III What is DOS?
DOS is a program that is divided into several sub-programs. There are three major portions of DOS.
The first two of the portions listed below are hidden from view. The three portions are:
The third sub-program is visible and actually replaceable. This program is called COMMAND.COM. The function of COMMAND.COM is to interface with you, the operator (user); in whatever human language you are most natural in using.
COMMAND.COM takes terse commands in your native human communication language, and interprets them into something that one of the hidden sub-programs can use.
This is why you will sometime hear COMMAND.COM referred to as a command interpreter. On other occasions you may hear COMMAND.COM referred to as a shell. In the days when DOS was the way to talk to the PC, a firm had created a replacement to COMMAND.COM, known as 4DOS. This was sold as a more powerful command shell/interpreter. For all practical purposes these days, though, you will be using the COMMAND.COM that shipped with your operating system.
Under the hood, the two hidden programs MSDOS.SYS and IO.SYS perform functions that the computer needs to do your bidding. IO.SYS is responsible for in and out functions.
This literally means in and out in ways you may not think of. As a case in point, think about the keyboard. Its purpose is to input into the computer, so it must talk to IO.SYS.
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