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
File Access Table(FAT)
- 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.
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.
