Making Utilities for MS-DOS
Michael Sokolov
English 4
Mr. Siedlecki
February 1, 1996
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
serve merely to access the internal structures.

These internal structures are extremely version-dependent. Each new major MS-DOS
version up to 4.00 introduced a significant change to these structures.

Applications using them will always be unportable and suffer compatibility
problems. Every computer science textbook would teach you not to mingle with
operating system internals. That’s exactly why these internal structures are

This bring another question, “Why does Microsoft rely on these structures in its
own applications?” To answer this question, we should take a look at an
important class of software products called utilities. Utilities are programs
that don’t serve end users directly, but extend an operating system to help
applications serve end users. To put it another way, utilities are helper
programs. Perhaps the best way to learn when you have to mingle with DOS
internals is to spend some time developing an utility for MS-DOS. A good example
is SteelBox, an utility for on-the-fly data encryption. This development project
have made me think about the use of DOS internals in the first place and it has
inspired me to write this paper.

Utilities like SteelBox, Stacker, DoubleSpace, new versions of SmartDrive, etc.

need to do the following trick: register with DOS as device drivers, get request
packets from it, handle them in a certain way, and sometimes forward them to the
driver for another DOS logical drive. The first three steps are rather
straightforward and do not involve any “illicit” mingling with MS-DOS internals.

The problems begin in the last step. MS-DOS doesn’t provide any documented
“legal” way to find and to call the driver for a logical drive. However, MS-DOS
does have internal structures, called Disk Parameter Blocks (DPBs) which contain
all information about all logical drives, including the pointers to their
respective drivers. If you think of it, it becomes obvious that MS-DOS must have
some internal structures like DPBs. Otherwise how would it be able to service
the INT 21h API requests? How would it be able to locate the driver for a
logical drive it needs to access?
Many people have found out about DPBs in some way (possibly through disassembly
of DOS code). In the online community there is a very popular place for
information obtained through reverse engineering, called The MS-DOS Interrupt
List, maintained by Ralf Brown. This list is for everyone’s input, and the
people who reverse engineer Microsoft’s operating systems often send their
discoveries to Ralf Brown, who includes them into his list. The DPB format and
the INT 21h call used to get pointers to DPBs are also in Interrupt List. As a
result, many programmers, including me, have used this information in their
utilities without much thinking.

However, this is not a good thing to do. DPBs exist since the first release of
MS-DOS as IBM PC-DOS version 1.00, but the DPB format has changed three times
throughout the history. The first change occured in MS-DOS version 2.00, when
the hard disk support, the installable device drivers and the UNIX-like nested
directories were introduced. The second change occured in MS-DOS version 3.00,
when the array of Current Directory Structures (CDSs), a new internal structure,
was introduced to support local area networks and JOIN/SUBST commands. The third
change occured in MS-DOS version 4.00, when 32-bit sector addressing was
introduced and an oversight with storing the number of sectors in a File
Allocation Table (FAT) was fixed. The reader can see that each new major MS-DOS
version up to 4.00 introduced a change in the DPB format. And this is typical
with all MS-DOS undocumented internal structures.

Although one can probably ignore DOS versions earlier than 3.10, he still would
have to deal with two different DPB formats. And prior to DOS version 5.00,
where DPBs were finally documented, no one could be sure that a new DOS version
wouldn’t change the DPB format once again. In the first version of SteelBox, my
utility that needs to know about DPBs in order to do its work, I simply compared
the DOS version number obtained via INT 21h/AH=30h with 4.00. If the DOS version
was earlier than 4.00, I assumed that it has the same DPB format as IBM PC-DOS
versions 3.10-3.30. If the DOS version was 4.00 or later, I assumed that it has
the same DPB format as IBM PC-DOS version 4.xx. However, there are problems with
such assumptions. First, there are some versions of MS-DOS other than IBM PC-DOS,
and some of them have their internal structures different from those of standard
MS-DOS and PC-DOS. For example, European MS-DOS 4.00 returns the same version
number as IBM PC-DOS version 4.00, but its internal structuresmuch more closely
resemble that of PC-DOS version 3.xx. Second, prior to Microsoft’s documenting
of DPBs in MS-DOS version 5.00, there was no guarantee that the DPB format
wouldn’t change with a new DOS version.

When I was developing a new version of SteelBox, I started to think about how to
use DPBs properly and in a version-independent manner. I justified the use of
DOS internals in the first place because I know that a lot of Microsoft’s own
utilities use them extensively. The examples are MS-DOS external commands like
SHARE, JOIN, and SUBST, Microsoft Network, Microsoft Windows, Microsoft CD-ROM
Extensions (MSCDEX), etc. Before we go any further, it should be noted that we
mustn’t be dumping unfairly on Microsoft. Originally I thought that DOS
internals are absolutely safe to use and that Microsoft doesn’t document them
intentionally in order to get an unfair advantage over its competitors. My
reasoning for this was that Microsoft’s own utilities have never stopped working
with a new DOS version.

To find the magic of “correct” use of DOS internals, I started disassembling
Microsoft’s utilities. First I looked at three DOS external commands, SHARE,
JOIN, and SUBST. All three programs check for exact DOS version number match.

This means that they can work only with one specific version of MS-DOS. This
makes sense, given that these utilities are bundled with MS-DOS and can be
considered to be parts of MS-DOS. One of the utilities, SHARE, unlike other DOS
external commands, accesses the DOS kernel variables by absolute offsets in
DOSGROUP, the DOS kernel data segment, in addition to getting pointers to
certain DOS internal structures and accessing their fields. SHARE not only
checks the MS-DOS version number, but also checks the flag at offset 4 in
DOSGROUP. In DOS Internals, Geoff Chappell says that this flag indicates the
format (or style) of DOSGROUP layout (501). If you look at the MS-DOS source
code (I’ll explain how to do it in a few paragraphs), you’ll see that programs
like SHARE access the kernel variables in the following way:
The kernel modules defining these variables in DOSGROUP are linked in with
SHARE’s own modules. Since the assembler always works the same way, the DOS
kernel variables get the same offsets in the SHARE’s copy of DOSGROUP as in the
DOS kernel’s copy. When SHARE needs to access a DOS kernel variable, it loads
the real DOSGROUP segment into a segment register, tells the assembler that the
segment register points to SHARE’s own copy of DOSGROUP, and accesses the
variable through that segment register. Although the segment register points to
one copy of DOSGROUP and assembler thinks that it points to another one,
everything works correctly because they have the same format. The reader can
drawn the following conclusion from this aside: MS-DOS designers have made the
MS-DOS internal structures accessible to other programs only for DOS’own use
(since linking DOS modules in with a program is acceptable only for the parts of
MS-DOS itself).

Having seen that DOS external commands are not a good example for a program that
wants to be compatible with all DOS versions, I turned to Microsoft Network. One
of its utilities, REDIR, is very similar to SHARE in its operation. Like SHARE,
it accesses the DOS kernel variables by absolute offsets. I thought that unlike
SHARE, REDIR is not tied to a specific DOS version. Unfortunatelly, I wasn’t
able to disassemble it, because as a high school student, I don’t have a copy of
Microsoft Network. However, Geoff Chappell says that it has separate versions
for different versions of DOS, just like SHARE. Therefore, I turned to another
utility again.

My next stop was MSCDEX, the utility for accessing the High Sierra and ISO-9660
file systems used by CD-ROMs. Unlike SHARE and REDIR, MSCDEX is not tied to one
specific DOS version. I’m using MSCDEX version 2.21 with MS-DOS version 5.00,
but the same version of MSCDEX can be used with PC-DOS version 3.30. However, it
accesses the DOS kernel variables by absolute offsets in DOSGROUP, just like
SHARE and REDIR. Of course, my question was “How does it do that in a version-
independent manner?” When I disassembled it, I saw that it takes the flag at
offset 4 in DOSGROUP and uses it to determine the absolute offsets of all the
variables it needs. If this flag equals 0, MSCDEX assumes that all offsets it’s
interested in are the same as in DOS versions 3.10-3.30. If this flag equals 1,
MSCDEX assumes that all offsets it’s interested in are the same as in DOS
versions 4.00-5.00. For all other values of this flag MSCDEX refuses to load.

Sharp-eyed readers might notice that this check already makes MSCDEX potentially
incompatible with future DOS versions. The comments in the source code for MS-
DOS version 3.30 (DOSMULT.INC file) refer to MSCDEX, therefore, it had existed
at the time of MS-DOS version 3.30. It is very doubtful that anyone, including
the author of MSCDEX, could know what offsets would the kernel variables in DOS
version 4.00 have at that time. If this is true, an MSCDEX version that predates
MS-DOS version 4.00 won’t run under DOS versions 4.00 and later.

MSCDEX uses the flag at offset 4 in DOSGROUP to determine not only the absolute
offsets of the kernel variables, but also the “style” of all other DOS internals
that had changed with DOS version 4.00. My first thought was that I can use this
flag in my utilities when I need to cope with different “styles” of DOS
internals. However, my next discovery really surprised me and gave me a real
understanding of what I’m doing when I mingle with DOS internals. MSCDEX version
2.21 refuses to run under DOS versions 6.00 and later. So much for the idea that
“Microsoft’s own utilities have never stopped working with a new DOS version.”
In fact, Geoff Chappell refers to this in DOS Internals (501).

The last utility I looked at was Microsoft SmartDrive version 4.00, which is
bundled with Microsoft Windows version 3.10. This utility also uses the DOS
internal structures, including the version-dependent ones. However, unlike
MSCDEX, SmartDrive doesn’t have a “top” DOS version number. It compares the DOS
version number with 4.00 and assumes that DOS similar to versions 3.10-3.30 if
it’s lower than 4.00 and to versions 4.00-5.00 if it’s 4.00 or higher.

SmartDrive assumes that all future DOS versions will be compatible with MS-DOS
version 5.00 at the level of the internal structures.

The lack of clear pattern in the usage of the undocumented DOS internal
structures by Microsoft’s own utilities made me think seriously about the
possibility of safe use of the DOS internals in the first place. Originally I
thought that Microsoft has some internal confidential document that explains how
to use the DOS internals safely, and that anyone having that magic document can
use the undocumented DOS internals as safely as normal documented INT 21h API.

However, the evidence I have obtained through reverse engineering of Microsoft’s
utilities puts the existence of that magic document under question. In
Undocumented DOS Andrew Schulman notes that it is possible that on some
occasions Microsoft’s programmers have found out about the MS-DOS internals not
from the source code or some other internal confidential documents, but from
general PC folklore, just like third-party software developers. For example, the
MWAVABSI.DLL file from the Microsoft Anti-Virus provides a function called AIO_
GetListofLists(). This function calls INT 21h/AH=52h to get the pointer to one
extremely important DOS internal structure. In the MS-DOS source code this
structure is called SysInitVars. However, in Ralf Brown’s Interrupt List and in
general PC folklore is called the List of Lists. This is an indication that
Microsoft’s programmers sometimes act just like third-party software developers
(Schulman et al., Undocumented DOS, 44).

On several occasions I have made references to the MS-DOS source code. However,
most programmers know that the MS-DOS source code is unavailable to non-
Microsoft employees. Therefore, before we go any further, I need to explain how
could I look at the MS-DOS source code. Microsoft gives it to certain companies,
mostly Original Equipment Manufactures (OEMs). Some people can claim that they
are OEMs and get the Microsoft’s documents available only to OEMs (however, this
costs a lot of money). And then some people who don’t care too much about laws
start distributing the confidential information they have. This is especially
easy in Russia, where copyright laws are not enforced. So one way or another,
knowledge of some parts of MS-DOS source code spreads among the people. The MS-
DOS OEM Adaptation Kit (OAK) contains commented source code for some MS-DOS
modules and include files and .OBJ files made from some other modules.

Let’s summarize what we’ve seen so far. MS-DOS, like any other operating system,
has internal structures. Every computer science textbook would teach you not to
rely on an operating system’s internals. In MS-DOS, the internal structures are
undocumented. Microsoft’s own utilities do rely on them. By reverse engineering
these utilities, looking at the MS-DOS source code, and thinking the problem
through one can come to the conclusion that there is absolutely no safe way of
using the MS-DOS internal structures. The only proper way of using them is not
using them at all.

Not later than I have come to this conclusion, my SteelBox development project
returned me back to the reality. No matter how bad it is to use of the MS-DOS
internals, utility developers like me have to do it because they have no other
choice. Now I’m almost sure that this is precisely why Microsoft uses the MS-DOS
internals itself. Before we go any further, I need to clarify one important

Once a programmer asked Microsoft to document the INT 2Fh/AH=11h interface,
generally known as the network redirector interface. Microsoft responded:
The INT 2fh interface to the network is an undocumented interface. Only INT 2fh,
function 1100h (get installed state) of the network services is documented.

Some third parties have reverse engineered and documented the interface (i.e.,
“Undocumented DOS” by Shulman sic, Addison-Wesley), but Microsoft provides
absolutely no support for programming on that API, and we do not guarantee that
the API will exist in future versions of MS-DOS.

This sounds like Microsoft saying, “Here’s where you get the info, but you
better not use it.” (Schulman et al., Undocumented DOS, 495). Some people might
think that Microsoft has internal confidential documents describing the MS-DOS
internals much better than Andrew Schulman’s Undocumented DOS, but there are
indications that the MS-DOS source code is the only “document” Microsoft has
(I’ll address this issue in a few paragraphs). Perhaps the Microsoft’s
programmers themselves use the same documentation as third parties.

So far we have seen that MS-DOS is not a perfect operating system, and it gives
utility developers no other choice but to use its undocumented version-dependent
internals. The reader might ask, “what can we do about it?” First of all, some
of the former undocumented DOS functionality was documented in DOS version 5.00.

The reason for that probably was that some INT 21h functions that were used by
DOS external commands like PRINT don’t actually deal with any DOS internals at
all, and Microsoft had simply underestimated the usefulness of these functions
originally. Microsoft has even documented the DPBs. However, Microsoft’s
documentation says that the DPBs are available only in DOS versions 5.00 and
later, but the reader should remember that the DPB format has changed several
times throughout the history. So in this case Microsoft even restricted
themselves in the ability of making changes in MS-DOS by documenting the DPBs.

However, there are still a lot of undocumented internals in MS-DOS. It should be
noted that documenting them is out of question. This would make it impossible to
make significant changes in MS-DOS, thereby stalling its enhancement. In
Undocumented DOS Andrew Schulman suggests that Microsoft could make an add-in to
MS-DOS that would provide “clean” documented services that would eliminate the
need for the use of DOS internals. Once Microsoft actually did this, when it
introduced the IFSFUNC utility in MS-DOS version 4.00. This utility converted
the “dirty” and extremely version-dependent redirector interface into a device-
driver-like interface. However, this utility was removed from MS-DOS versions
5.00 and later (I’ll explain why in a few paragraphs).

Fortunately, the ill-fated IFSFUNC utility was not the only effort to enhance
MS-DOS. In Microsoft Windows versions 3.00 through 3.11, there is a component
called Win386. It has got its name from Windows/386, its ancestor. In early beta
releases of Microsoft’s Chicago operating system this component was called
DOS386. When Chicago was renamed into Windows 95, this component was given
uninteresting name VMM32. Finally, the beta release of Microsoft C/C++ Compiler
version 7.00 included this component from Microsoft Windows under the name
MSDPMI. I think that the best name for this component is DOS386, so I’ll call it
this way.

Probably the reader would ask, “What this component is?” DOS386 is a
multitasking protected-mode operating system. A close inspection of DOS386
reveals that it has almost nothing to do with Windows, and has a lot to do with
DOS (that’s why I prefer the name DOS386 over Win386). Two of DOS386’s
subcomponents, DOSMGR and IFSMGR, are perhaps the heaviest users of DOS
internals. These modules know a lot about the internals of MS-DOS, and they
provide their own interfaces which in fact can help an utility avoid using DOS
internals. For example, let’s return to our SteelBox utility.

This utility needs to access a file from inside an INT 21h call. Most DOS
programmers know that DOS INT 21h API is non-reentrant. It means that no INT 21h
calls can be made while an INT 21h call is already being serviced. Therefore, an
utility like SteelBox would have to play tricks with DOS internals with all the
consequences. On the other hand, DOS386’s IFSMGR subcomponent provides an
interface that replaces INT 21h. Unfortunately, IFSMGR is documented only in the
Windows 95 Device Development Kit (DDK), and I don’t have a copy of it yet.

However, it is quite possible that the IFSMGR’s interface is reentrant. If it is,
all problems with SteelBox would be immediately solved, and it won’t contain a
single undocumented DOS call. Keep in mind, however, that DOS386 is relatively
new, and perhaps its current version doesn’t provide all the desired
functionality. But certainly DOS386 is definitely a good foundation for a new
operating system.

Although I definitely don’t want to overblame Microsoft, I have to say some
unpleasant truth about this company. In their run for profit, people at
Microsoft violates some principles of free enterprize. In other words, they try
to make a monopoly. One of the unfair things Microsoft does is called
discriminatory documentation. Although the source code for MS-DOS, Microsoft
Network, and other Microsoft products is supposedly unavailable to anyone,
Microsoft has made the source code of some utilities available to selected
vendors (Schulman et al., Undocumented DOS, 495).

Another example is the deliberate incompatibility of some Microsoft products
with Digital Research’s DR-DOS. Some programs, including Microsoft Windows
version 3.10 beta and Microsoft C Compiler version 6.00, contain special code
with sole purpose of making them incompatible with DR-DOS and other DOS
workalikes. Although I’m definitely not a supporter of DOS workalikes, I think
that Microsoft should use fair methods of competition.

Finally, there is a big problem with Microsoft’s packaging of MS-DOS and DOS386.

The most important problem with DOS386 is that it’s currently available to users
only as Win386 in Microsoft Windows. Furthermore, the usual Windows technical
documentation (except the DDK) doesn’t even mention the existence of Win386,
because it’s actually not a part of Windows. As a result, an amasing number of
programmers don’t even know about DOS386 (or Win386), and many of those how do
greatly underestimate its tremendous importance.

Now Windows 95 comes into play. In this package, MS-DOS, DOS386, and Windows are
thrown into one melting-pot. First of all, the integration of MS-DOS and DOS386
is a very good step. Given the volatility of DOS internals, the DOSMGR
subcomponent of DOS386 (which, remember, is perhaps the heaviest user of DOS
internals) cetainly should be tied to one specific DOS version. However, the tie
between DOS/DOS386 and Windows is largely artificial. Try a simple experiment.

Rename KRNL386.EXE file in your WINDOWSSYSTEM directory into something else,
and put something else (COMMAND.COM fits nicely) into that directory under the
name KRNL386.EXE. And then try to run Windows. But instead of running Windows,
this would load and activate Win386 without loading Windows. And there is no
magic in this simple experiment. KRNL386.EXE is the first module of Windows, and
Win386 runs it when it completes its initialization. By putting something else
in place of KRNL386.EXE, one can break the artificial tie between Windows and

At some point of time Microsoft probably throught of making a version of DOS386
which would not be tied to Windows. There was an utility called MSDPMI in the
beta release of Microsoft C/C++ Compiler version 7.00, which was that very
DOS386 without Windows. But now Microsoft is abandoning MS-DOS and everything
else that is not Windows. Microsoft tries to persuade users that Windows 95
doesn’t contain a DOS component, but this is not true. It is simply tied into

Now let’s summarize the above. Microsoft is ignoring the minority users who
don’t like Windows and who want to use MS-DOS and DOS386 without Windows,
because Microsoft cares only about its profit. One person cannot stop them doing
that. Therefore, we, the programmers, should unite. If I call Microsoft alone,
no one would listen to me. But if thousands of us do it together, we might
achieve something. If you have any questions or suggestions about creating an
association of programmers against Microsoft, please send E-mail to Michael
Sokolov at emailprotected
Brown, Ralf. The MS-DOS Interupt List. Not published on paper, available online
for free.

Chappell, Geoff. DOS Internals. New York: Addison-Wesley Publishing Company,

Microsoft Corporation. Microsoft Windows Device Development Kit. Computer
software. Redmond: Microsoft, 1990.

Pietrek, Matt. Windows Internals: The Implementation of the Windows Operating
Environment. New York: Addison-Wesley Publishing Company, 1993.

Schulman, Andrew. , Ralf Brown, David Maxey, Raymond J. Michels, Jim Kyle.

Undocumented DOS: A Programmer’s Guide to Reserved MS-DOS Functions and Data
Structures. New York: Addison-Wesley Publishing Company, 1994.

Schulman, Andrew. , David Maxey, Matt Pietrek. Undocumented Windows: A
Programmer’s Guide to Reserved Microsoft Windows API Functions. New York:
Addison-Wesley Publishing Company, 1992.



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