Making Utilities for MS-DOS

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Making Utilities for MS-DOS

These days, when computers play an important role in virtually all aspects of

our life, the issue of concern to many programmers is Microsoft's hiding of

technical documentation. Microsoft is by far the most important system software

developer. There can be no argument about that. Microsoft's MS-DOS operating

system has become a de facto standard (IBM's PC-DOS is actually a licensed

version of MS-DOS). And this should be so, because these systems are very well

written. The people who designed them are perhaps the best software engineers in

the world.

But making a computer platform that is a de facto standard should imply a good

deal of responsibility before the developers who make applications for that

platform. In particular, proper documentation is essential for such a platform.

Not providing enough documentation for a system that everyone uses can have

disastrous results. Think of it, an operating system is useless by itself, its

sole purpose is to provide services to applications. And who would be able to

develop applications for an operating system if the documentation for that

system is confidential and available only to the company that developed it?

Obviously, only the company that has developed that operating system will be

able to develop software for it. And this is a violation of the Antitrust Law.

And now I start having a suspicion that this is happening with Microsoft's

operating systems. It should be no secret to anyone that MS-DOS contains a lot

of undocumented system calls, data structures and other features. Numerous books

have been written on this subject (see bibliography). Many of them are vital to

system programming. There is no way to write a piece of system software, such as

a multitasker, a local area network, or another operating system extension,

without knowing this undocumented functionality in MS-DOS. And, sure enough,

Microsoft is using this functionality extensively when developing operating

system extensions. For example, Microsoft Windows, Microsoft Network, and

Microsoft CD-ROM Extensions (MSCDEX) rely heavily on the undocumented internals

of MS-DOS.

The reader can ask, "Why do they leave functionality undocumented?" To answer

that question, we should look at what this "functionality" actually is. In MS-

DOS, the undocumented "functionality" is actually the internal structures that

MS-DOS uses to implement its documented INT 21h API. Any operating system must

have some internal structures in which it keeps information about disk drives,

open files, network connections, alien file systems, running tasks, etc. And MS-

DOS (later I'll call it simply DOS) has internal structures too. These

structures form the core of undocumented "functionality" in MS-DOS. This

operating system also has some undocumented INT 21h API functions, but they

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