienced users (Flynn & McHoes, 265).
UNIX also has several very distinct disadvantages. First, novice users find its commands
are almost cryptic which is interpreted as being non-"user-friendly". Second, the fact
that there exist so many versions of the operating system means that software producers
must make several versions of their applications to cover the greatest number of
potential users. Third, UNIX is a large operating system, and depending on the number of
services installed and the functions used, space used on a hard drive may vary from 20 Mb
to 300 Mb (Osiris, 1).
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