`
`OLYMPUS EX. 1015 -1l393
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`OLYMPUS EX. 1015 - 1/393
`
`

`

`
`
`THE PROGRAMMER’8
`
`TECHNICAL
`
`REFERENCE .-
`
`MS-DOS, IBM PC & Compatibles
`
`Dave Williams '1
`
`SIGMA PRESS —- Wilmslow, United Kingdom '
`
`'.
`
`OLYMPUS EX. 1015 - 2/393
`
`

`

`
`
`
`
`Copyright ©, D. Williams, 1990
`
`All Rights Reserved. No part of this publication may be reproduced, stored in a
`retrieval system, or transmitted in any form or by any means, electronic, mechanical,
`photocopying, recording or otherwise, without prior written permission.
`
`First published in 1990 by
`
`Sigma Press, 1 South Oak Lane, Wilmslow, Cheshire SK9 6AR, England.
`Reprinted, 1992.
`
`British Library Cataloguing in Publication Data
`
`A CIP catalogue record for this book is available from the British Library.
`
`ISBN: 1-85058-199-1
`
`Typesetting and design by
`
`Sigma Hi-Tech Services Ltd
`
`Printed in Malta by
`Interprint Ltd.
`
`_ Distributed by
`
`John Wiley & Sons Ltd., Baffins Lane, Chichester, West Sussex, England.
`
`Acknowledgement of copyright names
`
`Within this book, various proprietary trade names and names, as listed below, are;
`protected by copyright and are mentioned for descriptive purposes:
`
`UNIX, AT&T, Allied Telephone and Telegraph; AST, RAMpage! AST Corporation;
`Atari, ST, Atari Computer; Borland, Turbo C, Turbo Pascal, Turbo Lightning,
`Borland; Amiga 2000. Commodore Business Machines; Compaq. Deskpro, Compaq
`Computer Corporation; Corona, Cordata, Cordata Computer; I 0-Net, Fox Research,
`Inc.; Smartmodem, Hayes; IBM, PC, Per, PC/XT, PC/AT, XT/286, PS/2, TopView.
`DOS. PC-DOS, Micro Channel 3270 PC, RT PC, Token Ring, IBM Corporation;
`Intel,
`iAPX286,_ iAPX386, LIM EMS, Communicating Applications Standard,
`Intel.
`Corporation; Logitech, Logimouse, Logitech, Inc.; Microsoft, MS. MS—DOS, 05/2,
`Xenix, Windows, Windows/286, Windows/386, Microsoft Networks, LIM EMS, XMA,
`Microsoft Corp.; Mouse Systems, Mouse Systems Corp.; Novell, NetWare,
`Novell Corp; Destiew, Quarterdeck Office Systems; ARC, SEAware,
`Inc.;
`DoubleDOS, Softlogic; TaskView, Sunny Hill Software; Tandy, Tandy Corp.; Zenith.
`Z-100, Zenith Radio Corporation; ShowPartner, Paintbrush, ZSoft Corporation;
`‘LIM 4.0’ and ‘Expanded Memory Specification’ are copyright Lotus Development
`Corp, Intel Corp, and Microsoft Corp; ‘EEMS',
`'AQA 3.1’ and 'Enhanced Expanded
`Mem0ry Specification’ are copyright by Ashton-Tate, Quadram, and AST-Various
`other names are trademarks of their respective companiesFull acknowledgment
`is
`hereby made of all such protection.
`
`OLYMPUS EX. 1015 - 3/393
`
`

`

`
`
`Preface
`
`
`This book is a technical reference. It is NOT a tutorial. It is intended to replace the various (ex-
`pensive) references needed to program for the DOS environment, that stack of magazines
`threatening to take over your work area, and those odd tables and charts you can never find
`when you need them.
`
`The various Microsoft and IBM publications and references don’t always have the same infor-
`mation. This has caused some consternation about the ‘undocumented’ features to be found in
`DOS. In general, if a call doesn’t appear in the IBM DOS 'Iechnical Reference it is considered
`‘undocumented’ although it may be in common use.
`-
`
`Microsoft’s offical policy toward DOS has been to put the burden of documenting and suppor-
`ting their product to their vendors. Microsoft will not answer any questions concerning DOS di-
`rectly since they don’t officially support it. This leaves what information IBM and other OEMs
`(DEC, Zenith, et al) have chosen to publish, and the information obtained from programmers
`who’ve poked around inside it.
`
`Now that Microsoft is sellingMSDOS 3.3 and 4.0 over the counter they seem to be dragging '
`their feet over whether they will have to support the generic version since it doesn’t have an .
`OEM name on it anymore. In view of their push to 08/2 (OS/2! Just Say N0!) further support of
`DOS seems unlikely.
`
`A project this size takes a LOT of time and effort. I’ve tried to verify as much of the information
`I’ve received as I could, but there’s just too much for absolute certainty.
`
`OLYMPUS EX. 1015 - 4/393
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`

`

`
`
`OLYMPUS EX. 1015 - 5/393
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`

`

`
`
`Contents
`
`Chapter 1: DOS and the IBM PC
`
`Chapter 2: CPU Port Assignments, System Memory Map, BIOS Data Area,
`Interrupts 00h to 09h
`
`Chapter 3: ThePC ROM BIOS
`
`Chapter4: DOS Interrupts and Function Calls
`
`Chapter 5: Interrupts 22h Through 86h
`
`Chapter 6: DOS Control Blocks and Work Areas
`
`Chapter7: DOS File Structure
`
`Chapters: DOS Disk Information
`
`Chapter 9: Ins tallable Device Drivers
`
`Chapter 10: Expanded and Enhanced Expanded Memory Specifications
`
`Chapter 11: Conversion Between MSDOS and Foreign Operating Systems
`
`Chapter 12: Microsoft Windows A.P.I.
`
`Chapter 13: Network Interfacing
`
`Chapter 14: Mouse Programming '
`
`Chapter 15: Register-Level Hardware Access
`
`Chapter 16: Video Subsystems and Programming
`
`Appendix 1: Keyboard Scan Codes
`
`Appendix 2: Standard ASCII Character Codes
`
`Appendix 3: ASCII Control Codes
`
`Appendix 4: IBM PC Interrupt Usage
`
`Appendixs: List of IBM PC—XT—AT-PS/Z Diagnostic Error Codes
`
`Appendix 6: Pinouts For Various Interfaces
`
`Appendix 7: ANSLSYS
`
`Bibliography
`
`Index
`
`10
`
`25
`
`54
`
`98
`
`130
`
`140
`
`151
`
`171
`
`185
`
`208
`
`210
`
`269
`
`300
`
`310
`
`.315
`
`328
`
`342
`
`345
`
`347
`
`349
`
`358
`
`370
`
`374
`
`380
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`OLYMPUS EX. 1015 - 6/393
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`

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`

`

`
`
`DOS and the IBM PC
`
`
`Some History
`
`Development of MS-DOS/PCDOS began in October 1980, when IBM began searching the mar—
`ket for an operating system for the yet-to-be-introduced IBM PC. Microsoft had no real opera-
`ting system to sell, but after some research licensed Seattle Computer Products’ 86-DOS opera-
`ting system, which had been written by a man named Tim Paterson earlier in 1980 for use on that
`company’s line of 8086, 8100 bus micros. 86-DOS (also called QDOS, for Quick and Dirty
`Operating System) had been written as more or less a 16-bit version of CPM, since Digital Re-
`search was showing no hurry in introducing CP/M-86.
`This code was hurriedly polished up and presented to IBM for evaluation. IBM had originally in-
`tended to use Digital Research’s CP/M operating system, which was the industry standard at the
`time. Folklore reports everything from obscure legal entanglements to outright snubbing of the
`IBM representatives by Digital. Irregardless, IBM found itself left with Microsoft’s offering of
`"Microsoft Disk Operating System 1.0". An agreement was reached between the two, and IBM
`‘ agreed to accept 86-DOS as the main operating system for their new PC. Microsoft purchased
`all rights to 86—DOS in July 1981, and "IBM PC-DOS 1.0" was ready for the introduction of the
`IBM PC in October 1981. IBM subjected the operating system to an extensive quality-assurance
`program, reportedly found well over 300 bugs, and decided to rewrite the programs. This is why
`PC-DOS is copyrighted by both IBM and Microsoft.
`
`It is sometimes amusing to reflect on the fact that the IBM PC was not originally intended to run
`MS-DOS. The target operating system at the end of the development was for a (not yet in exist-
`ence) 8086 version of CP/M. On the other hand, when DOS was originally written the IBM PC
`did not yet exist! Although PC-DOS was bundled with the computer, Digital Research’s CP/M—
`86 would probably have been the main operating system for the PC except for two things - Digi-
`tal Research wanted $495 for CP/M-86 (considering PC-DOS was essentially free) and many
`software develOpers found it easier to port existing CP/M software to DOS than to the new ver-
`sion of CP/M. Several computer magazines claimed that Digital Research aided IBM in writing
`DOS 4.0, which was subsequently licensed back to Microsoft, which has dropped further devel—
`opment of the operating system to tilt at the windmills of 08/2. 08/2? Not yet! After using DR-
`DOS 3.4 and noting its behaviour, I now tend to seriously doubt Digital had any dealings with
`PC—DOS 4.0.
`
`MS-DOS and PC-DOS have been run on more than just the IBM—PC and clones. Some of the
`following have been done:
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`

`

`
`
`2
`
`The Programmer’s Technical Reference
`
`Hardware PC Emulation:
`Commodore Amiga 2000
`IBM PC/AT
`Atari 400/800
`Apple Macintosh
`Atari ST
`Apple II
`
`Software PC Emulation:
`Atari ST
`
`Apple Macintosh
`
`DOS Emulation:
`
`05/2
`QNX
`SunOS
`X'enix
`
`)
`
`What is DOS?
`
`8088 or A2286D 80286 Bridge Board
`80286 AT adapter
`Co -Power 88 board
`AST 80286 board
`PC-Ditto II cartridge
`"IranSPC8088 board, QuadRam QuadLink
`
`PC-Ditto I
`
`So ftPC
`
`.
`
`DOS emulation in "Compatibility Box"
`DOS window
`DOS window
`DOS emulation with DOSMerge
`
`DOS exists as a high-level interface between an application program and the computer. DOS
`stands for "Disk Operating System“, which reflects the fact that'its main original purpose was to
`provide an interface between the computer and its disk drives.
`
`DOS now lets your programs do simple memory management, 1/0 from the system console, and
`assorted SyStem tasks (time and date, etc) as well as managing disk operations. Versions 3.1 and
`up also incorporate basic networking functions.
`
`With the introduction of installable device drivers and TSR (terminate but stay resident) pro—
`grams in DOS 2.0, the ba51c DOS functions may be expanded to cover virtually any scale of oper-
`ations required.
`
`Other Operating Systems
`
`There are a number of compatible replacements for MicroSoft’s MS-DOS. Some are:
`
`(multitasking, multiuser)
`Consortium Technologies MultiDOS
`(multitasking)
`Digital Research Concurrent DOS
`(for 80386 computers)
`Digital Research Concurrent DOS 386
`Digital Research Concurrent DOS XM (multitasking, multiuser)
`Digital Research DR-DOS 3.31 and 4.0
`(PC-DOS clones)
`PC-MOS/386
`(multitasking, multiuser)
`Wendin-DOS
`(multitasking, multiuser)
`VM/386
`(multitasking)
`A
`
`Various other operating systems are available for the IBM PC. These include:
`
`Digital Research CP/M-86
`Digital Research Concurrent CP/M-86 (multitasking)
`Minix (multitasking UNIX workalike)
`Pick (database-operating system)
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`

`

`
`
`DOS and the IBM PC
`
`3
`
`QNX (multitasking, multiuser)
`UNIX (various systems from IBM itself, Microsoft-5C0, Bell, and various UNIX clones, single
`and multi user) (AIX, Xenix, AT&T System V, etc.)
`
`"Shell" programs exist which use DOS only for disk management while they more or less com-
`prise a newo perating system. These include:
`
`Destiew
`Windows
`OmniView
`GEM
`
`TopView’
`- TaskView
`
`Specific Versions of MS/PC-DOS
`
`DOS 1.x is essentially 86-DOS. DOS 2.x kept the multiple file layout (the two hidden files and
`COMMANDCOM) but for all practical purposes is an entirely different operating system with
`backwards compatibility with 1.x. I seriously doubt there has been much code from 1.x retained
`in 2.x. DOS 3.x is merely an enhancement of 2.x; there seems little justification for jumping a
`whole version number. DOS 4.0, originating as it did from outside Microsoft, can justify a ver-
`sion jump. Unfortunately, 4.x seems to have very little reason to justify its existence - virtually all
`of its core features can be found in one version or another of DOS 3.x.
`
`DOS version nomenclature: major.minor.minor. The digit to the left of the decimal point indi-
`cates a major DOS version change. 1.0 was the first version. 2.0 added support for subdirec-
`tories, 3.0 added support for networking, 4.0 added some minimal support for Lotus—Intel-
`Microsoft EMS.
`
`The first minor version indicates customization for a major application. For example, 2.1 for the
`Per, 3.3 for the PS/23. The second minorversion does not seem to have any particular meaning.
`I The main versions of DOS are:
`
`,
`original release
`August 1981
`PC—DOS 1.0
`bugfix, double sided drive support
`May 1982
`PC-DOS 1.1
`for early compatibles
`June 1982
`MS-DOS 1.25
`for PC/XT, Unix-type subdirectory support
`March 1983
`PC-DOS 2.0
`for Per, bugfixes for 2.0
`October 1983
`PC-DOS 2.1
`compatible equivalent to PC-DOS 2.1
`MS-DOS 2.1 1 October 1983
`1.2 meg drive for PC/AT, some new system calls
`PC-DOS 3.0
`August 1984
`bugfix for 3.0, implemented network support
`PC-DOS 3.1
`November 1984
`compatible; extended foreign language support
`MS—DOS 2.25 October 1 985
`720k 3.5 inch drive support for Convertible
`PC-DOS 3.2
`December 1985
`for PS/Z series, 1.44 meg, multiple DOS partitions
`PC-DOS 3.3
`April 1987
`over-32 meg DOS partitions, new function calls
`MS-DOS 3.3 1 November 1987
`minor EMS support, some new function calls
`PC-DOS 4.0
`August 1988
`Microsoft version with some bugfixes
`MS-DOS 4.01
`January 1989
`. IBM’s PC-DOS is considered to be the "standard" version of DOS; Microsoft has sold MS-DOS
`over the counter only since version 3.2 (previously, Microsoft sold its versions only to OEMs).
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`

`

`
`
`4
`
`The Programmer’s Technical Reference
`
`Most versions of DOS functionally duplicate the external DOS commands such as DISKCOPY,
`etc. Although Microsoft announced that they would sell MS-DOS 4.0 only to OEMs, they ap-
`parently changed the policy and are now selling it over the counter.
`
`Some versions of MS—DOS varied from PC-DOS in the available external commands. Some
`
`OEMs only licensed the basic operating system code (the xDOS and xBIO programs, and
`COMMANDCOM) from Microsoft, and either wrote the rest themselves or contracted them
`from outside software houses like Phoenix. Most of the external programs for DOS 3.x and 4.x
`are written in "C" while the 1.x and 2.x utilities were written in assembly language. Other OEMs
`required customized versions of DOS for their specific hardware configurations, such as Sanyo
`55x and early Tandy computers, which were unable to exchange their DOS with the IBM version.
`
`At least two versions of DOS have been modified to be run entirely out of ROM. The Sharp
`PC5000 had MS-DOS 1.25 in ROM, and the Toshiba 1000 and some Tandy 1000 models have
`MS-DOS 2.11 in ROM. Digital Research has also announced its DR-DOS is available in a ROM
`version and Award Software is marketing DOS cards to OEMs as a plug-in.
`
`PC-DOS 3.0 was extremely buggy on release. It does not handle the DOS environment correctly
`and there are numerous documented problems with the batch file parser. The network support
`code is also nonfunctional in this DOS version. It is recommended that users upgrade to at least
`version 3.1.
`
`DEC MS-DOS versions 2.11 for the Rainbow had the ANSISYS device driver built into the
`
`main code. The Rainbow also used a unique quad density, single-sided floppy drive and its DOS
`had special support for it.
`
`IBM had a version 1.85 of PC-DOS in April 1983, after the“ introduction of DOS 2.0. It was evi-
`dently for internal use only, supported multiple drive file searches (a primitive form of PATH),
`built in MODE commands for screen support, a /P parameter for TYPE for paused screens, an
`editable command stack like the public domain DOSEDITCOM utility, and could be set up to
`remain completely resident in RAM instead of a resident/transient part like normal DOS. It is a
`pity some of the neat enhancements didn’t make it into DOS 2.0. IBM also had an "internal use
`only" version 3.4, evidently used while developing DOS 4.0.
`
`numbering system.
`Some versions of DOS used in compatibles do not maintain the 1.x, 2.x,
`Columbia Data Products computers labelled DOS 1.25 as DOS 2.0. Early Compaqs labelled
`DOS 2.0 as DOS 1.x. Other versions incorporated special features - Compaq DOS 3.31 and
`Wyse DOS 3.21 both support 32-bit file allocation tables in the same fashion as DOS 4.x.
`
`According to PC'Week Magazine, July 4, 1988, Arabic versions of MS-DOS are shipping with a
`hardware copy-protection system from Rainbow Technologies. This is similar to the short-lived
`system used by AutoCAD 2.52 and a very few other MS-DOS programs, where an adapter block
`is plugged into the parallel port and software makes use of coded bytes within the block. This
`type of copy protection has been common on Commodore products for several years, where it is
`called a "dongle".
`
`The AutoCAD dongle was defeated by a small program written within weeks of version 2.52’s
`debut. Version 2.62 was released 3 months later, without the dongle. The DOS dongle will, how-
`ever, prevent the system from booting at all unless it is found.
`
`This makes the Arabic version of MS-DOS the first copy-protected operating system, a dubious
`distinction at best. The modifications to the operating system to support the dongle are not
`known at this time. Frankly, it would seem that burning the operating system into ROMS would
`be cheaper and simpler.
`
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`
`

`

`
`
`DOS and the IBMPC
`
`'
`
`5
`
`Versions of DOS sold in Great Britain are either newer than those sold in the US or use a differ—
`ent numbering system. DOS 3.4, 4.0, 4.1, 4.2, and 4.3 had been released here between the US re-
`leases of 3.3 and 4.0.
`
`MicrosOft changed their OEM licensing agreements between DOS versions 2.x and 3.x. OEM
`versions of DOS 3): must maintain certain data areas and undocumented functions in order to
`
`provide compatibility with the networking features of the operating system. For this reason,
`resident programs will be much more reliable when operating under DOS 3.x.
`
`IBM’s release of DOS 4.0 (and the immediate subsequent release of a bugfix) is a dubious step
`"forward". DOS 4.0 is the first version of DOS to come with a warranty; the catch is that IBM
`warrants it only for avery slim list of IBM-packaged software. 4.0 has some minor EMS support,
`support for large hard disks, and not much else. \Vith its voracious RAM requirements and lack
`of compatibility with previous versions of DOS (many major software packages crash under
`DOS 4.0), plus the increase in price to a cool $150, there has been no great rush to go to the ne-
`west DOS
`
`The Operating System Hierarchy
`
`The Disk Operating System (DOS) and the ROM BIOS serve as an insulating layer between the
`application program and the machine, and as a source of services to the application program.
`
`As the term ‘system’ might imply, DOS is not one program but a collection of programs de—
`signed to work together to allow the user access to programs and data. Thus, DOS consists of
`several layers of "control" programs anda set of "utility" programs.
`
`The system hierarchy may be thought of as a tree, with the lowest level being the actual hard-
`ware. The 8088 or V20 processor sees the computer’s address space as a ladder two bytes wide
`and one million bytes long. Parts of this ladder are in ROM, parts in RAM, and parts are not as-
`signed. There are also various "ports" that the processor can use to control devices.
`
`The hardware is normally addressed by the ROM BIOS, which will always know where every-
`thing is in its particular system. The chips may usually also be written to directly, by telling the
`processor to write to a specific address or port. This sometimes does not work as the chips may
`not always be at the same addresses or have the same functions from machine to machine.
`
`DOS Structure
`
`DOS consists of four components:
`The boot record
`
`The ROM BIOS interface (IBMBIO.COM or IO.SYS)
`The DOS program file (IBMDOS.COM or MS-DOS.SYS)
`The command processor (COMMANDCOM or aftermarket replacement)
`
`The Boot Record
`
`The boot record begins on track 0, sector 1, side 0 of every diskette formatted by the DOS FOR-
`MAT command. The boot record is placed on diskettes to produce an error message if you try to
`start up the system with a non-system diskette in drive A. For hard disks, the boot record resides
`
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`

`

`
`
`6
`
`The Programmer’s Technical Reference
`
`on the first sector of the DOS partition. All media supported by DOS use one sector for the boot
`record.
`’
`
`Read only Memory (ROM) BIOS Interface and Extensions
`
`The file IBMBIO.COM or IO.SYS is the interface module to the ROM BIOS. This file provides
`a low—level interface to the ROM BIOS device routines and may contain extensions or changes
`to the system board ROMs. Some compatibles do not have a ROM BIOS to extend, and load the
`entire BIOS from disk (Sanyo 55x, Viasyn machines). Some versions of MS—DOS, Such as those
`supplied to Tandy, are named IBMBIO.COM but are not IBM files.
`
`These low-level interface routines include the instructions for performing operations such as
`displaying information on the screen, reading the keyboard, sending data out to the printer,
`operating the disk drives, and so on. It is the operating system’s means of controlling the hard-
`ware. IBMBIO.COM contains any modifications or updates to the ROM BIOS that are needed
`to correct any bugs or add support for other types of hardware such as new disk drives. By using
`IBMBIO.COM to update the ROM BIOS on the fly when the user turns on their computer,
`IBM does not need to replace the ROM BIOS chip itself, but makes any corrections through the
`cheaper and easier method of modifying the IBMBIO.COM file instead.
`
`IBMBIO.COM also keeps track of hardware operations on an internal stack or "scratch pad"
`area for the operating system to save information such as addresses it will need, etc. An example
`of the use for this stack can be seen when running a program such as a word processor. If you
`have told the word processor to save your letter, it will write the data to your disk. During this
`time, if you start typing some more information, the keyboard generates a hardware interrupt.
`Since you don’t want the process of writing the information to the disk to be interrupted, DOS
`allocates a slot in the stack for the keyboard’s hardware interrupt and when it gets a chance,
`(probably after the data has been written to the disk), it can process that interrupt and pick up
`the characters you may have been typing. The STACKS= command in DOS 3.2+‘s
`CONFIGSYS file controls the number of stack frames available for this purpose.
`
`IBMBIO.COM also reads your CONFIGSYS file and installs any device drivers (Le.
`DEVICE=ANSI.SYS) or configuration commands it may find there.
`
`The DOS Program
`
`The actual DOS program is the file IBMDOS.COM or MS—DOS.SYS. It provides a high-level
`interface for user (application) programs. This program consists of file management routines,
`data blocking/deblocking for the disk routines, and a. variety of built—in functions easily
`accessible by user programs.
`
`‘
`
`When a user program calls these function routines, they accept high-level information by way of
`register and control block contents. When a user program calls DOS to perform an operation,
`these functions translate the requirement
`into one or more calls to IBMBIO.COM,
`MS-DOS.SYS or system hardware to complete the request.
`
`The Command Interpreter
`
`The command interpreter, COMMAND.COM, is the part you interact with on the command
`line. COMMAND.COM has three parts. IBM calls them the "resident portion", the "initializa-
`tion portion" and the "transient portion".
`
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`

`

`
`
`DOS and the IBM PC
`
`7
`
`IBM’s original documentation spoke of installing alternate command interpreters (programs
`other than COMMAND.COM) with the SHELL: statement in CONFIG.SYS. Unfortunately,
`IBM chose not to document much of the interaction between IBMDOS.COM and IBM-
`BIO.COM. By the time much of the interaction was widely understood, many commercial soft-
`ware programs had been written to use peculiarities of COMMAND.COM itself.
`
`Two programs exist that perform as actual "shells" by completely replacing COMMAND.COM
`and substituting their ov'vn command interpreter to use with the hidden DOS files. These are
`Command Plus, a commercial package, and the very interesting shareware 4DOS package. Both
`supply greatly enhanced batch language and editing capabilities.
`
`Note: DOS 3.3 + checks for the prescnce of a hard disk, and will default to COMSPEC= C:\. Pre-
`- vious versions default to COMSPEC=A:\. Under some DOS versions, if COMMAND.COM is
`not immediately available for reloading (i.e., swapping to a floppy with COMMAND.COM on
`it) DOS may crash.
`
`Resident Portion
`
`The resident portion resides in memory immediately following IBMDOS.COM and its data
`area. This portion contains routines to process interrupts 22h (Terminate Address), 23h (Ctrl-
`Break Handler), and 24h (Critical Error Handler), as well as a routine to reload the transient
`portion if needed. For DOS 3.x, this portion also contains a routine to load and execute external
`commands, such as files with extensions of COM or EXE.
`
`When a program terminates, a checksum is used to determine if the application program over-
`laid the transient portion of COMMAND.COM. Ifso, the resident portion will reload the tran-
`sient portion from the area designated by COMSPEC= in the DOS environment. If COM—
`MAND.COM cannot be found, the system will halt.
`
`All standard DOS error handling is done within the resident portion of COMMAND.COM.
`This includes displaying error messages and interpreting the replies to the "Abort, Retry, Ig-
`nore, Fail?" message.
`
`Since the transient portion of COMMAND.COM is so large (containing the internal com-
`mands and all those error messages), and it is not needed when the user is running an applica-
`tion it can be overlaid that program if that application needs the room. When the application is
`through, the resident portion of COMMAND.COM brings the transient portion back into
`memory to show the prompt. This is why you will sometimes see the message "Insert disk with
`COMMAND.COM". It needs to get the transient portion off the disk since it was overlaid with
`the application program.
`
`The initialization portion of COMMAND.COM follows the resident portion and is given con-
`trol during the boot-up procedure. This section actually processes theAUTOEXEC.BAT file. It
`also decides where to load the user’s programs when they are executed. Since this code is only
`needed during start-up, it is overlaid by the first program which COMMAND.COM loads.
`The transient portion is loaded at the high end of memory and it is the command processor it-
`self. It interprets whatever the user types in at the keyboard, hence messages such as ‘Bad com-
`mand or file name’ forwhen the user misspells a command. This portion contains all the internal
`commands (i.e. COPY, DIR, RENAME, ERASE), the batch file processor (to run .BAT files)
`and a routine to load and execute external commands which are either .COM or .EXE files.
`
`The transient portion of COMMAND.COM produces the system prompt, (C), and reads what
`
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`

`
`
`8
`
`The Programmer’s Technical Reference
`
`the user types in from the keyboard and tries to do something with it. For any .COM or .EXE
`files, it builds a command line and issues an EXEC function call to load the program and trans-
`fer control to it.
`
`DOS Initialization
`
`The system is initialized by a software reset (Ctrl-Alt-Del), a hardware reset (reset button), or by
`turning the computer on. The Intel 80x8x series processors always look for their first instruction
`at the end of their address space (OFFFFOh) when powered up or reset. This address contains a
`jump to the first instruction for the ROM BIOS.
`
`Built-in ROM programs (Power-On Self-Test, or POST, in the IBM) check machine status and
`run inspection programs of various sorts. Some machines set up a reserved RAM area with bytes
`indicating installed equipment (AT and PCj r).
`
`When the ROM BIOS finds a ROM on an adapter card, it lets that ROM take control of the sys-
`tem so that it may perform any set up necessary to use the hardware or software controlled by
`that ROM. The ROM BIOS searches absolute addresses OCSOOOh through OEOOOOh in 2K in-
`crements in search of a valid ROM. A valid ROM is determined by the first few bytes in the
`ROM. The ROM will have the bytes 55h, OAAh, a length indicator and then the assembly lan-
`guage instruction to CALL FAR (to bring in a ‘FAR’ routine). A checksum is done on the ROM
`to verify its integrity, then the BIOS performs the CALL FAR to bring in the executable code.
`The adapter’s ROM then performs its initialization tasks and hopefully returns control of the
`computer back to the ROM BIOS so it can continue with the booting process.
`
`The ROM BIOS routines then look for a disk drive at A: or an option ROM (usually a hard disk)
`at absolute address C:800h. If no floppy drive or option ROM is found, the BIOS calls int 19h
`(ROM BASIC if it is an IBM) or displays an error message.
`
`If a bootable disk is found, the ROM BIOS loads the first sector of data from the disk and then
`- jumps into the RAM location holding that code. This code normally is a routine to load the rest
`of the code off the disk, or to ‘boot’ the system.
`
`The following actions occur after a system initialization:
`
`1. The boot record is read into memory and given control.
`
`2. The boot record then checks the root directory to assure that the first two files are
`IBMBIO.COM and IBMDOS.COM. These two files must be the first two files, and they
`must be in that order (IBMBIO.COM first, with its sectors in contiguous order).
`Note: IBMDOS.COM need not be contiguous in version 3.x+.
`
`The boot record loads IBMBIO.COM into memory.
`
`The initialization code in IBMBIO.COM loads IBMDOS.COM, determines equipment
`status, resets the disk system, initializes the attached devices, sets the system parameters
`and loads any installable device drivers according to the CONFIGSYS file in the root
`directory (if present), sets the low—numbered interrupt vectors, relocates IBMD OS.COM
`downward, and calls the firstbyte of DOS.
`Noreb CONFIG.SYS may be a hidden file.
`
`5.
`
`V DOS initializes its internal working tables, initializes the interrupt vectors for interrupts
`20h through 27h, and builds a Program Segment Prefix for COMMANDCOM at the lowest
`available segment. For DOS versions 3.10 up, DOS also initializes the vectors for interrupts
`
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`

`

`DOS and the IBM PC
`
`'
`
`'
`
`9
`
`OFh through 3Fh. An initialization routine is included in the resident portion and assumes
`control during start-up. This routine contains the AUTOEXECBAT file handler and
`determines the segment address where user application programs may be loaded. The
`initialization routine is then no longer needed and is overlaid by the first program
`CONHVIANDCOM loads.
`
`Note: AUTOEXECBAT may be a hidden file.
`
`6.
`
`IBMBIQCOM uses the EXEC function call to load and start the top-level command
`processor. The default command processor is COMMAND.COM in the root directory of
`the boot drive. If COMMANDCOM is in a subdirectory or another command processor is
`to be used, it must be specified by a SHELL: statement in the CONFIG.SYS file. A
`transient portion is loaded at the high end of memory. This is the command processor itself,
`containing all of the internal command processors and the batch file processor. For
`DOS 2.x, this portion also contains a routine to load and execute external commands, such
`as files with extensions of COM or EXE. This portion of COMMAND.COM also produces
`the DOS prompt (such as ‘A’), reads the command from the standard input device (usually
`the keyboard or a batch file), and executes the command. For external commands, it builds a
`command line and issues an EXEC function call to load and transfer control to the
`program.
`
`Note 1.
`
`COMMAND.COM may be a hidden file.
`
`2.
`
`For IBM DOS 2.x, the transient portion of the command processor contains the
`EXEC routine that loads and executes external commands. For MS-DOS 2.x+ and
`
`IBM DOS 3.x+, the resident portion of the command processor contains the
`EXEC routine.
`
`3.
`
`IBMBIO only checks for a file named COMMANDCOM. It will load any file of
`that name if no SHELL: command is used.
`
`That pretty much covers the boot-up process. After COMMANDCOM is loaded, it runs the
`AUTOEXECBAT file and then the user gets a prompt to begin working.
`
`
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`

`
`
`CPU Port Assignments, System
`Memory Map, BIOS Data Area,
`Interrupts 00h to 09h
`
`Introduction
`
`For consistency in this reference, all locations and offsets are in hexadecimal unless otherwise
`specified. All hex numbers are prefaced with a leading zero if they begin with an alphabetic char—
`acter, and are terminated with a lowercase H (h). The formats vary according to common usage.
`
`System Memory Map
`
`The IBM PC handles its address space in 64k segments, divided into 16k fractions and then fur-
`then as necessary.
`start
`start
`end
`addr. addn
`addr
`(dec)
`(hex)
`
`usage
`
`start of RAM, first K is interrupt vector table
`PC—O system board RAM ends
`
`PC—l system board RAM ends
`
`*640k RAM Ar'ea*
`
`0k
`16k
`32k
`48k
`64k
`80k
`96k
`112k
`128k
`144k
`160k
`176k
`
`192k
`208k
`224k
`240k
`
`00000—03FFF
`04000—07FFF
`08000—0BFFF
`10000—13FFF
`14000—17FFF
`18000—1BFFF
`1C000-1FFFF
`20000—23FFF
`24000—27FFF
`28000—ZBFFF
`2C000—2FFFF
`
`30000—33FFF
`34000—37FFF
`38000—BBFFF
`3C000—3FFFF
`
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`

`
`
`CPUPorts Assignments, system Memory Data, BIOS Data Area
`40000—43FFF
`PC-2 system board RAM ends
`44000—47FFF
`48000—4BFFF
`4C000-4FFFF
`
`I]
`
`‘50000—53FFF
`54000-57FFF
`58000-SBFFF
`SCOOO-SFFFF
`
`60000-63FFF
`64000-67