No major operating system places this restriction on

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Unformatted text preview: r so that they can be easily ported to other machines. 3. Shell. This layer has the command interpreter. MS-DOS provides a command-line interface. The MS-DOS shell, known as command.com, has about 70 commands that are partitioned into two categories - internal commands and external commands. Internal commands are those that are always memory resident, whereas external commands are those, which normally reside on disk and are loaded into the memory whenever they have to be executed. Obviously, external commands are executed much more slowly than internal ones. Later a GUI known as DOS shell was also added to this layer, which was largely a menudriven user interface. To give an idea of what the shell commands look like and what they do, some of the shell commands are given in Figure 14.28. Both the MS-DOS and Unix versions have been given to show that the commands names normally signify their task so that they can be easily remembered and used. Moreover, note that the commands are usually short so that you have to type only a few characters to invoke a task. Most of these commands require arguments, which are specific pieces of information required to complete the command. For example, the del command requires the name of the file you want to delete. Limitations of MS-DOS In spite of its wide popularity in the 1980s, MS-DOS could not continue to be popular like Unix for a long period. The following are some of the reasons often quoted for its failure: 1. It was a single user, single task operating system (did not support multitasking). That is, it assumed that only one user uses the machine and runs only one program at a time. 2. Since it was a single user, single task operating system, it did not have any protection features to protect two or more programs stored in memory from one another. 3. It had lintited program addressability to 640 Kbytes. That is, it was designed, to support programs whose maximum size was 640 Kbytes. At first, this was not a problem, since those days machines had only 64 Kbytes of R...
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