DOS stands for Disk Operating System. In the early 1980s, Microsoft got the right to QDOS (Quick and Dirty Operating System) developed by a small company, Seattle Computer, and then has been sold it under the name MS-DOS. Microsoft licenses a version called PC-DOS to IBM (International Business Machines) for its IBM personal computers, and its version, MS-DOS, to many other PC manufacturers.

Advantages and Disadvantages

• Advantages: DOS has many advantages. The advantages will be the reasons for learning DOS. DOS is the most popular microcomputer operating system for IBM computers and compatibles ever sold worldwide. It runs thousands of applications, and is easy to use. DOS is easy for novices to use, and many publications and books are available for the operating system. DOS runs on low- priced IBM computers and compatibles. DOS does not require an expensive computer system while some others still do.

• Disadvantages: DOS has some drawbacks, but there is no doubt that we will continue to see DOS used in the 1990s. It will then slowly be replaced as the dominant operating system. The hardware has evolved in significant ways. New microcomputers have more capacity and faster electronics than the old IBM PC and compatibles. This makes DOS an old operating system.
  DOS has direct access to only 640 kilobytes of primary storage, although new microcomputers have much more primary storage. This restriction is an inherent limitation of DOS. Today's new software for word processing, graphics, spreadsheets, and database management requires more primary storage. In addition, DOS is a single tasking operating system. It can support only one user and one program at a time.
  Another disadvantage is that DOS has a character-based interface. A graphical user interface is easier to the users than the character-based interface.

File Access Table(FAT)

DOS creates a FAT for each disk during formatting. Every sector on the disk is represented by an entry in the FAT as part of a cluster. DOS looks for available clusters when a file is enlarged or created. When DOS allocates files on a freshly formatted disk, DOS uses the first cluster and sequences through a connected series of clusters, leaving many never-used clusters at the end of FAT. When a file is erased or shortened, DOS marks the released clusters in the FAT as available again. When a file is allocated more than one cluster, each cluster points to the next cluster that contains more of the files. The pointer is the next cluster number. The result is a chain of clusters that comprise the map of a file's disk storage. Thus, FAT acts as a storage map and tells DOS exactly where to go on the disk to get all parts of a file.

How Application Programs Run in DOS

When the user specifies the application software to work on, the RAM part of memory is filled with that image of that software. For example, if the user wants to use WordPerfect, the command is taken from the command line and the corresponding software is loaded on to the RAM and it works like a word processor.

Files on DOS Disks

The files on DOS disks have specific purposes. A COM file extension identifies a command file. Command files are the names of external DOS commands. Files with CPI extensions operate the display screen. A file with the DAT extension is a data file. Files with BAT extension are batch files. AUTOEXEC.BAT is a special batch file that runs automatically when a computer is started. EXE files are executable program files. SYS files are system files. SYS files are used to add or modify hardware support to the basic PC operation.

This is a graphics-based operating environment from Microsoft. This operating system runs under DOS. Thus, it is usually called "DOS with windows." Windows 3.x allows multiple tasking, allowing users to open several applications simultaneously and shift between them. Windows operating system is very similar to the Macintosh desktop environment.
Windows 3.x is a major upgrade of Microsoft's earlier versions. It provides a DOS extender that allows Windows 3.x applications to run in up to 16MB of memory. In the Windows, users can run DOS applications and change data between them.

Modes

Windows 3.x has three different operating modes.

• Real Mode: This mode is for computers with less than 1MB of memory. It uses expanded memory if available, and does not use extended memory.

• Standard Mode: This is a normal Windows 3.0 operating mode. It works with extended memory (XMS). This mode allows users to use DOS applications in the window environment by allowing users to switch between DOS applications and windows applications. The mode also uses expanded memory (EMS) if available.

• 386 Enhanced Mode: This mode is the extension of Windows/386 version that runs on 386 and higher CPUs. This mode allows multitasking of Windows and standard DOS applications. It converts character-based programs (DOS applications) to bit-mapped graphics (Windows applications). It activates EMM386.SYS memory manager to use EMS memory.

Advantages and Disadvantages

• Advantages: Windows allows multitasking capability and more primary memory. Windows provides a memory manager to allow users to access more than 640KB of conventional memory. Another benefit of it is that it has a dynamic data exchange function. Dynamic data exchange allows one program (e.g., a word processing program) to request data or information from other programs (e.g., a spreadsheet program or a database program etc.) currently running and use those information. A graphical user interface (GUI) against character- based approach is another advantage. This allows users to use Windows environment such as mouse, pointer, clicking, icon, or pull-down menu.

• Disadvantages: Windows 3.x can handle multitasking, but it is really switching between tasks. Windows 3.x is not a multithreaded operating system. This works well for many applications, but for some information and data can be lost if Windows does not get back to a task soon enough. There are many application programs for Windows, but users frequently demand more powerful application programs that exceed the capabilities of current Windows programs. The other disadvantage is that Windows 3.x was not originally intended for networks and is not very efficient in that environment although it has a network version, Windows for Workgroup 3.11.

Windows 95 upgrades its earlier versions (Windows 3.x) in many ways. It has a new 3-D interface. It is mostly a 32- bit system although it has 16-bit components included for compatibility with Windows 3.x. Windows 95 does not need to have a separate DOS. It integrates all DOS services. A Windows 95 system runs in protected mode. This means that it speeds up the processors and provides more safety.
Windows 95 serves two purposes: It will move developers to the Win32 API and will ease the transition for users whose hardware is not yet ready to handle the demands of Windows NT.
Windows 95 offers better preemptive multitasking. Although for most things, cooperative multitasking is good. However, if users want to do several things simultaneously, they are going to find that preemptive multitasking provides smoother operation and better speed.
Because Windows 95 is based on Win32 and has borrowed some of NT's features, some people are confused over which Windows to use. Windows 95 is for anyone who has a lower capability computer that does not enable to use Windows NT. Windows 95 is likely to perform better than NT as a desktop system, especially when running older 16-bit Windows applications.

Advantages and Disadvantages

• Advantages: This operating system provides compatibility with existing Windows and DOS programs. It also offers compatibility with existing Windows and DOS device drivers. It works as in standalone computer and in networked environment for both desktop and mobile users. It supports Windows for Workgroups, Novell and UNIX-style networks.
  It is easier to learn and use, gives users better performance, has more capacity, integrates applications better, connects better, and crashes less than its predecessor, Windows 3.x.

• Disadvantages: Windows 95 still has limits from 16-bit components retained for backward compatibility with 16-bit application. It does not give much benefit to the 16-bit applications. To get benefits from Windows 95, users must upgrade their applications to 32-bit versions that support OLE and can interact directly with Windows 95's shell. It will cost users significantly.

Difference of 95 with Windows NT

Windows 95 has a different user interface with Windows NT. The interface of NT is much better. Windows 95 uses a different model for device drivers than NT. Thus, not all Windows 95 applications run on NT without modification, and vice versa. Windows 95 is a mix of 16- and 32-bit application programming interface, but NT is solely a 32-bit interface.

Window NT is a platform for 32-bit Windows applications. This is a powerful and one of the most advanced operating systems available today.
Windows NT was originally designed with big-system features. Beginning with version 3.5, Microsoft began repositioning Windows NT as a workstation and server operating system. It also has a sophisticated multiuser security system, so Windows NT performs excellently as a disk server. NT has support for network services useful to applications like client/server database engines. Still, all the features of Windows NT are suitable for use in demanding applications of desktop users.

Advantages and Disadvantages

• Advantages: Windows NT allows users to run all applications, including Windows applications, from the command line. It has better speed. Windows NT does not have arbitrary resource limits. Computer system reliability is improved by the result of this. It has the familiar Windows 3.x-style user interface, so users do not need to spend much time for learning. NT has a preemptive multitasking capability, and the users can multitask applications within their own memory spaces. It has much more reliable networking capability than that provided by Windows 3.x or Windows 95.

• Disadvantages: Windows NT is a platform for 32-bit Windows applications. 32- bit applications are not widely available yet. Still, 16-bit applications are prevailing in the software market. This operating system requires much resource. Windows NT workstation requires 12MB of memory when run on Intel 486/pentium computers (It runs in 8MB, but Microsoft recommends 12MB for NT to run adequately). Windows NT server requires 16MB. Windows NT system that is run on an RISC workstation also demands 16MB of memory. The NT requires at least 70MB of hard disk space only for the operating system. After all, this operating system requires too much for normal microcomputer users

OS/2 stands for Operating System 2. This is another operating systems for powerful microcomputers and networking. OS/2 was designed to avoid some of the most serious limitations of DOS. OS/2 runs in 4MB of memory, although it runs well in an 8MB system. It requires about 30MB of hard disk space. OS/2 WARP, the window version of OS/2, performs reasonably well in 4MB system and requires less disk space.
Users do not need to abandon DOS and Windows to try out OS/2. OS/2 provides a dual boot feature that allows users to boot up in either DOS or OS/2. OS/2 uses folders much like those on the Macintosh and on some UNIX GUIs.

Advantages and Disadvantages

• Advantages: It is not affected by requirements of DOS and Windows that certain device drivers live in the first 640KB of memory. Instead, OS/2 can load network adapter drivers, multimedia interfaces, CD-ROM software or video drivers into memory as required. While OS/2 works very nicely with the same FAT file system used by DOS, its HPFS has a number of benefits. It is significantly faster, it allows a long file name, and it supports disk drivers as large as 256 gigabytes.
  This operating system allows multitasking and dynamic data interchange. It also provides common graphical user interface with mainframes, minicomputers, and microcomputers by OS/2 WARP. OS/2 was developed to assist in the sharing of data and programs among several microcomputers. OS/2 solidly supports Novell Netware and TCP/IP. Most network interface cards are supported.

• Disadvantages: OS/2 Shell or WARP is similar to Windows, but is not identical. It may take users a while to adjust to some of the differences. The application programs developed specifically for OS/2 are not as many as those for Windows, although more and more application developers are going to converting and developing programs for OS/2 thesedays.

UNIX was originally developed in 1969 by K. Thompson, R. Canaday and D. Ritchie for minicomputers in the Bell Laboratories in the USA owned and run by AT& T. By the mid-1970s, UNIX had been introduced to University of California at Berkeley by Thompson, then widespread into an academic world. Most computer science departments of universities have been used UNIX. UNIX initially became popular in industry because for many years AT& T licensed the system to universities for a nominal fee. The effect of this was that UNIX was carried by recent computer science and engineering graduates to their new places of employment. Rapid commercialization of UNIX followed on from this. This rapid commercialization caused the lack of standards. Many hardware and software vendors developed their own versions.
After UNIX was widespread, it had been used by scientists and engineers. It is because of its scientific and technical orientation. It is less well known with business people. All that, however, is probably about to change. The reason is that with the arrival of very powerful microcomputers using the newer chips such as pentium and pentium-pro chips, UNIX has become a major player in the microcomputer world. The difficulty of learning is also about to change because of the graphical user interface.

This is a multiuser, multitasking operating system that runs on many different computer systems from microcomputer to mainframe, because UNIX is written in C programming language, which is a language designed for system-level programming. UNIX is consisted of a kernel, the file system, the user interface. The kernel is the heart of the operating system. The file system has a hierarchical directory method for organizing files on the disk and the shell.

Advantages and Disadvantages

• Advantages: UNIX is the portable operating system that can be used with many different computers from mainframe to microcomputer. The other operating systems such as DOS, Windows, Mac operating system, and OS/2 cannot be used for other systems. One of many strong features of UNIX is multitasking. UNIX allows users to run multiple programs simultaneously.
  UNIX also shares it among multiple users. UNIX is not restrained by the computer systems such as conventional memory as the DOS and OS/2 systems are. It can accomplish many operations that were formerly performed on minicomputers or mainframes. It is important, because industry can achieve the performance and benefits of large computers from microcomputers by using UNIX.
  Networking is another strong feature of a UNIX operating system. UNIX can be connected through several different kinds of equipment. It can share files among them.

• Disadvantages: Although UNIX can do many things, it can be difficult for novice microcomputer users to understand. This is a reason it has had a limited impact to date. UNIX was a minicomputer operating system used by programmers and computer science professionals some time before the rise of the microcomputer. This means that it has certain qualities making it useful to programmers - many supporting utility programs and documentation, for instance. Some of its features make it difficult for end users. However, it is about to change, because a set of standards for a UNIX GUI have come into use and most of the UNIX operating system incorporates graphical user interface.
  Limited application programs are another disadvantage for UNIX. UNIX off-the-shelf programs for microcomputers are limited. However, this situation is going to change, because many software vendors are rewriting DOS and Windows applications for UNIX.
  As stated earlier, there is not a standard for UNIX. This may be the biggest problem for the popularity of UNIX. The principal microcomputer versions are the AT& T UNIX System V, the UC Berkeley UNIX, and Xenix developed by Microsoft for a microcomputer version. AIX from IBM, Solaris, and Linux are other variants of UNIX. An application program written for one version of UNIX may not run on other versions.
  There is also not a standard GUI for many UNIXs. There were several attempts to combine the Sun, AT& T, and U.C. Berkeley versions of UNIX to produce a standard graphical user interface. The one was Open Look (Sun/Open Windows) by Sun, AT& T and Berkeley. Another graphical user interface was an OSF/Motif by the Open Software Foundation (OSF). OSF/Motif is now in severe disarray.

It uses a graphics screen that places familiar office objects on a display screen. Files, folders, programs, and disks are represented by icons. It has a hierarchical file system that lets users drag document icons into and out of folder icons. Folders can also contain other folders and so on.
In IBM computers and IBM compatible computers, software developers usually decide how they make a user interface. In contrast, Macintosh application developers usually conform to the Macintosh user interface. This consistent user interface makes users easy to learn new programs from the start. This also makes Macintosh operating system and its application programs indistinguishable.
The Macintosh user interface style has been adapted to many other operating systems. For example, OS/2 Presentation Manager and WARP, New Wave, most UNIX systems, and Windows look very similar to the Macintosh graphics user interface.
The Macintosh operating system has two major files. They are the System file and the Finder. The system file manages the user interface. Both files work together to achieve the operating system procedures such as formatting disks, copying files, erasing files, and running application programs.

Advantages and Disadvantages

• Advantages: The ease of use is the primary charm of the Macintosh. The graphics interface such as menus, screen display, and dialog boxes are quite more consistent across applications than they are in Windows. It also offers easy plug and play. When install a board in a Mac, users need to plug it into a slot, drag its driver icon into the System Folder, and reboot the computer.
  It provides long filenames, integrated program and file management, desktop icons, and shortcuts. It provides a high quality graphics processing that makes Macintosh popular for desktop publishing (Mac is the standard computer in the publishing industry). It has a great video architecture that allows users to add multiple monitors to a computer by adding another video board and hook up another monitor.
  The later versions of Macintosh operating system (System 7.x series) enable users to do multitasking. That is, multiple programs can run simultaneously, each sharing the CPU (cooperative multitasking like Windows 3.x). System 7.x allows applications programs to exchange and share data and commands with other application programs.
  It even provides built-in sound, SCSI, and Ethernet that allow users to work easily with multimedia. It has much better multimedia toolkit than Windows has. The IBM and compatible computers, PC, cannot match the Macintosh's toolkits for multimedia, and Windows multimedia applications do not work together as smoothly as their Macintosh counterparts.

• Disadvantages: The first disadvantage of the Macintosh platform is the incompatibility with DOS and Windows (PC) applications. This has been made Macintoshes less attractive to corporate users that want to have compatibility and connectivity within company and between companies. However, software (e.g., Insignia's SoftWindows) are now available for the Mac to allow it to run Windows on PowerMac although it runs slowly. This is a great utility for a Mac user who needs to run DOS or Windows applications occasionally. Another solution for this is the fact that networks connecting Macintoshes to PCs are available these days.

Window

A window was originally a general name of viewing area on a display screen provided by software. Many operating systems can provide multiple windows on a display screen. Under this environment, users usually can work several tasks with several application programs on a screen at the same time. Microsoft makes Windows the brand name of its operating system. They are Windows/286, Windows/386, Windows 3.x, Windows 95 and Windows NT. Users need to be careful not to think of only Microsoft's Windows has windows. Many operating systems such as OS/2 Presentation Manager, OS/2 WARP, many UNIX systems, and Macintosh operating system have windows.

XMS

It is an abbreviation of eXtended Memory Specification. This is an interface that allows DOS applications to use extended memory. This allows DOS applications to only allocate extended memory, but does not allow them to run in extended memory.

EMS

Expanded Memory Specification is a technique for expanding memory beyond one megabyte under DOS. EMS and XMS are different techniques to enlarge memory running under DOS. XMS is a normal memory beyond one megabyte on 286 and higher computers, but EMS is separate memory that can be installed in any computer system.

Win32 API

It is a 32-bit Windows Application Programming Interface. This enables applications to be more responsive, has larger document capacities, and handles CPU- intensive tasks more quickly than Win16 API. When a program marked with Windows 95 ready on the software package, it is designed using Win32 API.

Preemptive/Cooperative Multitasking

Preemptive multitasking means that one Windows application can get control of CPU without the knowledge of another Windows application. In contrast, Cooperative multitasking means that an application can assume it has full control until it yields to other applications. Many operating systems such as Windows NT, Windows 95, or OS/2 have preemptive multitasking function.

Resource Limit

Windows 3.x has three 64KB resource heaps that must be shared by all applications in the system. When those three heaps are exhausted by many applications, errors like "out of system resource" occur. Windows NT does not have that kind of limit.

HPFS

This refers a High Performance File System introduced with OS/2. The system handles larger disks, long file names, and can launch the program by referencing the data. It coexist with the existing FAT system.

FAT

FAT is an abbreviation of File Allocation Table that is the part of DOS and OS/2 file system. It keeps tracks of where the data is stored on a disk. It is a table with an entry for each cluster. The directory contains file ID that points to the FAT entries where the files start.

Linux

Linux is a great variant of UNIX. It has most of the features and, in some cases, better features of UNIX. Furthermore, it runs on lower end machines - 386- class system or higher with 8MB of memory. It requires only as small as 20MB of hard disk space to install it. It requires a CD-ROM drive. Users can get all these features at only $30 to $50 (Information: http://www.cdrom.com or http://www.morse.net). Users who are not decided to buy can even download Linux free from several ftp sites (e.g., sunsite.unc.edu, tsx-11.mit.edu, ftp.uu.net, and wuarchive.wustl.edu).