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I
UNDOCUMENTED
A PROGRAMMER’S GUIDE
D RESERVED MS-DOS FUNCTIONS
AND DATA STRUCTURES
LNDREW SCHULMAN, RAYMOND J. MICHELS, JIM KYLE,
TIM PATERSON, DAVID MAXEY, AND RALF BROWN;0]
UNDOCUMENTED DOS
UNDOCUMENTED DOS
A PROGRAMMER'S GUIDE
TO RESERVED MS-DOS® FUNCTIONS
AND DATA STRUCTURES
ANDREW SCHULMAN, RAYMOND J. MICHELS,
JIM KYLE, TIM PATERSON, DAVID MAXEY, RALF BROWN
TT
Addison-Wesley Publishing Company, Inc.
Reading, Massachusetts Menlo Park, California New York
Don Mills, Ontario Wokingham, England Amsterdam Bonn
Sidney Singapore Tokyo Madrid San Juan Paris
Seoul Milan Mexico City Taipei
Many of the designations used by manufacturers and sellers to distinguish their
products are claimed as trademarks. Where those designations appear in this
book and Addison-Wesley was aware of a trademark claim, the designations
have been printed in initial capital letters.
Library of Congress Cataloging-in-Publication Data
Undocumented DOS : a programmer's guide to reserved MS-DOS
functions and data structures / by Andrew Schulman (general
editor)... [etal.].
p. cm.
Includes bibliographical references (p. 679) and index.
ISBN 0-201-57064-5
1. MS-DOS (Computer operating system) I. Schulman, Andrew.
QA76.76.063U53 1990
005.4'46—dc20 90-46992
Copyright © 1990 by Andrew Schulman, Raymond J. Michels, Jim Kyle, Tim
Paterson, David Maxey, and Ralf Brown
All rights reserved. No part of this publication may be reproduced, stored in a re-
trieval system, or transmitted, in any form or by any means, electronic, mechani-
cal, photocopying, recording, or otherwise, without the prior written permission
of the publisher. Printed in the United States of America. Published simulta-
neously in Canada.
Managing Editor: Amorette Pedersen
Copy Editor: Barbara Tilly
Set in 10.5-point Palatino by Benchmark Productions
3 4 5 6 78 9 10 -MW-94939291
Third printing, June 1991
Table of Contents
Introduction ix
Chapter 1
Regarding the Use of Undocumented DOS 1
Why Leave Functionality Undocumented?... Why Is Undocumented
DOS Important?... Permission, But Not Support... Fear of the
Undocumented... Reserved and Undocumented 80x86 Features...
Where Angels Fear to Tread: Programs That Use Undocumented DOS...
Ain't Misbehavin'... Simulated DOS... Categories of Undocumented
DOS... The Case of the Missing One-Quarter...
Chapter 2
29
Programming for Documented and Undocumented DOS: A Comparison
Using Documented DOS Functions... Using Undocumented DOS ...
When Not to Use Undocumented Features... Verifying Undocumented
DOS... An Important Special Case: Novell NetWare...
Undocumented DOS Calls from Protected Mode...
v
vi UNDOCUMENTED DOS
Chapter 3
MS-DOS Resource Management: Memory, Processes, Devices 81
Memory Management... Process Management... DOS Termination
Address... Device Management... Loading Device Drivers from the
DOS Command Line...
Chapter 4
The DOS File System and Network Redirector 153
The Physical Disk: How DOS Sees It... The List of Lists... Current
Directory Structure (CDS)... System FCBs... System File Tables (SFTs)
and Job File Table (JFT)... Making Alterations... Indirect Server Call...
The MS-DOS Network Redirector... Conclusion...
Chapter 5
Memory Resident Software: Pop-ups and Multitasking 261
TSR: It Sounds Like a Bug, But It's a Feature... Where Does
Undocumented DOS Come In?... MS-DOS TSRs... The Generic TSR...
TSR Programming in Microsoft C... Jiggling the Stack... Undocu-
mented DOS Functions for TSRs... Inside the Generic TSR... Writing
TSRs with the DOS Swappable Data Area (SDA)... Removing a TSR...
Sample TSR Programs... Multitasking TSR...
Chapter 6
Command Interpreters 353
Requirements of a Command Interpreter... How COMMAND.COM
Works... Alternatives to COMMAND.COM... Sample Program:
Master Environment Editor... Conclusion...
Chapter 7
The MS-DOS Debugger Interface 427
Loading Without Executing... Debuggers and Windows Memory
Movement... Conclusion...
Table of Contents vii
Chapter 8
INTRSPY: A Program for Exploring DOS 451
Why a Script-Driven, Event-Driven Debugger?... A Guided Tour...
INTRSPY User's Guide... Using INTRSPY... Writing a Generic
Interrupt Handler... The Problem with Intel's INT...
Implementation...
Appendix A
Undocumented DOS Functions 495
Appendix B
Annotated Bibliography 679
Index 685
Introduction
There is a story behind this book:
For months, a coworker of mine, having been misinformed that I was some
sort of "DOS hotshot," had been pestering me to write a program to convince MS-
DOS that it no longer had an L: drive. I never quite figured out what the program
was for, but apparently customers were clamoring for a way to remove the
Microsoft CD-ROM Extensions (MSCDEX) from memory, and this program had
something to do with it. Anyway, I had tried several different approaches, includ-
ing using MS-DOS's Cancel Device Redirection function (INT 21h Function 5Fh
Subfunction 04h), all without success.
I then came across Ralf Brown's "Interrupt List," part of which now forms the
appendix to this book. One of the DOS functions listed by Ralf, Get List Of Lists
(INT 21h Function 52h), was marked "DOS 2+ internal." It was not listed in the
official DOS documentation: IBM's DOS 3.3 Technical Reference jumps straight
from Find Next (INT 21 h Function 4Fh) to Get Verify Setting (Function 54h), with
no mention of any functions between 4Fh and 54h. Even Ray Duncan's Advanced
MS-DOS Programming simply lists Function 52h as "Reserved." Anyway, the un-
documented DOS function described in Ralf's list turned out to be exactly what
was needed to write a program to "cancel" the L: drive. Once I knew about INT
21 h Function 52h, writing DRVOFF (as the utility was called) was trivial. Without
this information, it was impossible.
IX
x UNDOCUMENTED DOS
This introduction is not the place to go into the details of DRVOFF. The
source code for a similar program, and a full explanation, appear in chapter 4 of
this book. For now, the point is simply that here was a very real need that could be
met only using a DOS function that doesn't appear in Microsoft's or IBM's documentation.
Of course, I was already aware, like most DOS programmers, that there are
undocumented DOS functions. In fact, I had accumulated a folder marked
"Undocumented DOS" that contained various clippings from PC Magazine, Dr.
Dobb's Journal, and Programmer's Journal, printouts of source files downloaded
from bulletin boards, and printouts of discussions held in on-line conferences
such as ibm.dos/secrets on BIX. I also found a number of books that contained
discussions of undocumented DOS. But its randomness bothered me, and led to
this book. The snippets I had collected were not only random, they were also
sometimes contradictory, and they never seemed to take sufficient account of dif-
ferences between DOS versions. It was clear that something more comprehensive
was needed: a book that attempted to list all undocumented DOS functions and
data structures in one place, detailing the DOS version differences (even for such
odd versions of DOS as the compatibility box in OS/2), that frankly acknowl-
edged the quirks of working with undocumented DOS, and that showed how to
use undocumented DOS safely when writing programs.
Who better to write such a book than the software engineers who had al-
ready written elsewhere on the subject? Jim Kyle was an obvious choice, because
he had prepared the material on undocumented DOS in the second edition of
Que Corporation's popular DOS Programmer's Reference. Ray Michels was too, be-
cause he had written the chapter on "Undocumented DOS" in the Waite Group's
MS-DOS Papers. Ralf Brown maintained the one truly definitive, absolutely
reliable list of DOS calls, and it was clear that this "Interrupt List" had to be trans-
formed into the book's appendix. Tim Paterson had not so much written about
MS-DOS; he in fact wrote MS-DOS itself (Version 1.0) and was the perfect person
not only to describe one important facet of undocumented DOS, but also to act as
technical advisor for the entire book. It was also clear that the book needed to
give readers a utility that would allow them to explore DOS without disassembling,
and David Maxey, toolsmith extraordinaire, was the obvious choice to write
INTRSPY.
