Danny Amor's Online Library/Danny Amor's Online Library - Volume 1.iso/html/faqs/faq/msdos/programmer-faq.faq

Text File | 1995-07-25 | 135.4 KB | 3,001 lines

Subject: comp.os.msdos.programmer FAQ
Newsgroups: comp.os.msdos.programmer,comp.answers,news.answers
From: [e-mail] (Jeffrey Carlyle)
Date: 15 Nov 1994 10:08:24 GMT

Archive-name: msdos-programmer-faq/faq
Comp-os-msdos-programmer-archive-name: dos-faq
Posting-frequency: monthly
Last-modified: 05 Sep 1994

comp.os.msdos.programmer FAQ Version 2.01
-----------------------------------------
Copyright (C) 1994 by Jeffrey Carlyle, Perplexic Software.  All rights
reserved.  This article is not  in the public  domain,  but it  may be
redistributed  so long as  this notice,  the acknowledgments,  and the
information on obtaining the latest copy of this list are retained and
no fee is charged. The code fragments may be used freely; credit would
be polite.

For October's FAQ:
I'll be chacking out all the FTP sites listed in the FAQ and see that
they are still working.

TABLE OF CONTENTS
-----------------

Section 1.  General FAQ and Newsgroup Information
-------------------------------------------------
  1.01 - What is this article for? (Revised: 05/09/1994)
  1.02 - Who has contributed to this article?
  1.03 - How can I search this article for a particular topic?
  1.05 - Are the answers guaranteed to be correct and complete?
         (Revised: 05/09/1994)
  1.06 - What is comp.os.msdos.programmer about? (Revised: 05/09/1994)
  1.07 - Is comp.os.msdos.programmer just for C programmers?
  1.08 - What is comp.sys.ibm.pc.programmer?
  1.09 - Is comp.os.msdos.programmer available as a mailing list?
  1.10 - What's this "netiquette"?
  1.11 - How can I learn more about Usenet?
  1.12 - What other technical newsgroups should I know about?
  1.13 - Where are FAQ lists archived?
  1.14 - Where can I get the latest copy of this FAQ list?
  1.15 - Are there any good on-line references for PC hardware components?
  1.16 - Are there any good on-line references for PC interrupts?
  1.17 - What and where is "Ralf Brown's interrupt list"?
         (Revised: 05/09/1994)
  1.18 - Where can I find lex, yacc, and language grammars?
  1.19 - What's the best book to learn programming?
  1.20 - How do I use ftp?
  1.21 - Why won't my code work?

Section 2.  Compile and Link
----------------------------
  2.01 - What the heck is "DGROUP > 64K"?
  2.02 - How do I fix "automatic data segment exceeds 64K" or "stack
         plus data exceed 64K"?
  2.03 - Will Borland C code and Microsoft C code link together?
  2.04 - Why did my program bomb at run time with "floating point
         formats not linked" or "floating point not loaded"?
  2.05 - How can I change the stack size in Borland's C compilers?
  2.06 - What's the format of an .OBJ file?
  2.07 - What's the format of an .EXE header?
  2.08 - What's the difference between .COM and .EXE formats?

Section 3.  Keyboard
--------------------
  3.01 - How can I read a character without echoing it to the screen,
         and without waiting for the user to press the Enter key?
  3.02 - How can I find out whether a character has been typed, without
         waiting for one?
  3.03 - How can I disable Ctrl-C/Ctrl-Break and/or Ctrl-Alt-Del?
  3.04 - How can I disable the print screen function?
  3.05 - How can my program turn NumLock (CapsLock, ScrollLock) on or
         off?
  3.06 - How can I speed up the keyboard's auto-repeat?
  3.07 - What is the SysRq key for?
  3.08 - How can my program tell what kind of keyboard is on the system?
  3.09 - How can I tell if input, output, or stderr has been redirected?
  3.10 - How can I increase the size of the keyboard buffer?
  3.11 - How can I stuff characters into the keyboard buffer?

Section 4.  Disks and files
---------------------------
  4.01 - What drive was the PC booted from?
  4.02 - How can I boot from drive b:?
  4.03 - Which real and virtual disk drives are valid?
  4.04 - How can I make my single floppy drive both a: and b:?
  4.05 - How can I disable access to a drive?
  4.06 - How can a batch file test existence of a directory?
  4.07 - Why won't my C program open a file with a path?
  4.08 - How can I redirect printer output to a file?
  4.09 - How can I redirect the output of a batch file?
  4.10 - How can I redirect stderr?
  4.11 - How can my program open more files than DOS's limit of 20?
  4.12 - How can I read, create, change, or delete the volume label?
  4.13 - How can I get the disk serial number?
  4.14 - What's the format of .OBJ, .EXE., .COM files?
  4.15 - How can I flush the software disk cache?

Section 5.  Serial ports (COM ports)
------------------------------------
  5.01 - How do I set my machine up to use COM3 and COM4?
  5.02 - How do I find the I/O address of a COM port?
  5.03 - But aren't the COM ports always at I/O addresses 3F8, 2F8, 3E8,
         and 2E8?
  5.04 - How do I configure a COM port and use it to transmit data?
         (Revised: 05/09/1994)

Section 6.  Other hardware questions and problems
-------------------------------------------------
  6.01 - Which 80x86 CPU is running my program?
  6.02 - How can a C program send control codes to my printer?
  6.03 - How can I redirect printer output to a file?
  6.04 - Which video adapter is installed?
  6.05 - How do I switch to 43- or 50-line mode?
  6.06 - How can I find the Microsoft mouse position and button status?
  6.07 - How can I access a specific address in the PC's memory?
  6.08 - How can I read or write my PC's CMOS memory?
  6.09 - How can I access memory beyond 640K?
  6.10 - How can I use the protected mode? (New: 05/09/1994)

Section 7.  Other software questions and problems
-------------------------------------------------
  7.01 - How can a program reboot my PC?
  7.02 - How can I time events with finer resolution than the system
         clock's 55 ms (about 18 ticks a second)?
  7.03 - How can I find the error level of the previous program?
  7.04 - How can a program set DOS environment variables?
  7.05 - How can I change the switch character to - from /?
  7.06 - How can I write a TSR (terminate-stay-resident utility)?
  7.07 - Why does my interrupt function behave strangely?
  7.08 - How can I write a device driver?
  7.09 - What can I use to manage versions of software?
  7.10 - What's this "null pointer assignment" after my C program
         executes?
  7.11 - How can a batch file tell whether it's being run in a DOS box
         under Windows?
  7.12 - How can my program tell if it's running under Windows?
  7.13 - How can a program tell whether ANSI.SYS is installed?
  7.14 - How do I copyright software that I write?

Section 8.  Downloading
-----------------------
  8.01 - What are SimTel, Garbo, and Wustl?
  8.02 - I have no ftp access.  How can I get files from the archives?
  8.03 - Can I get archives on CD-ROM?
  8.04 - Where do I find program <mumble>?
  8.05 - How can I check SimTel or Garbo before I post a request for a
         program?
  8.06 - How do I download and decode a program I found?
  8.07 - Where is UUDECODE?
  8.08 - Why do I get errors when extracting from a ZIP file I downloaded?

Section 9.  Vendors and products
---------------------------------
  9.01 - How can I contact Borland?
  9.02 - How can I contact Microsoft?
  9.03 - What's the current version of UNZIP? (Revised: 05/09/1994)
  9.04 - What is the phone number for a vendor's BBS?

THE ANSWERS
-----------

Section 1.  General FAQ and Newsgroup Information
-------------------------------------------------

    1.01 - What is this article for?
    --------------------------------
    (Revised: 05/09/1994)

    This is the FAQ (Frequently Asked Questions) list for the newsgroup
    comp.os.msdos.programmer. This list is posted monthly; new questions
    and questions with revised answers are marked with "New:" or
    "Revised:" and the date. When any changes occur a seperate post
    with the subject line "comp.os.msdos.programmer FAQ diffs".

    FAQ lists are intended to reduce the noise level in their newsgroups
    that results from the repetition of the same questions, correct
    answers, wrong answers, corrections to the wrong answers,
    corrections to the corrections, debate, etc.

    This list should serve as a repository of the canonical "best"
    answers to the questions in it.  The names of folks who have helped
    to improve this FAQ list are listed in "1.02 - Who has contributed to
    this article?"

    1.02 - Who has contributed to this article?
    -------------------------------------------

    This list is maintained and edited by Jeffrey Carlyle, Perplexic
    Software, Bowling Green, Kentucky, USA. ([e-mail])

    The major contributor to this list is Stan Brown, the former list
    maintainer. Most of this list was written by Stan. He can be contacted
    at [e-mail].

    Many articles posted in comp.os.msdos.programmer sparked ideas or
    provided information for the first version of this list.  Though
    they are not responsible for any errors, thanks are due to the
    following persons for posted articles or private email that led to
    improvements in this FAQ list:

    Jamshid Afshar, Mark Aitchison, Sanjay Aiyagari, George Almasi,
    Aaron Auseth, Robert Baker, Preston Bannister, Scott Barman, Denis
    Beauregard, Per Bergland, Mike Black, Chris Blum, Ron Bodkin, Mark
    Brader, Jon Brinkmann, Andrew James Bromage, Glynn Brooks, Paul
    Brooks, Ralf Brown, Shaun Burnett, D'Arcy J.M. Cain, Raymond Chen,
    Denny de Jonge, Eric DeVolder, Alan Drew, Paul Ducklin, Gary Dueck,
    Roland Eriksson, Mark Evans, Markus Fischer, George Forsman, Roger
    Fulton, Vincent Giovannone, Robert Grunloh, B.Haible, Janos Haide,
    Klaus Hartnegg, Kris Heidenstrom, Tom Haapanen, Joel Hoffman, Ari
    Hovila, Chin Huang, Joe Huffman, Michael Holin, Mike Iarrobino,
    Byrial Jensen, Rune Jorgensen, Ajay Kamdar, Everett Kaser, Jeff
    Kellam, Jen Kilmer, Reinhard Kirchner, Dave Kirsch, Chad Knudsen,
    Samuel Ko, Janne Kukonlehto, Benjamin Lee, Stephen Lee, Jim Lynch,
    Greg Malknecht, Sidney Markowitz, Jim Marks, Dimitri Matzarakis,
    Fred McCall, Ken McKee, Doug Merrett, Tom Milner, Bill Moore, Duncan
    Murdoch, Steve Murphy, Daniel Neri, Mert Nickerson, David Nugent,
    John Oldenburg, David Pape, Keith Petersen, Karl Riedling, Arthur
    Rubin, Gerald Ruderman, Timo Salmi, Tapio Sand, John Schmid, Russell
    Schulz, Huseyin Sevay, Ajay Shah, Bob Smith, Sean Sullivan, Steve
    Summit, Tom Swingle, Anders Thulin, Curt Tilmes, Rick Watkins,
    Ya-Gui Wei, Joe Wells, Scott Winder, Gregory Youngblood,
    [e-mail]

    1.03 - How can I search this article for a particular topic?
    ------------------------------------------------------------

    To search for a particular topic, save this article to a text file
    and use your favorite editor's find function to search for a word.

    You can use the MS-DOS Editor to search for a topic. Run the editor
    and load the FAQ. Press ALT+S to activate the search menu then press
    F. In the box that says find what enter the word you want to look for.
    When you are completed, press enter. If a match is found EDIT will
    highlight it. If that is not what you were looking for press F3 and
    EDIT will show you the next match.

    1.05 - Are the answerers guaranteed to be correct and complete?
    --------------------------------------------------------------
    (Revised: 05/09/1994)

    I have tried to test all of the facts, but THERE IS NO WARRANTY ON
    THE CODE OR ON THE TECHNIQUES DESCRIBED HEREIN. Please send corrections
    to [e-mail]. All the code has been tested; but the
    testing may not have been perfect, and machines and configurations vary.
    (Except where otherwise noted, C code was tested with MSC 5, BC++ 2.0,
    or BC++ 4.0.)

    The mention of particular books or programs must not be construed to
    reflect unfavorably on any that are not mentioned.

    1.06 - What is comp.os.msdos.programmer about?
    ----------------------------------------------
    (Revised: 05/09/1994)

    comp.os.msdos.programmer (comp.sys.ibm.pc.programmer until September
    1990) concerns programming for MS-DOS systems.  The article "USENET
    Readership report for Jul 94" in news.lists shows 120,000 readers of
    this newsgroup worldwide.  Traffic (exclusive of crossposts) was 1981
    articles aggregating 3.1 Megabytes. It ranked as the 79th most popular
    newsgroup.

    Much of our traffic is about language products (chiefly from Borland
    and Microsoft).  More programming topics focus on C than on any one
    other language, but we are not just for C programmers (see "1.07 - Is
    comp.os.msdos.programmer just for C programmers?").

    Since most MS-DOS systems run on hardware that is roughly compatible
    with the IBM PC, on Intel 8088, 80188, or 80x86 chips, we tend to
    get a lot of questions and answers about programming other parts of
    the hardware.

    1.07 - Is comp.os.msdos.programmer just for C programmers?
    ----------------------------------------------------------

    No, it is for all programmers who want to share information about
    programming in MS-DOS and DOS replacements like 4DOS.  Programs and
    questions are also posted in Pascal, assembly, and other languages
    (including MS-DOS batch programming.)

    Why does the newsgroup seem to be so C-oriented sometimes?  There
    are two reasons.  First, comp.lang.c and comp.lang.pascal have
    evolved in different directions.  comp.lang.pascal welcomes vendor-
    specific discussion, such as Turbo Pascal.  Since so many of TP's
    features are tailored to programming on PCs and in MS-DOS, Turbo
    Pascal programmers tend to find DOS questions welcomed there, so
    that comp.os.msdos.programmer gets less of the "DOS in Turbo Pascal"
    traffic.  On the other hand, comp.lang.c has stayed closer to
    talking only about the C language, and vendor-specific or operating-
    system-specific questions are not welcome.  This tends to push
    questions about disks, DOS file structure, video, the keyboard,
    TSRs, etc. to comp.os.msdos.programmer even when those programs are
    written in C.

    This FAQ is definitely C-oriented, not because that's necessarily
    best but because I tried to stick to what I could verify personally.
    As a C programmer (with some assembler), I could most carefully
    verify solutions in C or assembler.  I felt that short, clear
    programs could be published in just one language and programmers
    could translate them into their languages of choice.  But the FAQ
    list also contains several long programs written only in C; this is
    a defect with no obvious remedy.  Most answers that point to source
    code at archive sites include both C- and Pascal-language source
    when available.

    1.08 - What is comp.sys.ibm.pc.programmer?
    ------------------------------------------

    comp.sys.ibm.pc.programmer is the old name of comp.os.msdos.programmer,
    and has been obsolete since September 1990.  However, many systems have
    not removed the old group, or have removed it but aliased it to the
    new name.  This means that some people still think they're posting
    to comp.sys.ibm.pc.programmer even though they're actually posting to
    comp.os.msdos.programmer.

    You can easily verify the non-existence of comp.sys.ibm.pc.programmer by
    reference to the "List of Active Newsgroups" posted to news.groups.
    It's available as /pub/usenet/news.answers/active-newsgroups/part1
    from the archives (see "1.13 - Where are FAQ lists archived?").

    1.09 - Is comp.os.msdos.programmer available as a mailing list?
    ---------------------------------------------------------------

    Sorry, no.

    1.10 - What's this "netiquette"?
    --------------------------------

    Netiquette is good Usenet etiquette.  It includes basic rules like
    the ones below.  (See also "1.11 - How can I learn more about
    Usenet?")

    - Always read a newsgroup for a reasonable time before you post an
      article to it.

    - Pick the one right group for your article; don't crosspost unless
      absolutely necessary.  If you absolutely must post an article to
      more than one group, do crosspost it and don't post the same
      article separately to each group.  See "1.12 - What other technical
      newsgroups should I know about?" when considering where to post an
      article.

    - Before you post a question, make sure you're posting to the right
      newsgroup; the best way to do that is to observe the preceding
      rule.  Check the group's FAQ list (if it has one) to make sure
      that your question isn't already answered there; see "1.13 - Where
      are FAQ lists archived?".

    - When you post a question, if you ask for email responses then
      promise to post a summary.  Keep your promise.  And make it a real
      summary: don't just append all the email you got.  Instead, write
      your own (brief) description of the solution:  this is the best
      way to make sure you really understand it.

    - Before you post a follow-up, read the other follow-ups.  Very
      often you'll find that someone else has already made the point you
      had in mind.

    - When someone posts a question, if you want to know the answer
      don't post a "me, too".  Instead send email to the poster asking
      him or her to share responses with you.

    - When posting a follow-up to another posted article, remove all
      headers and signature lines from the old article; just keep the
      line "In <article>, so-and-so writes:".  Also cut the original
      article down as much as possible; just keep enough of it to remind
      readers of the context.

    - Keep lines in posted articles to 72-75 characters.  Many
      newsreaders chop off column 81 or arbitrarily insert a newline
      there, which makes longer lines difficult or impossible to read.
      But you need to keep well below 80 characters per line to allow
      for the > characters that get inserted when other people post
      follow-ups to your article.

    - Keep your signature to 4 lines or less (including any graphics),
      and for heaven's sake make sure it doesn't get posted twice in
      your article.

    - Don't post email without first obtaining the permission of the
      sender.

    1.11 - How can I learn more about Usenet?
    -----------------------------------------

    There are two important newsgroups for learning about how Usenet and
    newsreader software works:

    - news.announce.newusers contains periodic postings that everybody
      is asked to read before posting anything to Usenet.  (In theory,
      all new users are subscribed to news.announce.newusers
      automatically.  But in practice not all newsreader software does
      that, so that many people violate the guidelines given there
      simply because they don't know about them.)

    - news.newusers.questions is described as "Q & A for users new to
      the Usenet".  But new and long-time users can ask or answer
      questions about Usenet and newsreader software there.  There's an
      important article, "Welcome to news.newusers.questions! (weekly
      posting)", that everyone is asked to read before posting to
      news.newusers.questions.  (See below for ways to get a copy of
      that article.)

    The following postings in news.announce.newusers might be considered
    the "mandatory course" for new users:

        Introduction to news.announce.newusers
        What is Usenet?
        Answers to Frequently Asked Questions
        Rules for posting to Usenet
        A Primer on How to Work With the Usenet Community
        Hints on writing style for Usenet
        Emily Postnews Answers Your Questions on Netiquette

    The articles mentioned above are downloadable via ftp from
    rtfm.mit.edu in the following files:

        /pub/usenet/news.answers/news-newusers-intro
            Welcome to news.newusers.questions! (weekly posting)
        /pub/usenet/news.answers/news-announce-intro/part1
            Introduction to news.announce.newusers
        /pub/usenet/news.answers/what-is-usenet/part1
            What is Usenet?
        /pub/usenet/news.answers/usenet-faq/part1
            Answers to Frequently Asked Questions
        /pub/usenet/news.answers/posting-rules/part1
            Rules for posting to Usenet
        /pub/usenet/news.answers/usenet-primer/part1
            A Primer on How to Work With the Usenet Community
        /pub/usenet/news.answers/usenet-writing-style/part1
            Hints on writing style for Usenet
        /pub/usenet/news.answers/emily-postnews/part1
            Emily Postnews Answers Your Questions on Netiquette

    For rtfm.mit.edu instructions, see "1.13 - Where are FAQ lists
    archived?"

    1.12 - What other technical newsgroups should I know about?
    -----------------------------------------------------------

    Caution:  Some of these newsgroups have specialized charters; you'll
    probably get flamed (and deserve it) if you post to an inappropriate
    group.  Most groups have FAQ lists that will tell you what's
    appropriate.  Don't post a request for the FAQ list; instead,
    retrieve it yourself: see "1.13 - Where are FAQ lists archived?"

    - misc.forsale.computers.d and misc.forsale.computers.pc-clone are
      where you post notices of equipment, software, or computer books
      that you want to sell.  Please don't post or crosspost those
      notices to comp.os.msdos.programmer.

    - comp.os.ms-windows.programmer.* groups (formerly part of
      comp.windows.ms.programmer):  Similar to comp.os.msdos.programmer,
      but focus on programming for the MS-Windows platform.

    - comp.sys.ibm.pc.hardware is for more hardware-oriented discussions
      of the machines that run DOS.

    - the various comp.lang.* groups for articles and questions on the
      programming languages.  Caution:  some groups welcome discussions
      that are operating-system dependent or vendor specific; others do
      not.  For example, comp.lang.c is definitely _not_ for questions
      about programming DOS or PC system features, even if the programs
      are written in C.  On the other hand, comp.lang.pascal contains
      much discussion that is specific to Turbo Pascal.

    - comp.binaries.ibm.pc.wanted: AFTER you have looked in the other
      groups, this is the place to post a request for a particular
      binary program.

    - comp.archives.msdos.announce (moderated) explains how to use the
      archive sites, especially Garbo and SimTel, and lists files
      uploaded to them.  Discussions belong in comp.archives.msdos.d,
      which replaced comp.binaries.ibm.pc.archives in December 1992.

    - MSDOS-Ann mailing list, for those who cannot subscribe to the
      comp.archives.msdos.announce newsgroup, lists SimTel and Garbo
      uploads in digest format.  Instructions are downloadable as

        /pc/MSDOSANN.ZIP from Garbo
        /pub/msdos/info/msdosann.zip from SimTel.

    - comp.binaries.ibm.pc.d is for discussions about programs posted in
      comp.binaries.ibm.pc, and only those programs.  This is a good
      place to report bugs in the programs, but not to ask where to find
      them (see cbip.wanted, above).  comp.binaries.ibm.pc.d is NOT supposed
      to be a general PC discussion group.

    - comp.sources.misc: a moderated group for source code for many
      computer systems.  It tends to get lots of Unix stuff, but you may
      also pick up some DOS-compatible code here.

    - alt.sources: an unmoderated group for source code.  Guidelines are
      posted periodically.

    - comp.os.msdos.programmer.turbovision is specifically for
      programming in Turbo Vision.

    - rec.games.programmer discusses many graphics programming topics.

    1.13 - Where are FAQ lists archived?
    ------------------------------------

    Very possibly the FAQ list you want is already at your site.  Check
    the newsgroup news.answers; if your site doesn't carry news.answers,
    check comp.answers, rec.answers, etc., according to the top-level
    name in the FAQ list's "home" newsgroup.  Articles are posted to the
    *.answers groups in a way that should make them last until the next
    versions are posted.  If they expire sooner at your site, you might
    want to lobby your sysadmin to treat the moderated *.answers groups
    as a special case and grant them longer expiration times than other
    groups.

    To ftp most FAQ lists, connect to rtfm.mit.edu, and change to
    directory /pub/usenet/news.answers.  The name of the file that you
    want is the Archive-name from the top of the article.  For instance,
    if the Archive-name is software-eng/part1 you would retrieve
    /pub/usenet/news.answers/software-eng/part1.

    By email (only if you have no ftp access, please), the server is
    [e-mail] .  It accepts "send" commands that omit the
    leading "/pub/" from file names; for example:

        send usenet/news.answers/software-eng/part1

    For full instructions about the mail server, send it a message
    consisting of these two lines:

        help
        index

    Not just FAQ lists, but every article listed in the "List of
    Periodic Informational Postings" (LoPIP) can be obtained by ftp or
    email from rtfm.mit.edu.  If you have an old copy of an
    informational article, look for an "Archive-name" at the beginning;
    rtfm.mit.edu stores it under that name in /pub/usenet/news.answers.
    If the article has no Archive-name, check the first name on the
    Newsgroups line and change to that directory under /pub/usenet.

    Or send email >>> with valid reply-to address << to [e-mail]
    and you'll receive my canned instructions (about 8K) for retrieving
    FAQ lists for most newsgroups, including a few that aren't archived
    at rtfm.mit.edu.  (This offer may be withdrawn without notice
    depending on system constraints.)

    1.14 - Where can I get the latest copy of this FAQ list?
    --------------------------------------------------------

    You can retrieve the latest version of this list as

        /pub/msdos/info/faqp*.zip from SimTel
        /pc/doc-net/faqp*.zip from Garbo.

    Check the date before downloading, to make sure that the archived
    version is actually newer than what you have.

    Like most FAQ lists, this one is archived at rtfm.mit.edu and a
    number of mirror sites.  If you have ftp access, retrieve either of
    these identical files:

        /pub/usenet/news.answers/msdos-programmer-faq/faq
        /pub/usenet/comp.os.msdos.programmer/dos-faq

    If you have no ftp access, see "1.13 - Where are FAQ lists archived?"
    for instructions on retrieving it by email.

    1.15 - Are there any good on-line references for PC hardware components?
    ------------------------------------------------------------------------

    Good reports of HELPPC21 have been posted.  It is downloadable as

        /pub/msdos/info/helppc21.zip at SimTel
        /pc/programming/helppc21.zip at Garbo.

    This hypertext system contains much information on ports and other
    hardware, as well as some overlap with Ralf Brown's interrupt list
    (1.17).  It is shareware ($25).

    1.16 - Are there any good on-line references for PC interrupts?
    ---------------------------------------------------------------

    The definitive work is Ralf Brown's interrupt list; see 1.17.

    1.17 - What and where is "Ralf Brown's interrupt list"?
    -------------------------------------------------------
    (Revised: 05/09/1994)

    This is megabytes of information on documented and (officially)
    undocumented BIOS and DOS interrupts, DOS tables, and interrupts
    hooked by many software packages.

    inter42a.zip, inter42b.zip, and inter42c.zip is the actual
    list. inter42d.zip is a collection of utilities and conversion
    programs for the list. The list is downloadable from:

        /pub/msdos/info/ from SimTel
        /pc/programming/ from Garbo.

    These versions were uploaded in early August 1994; updates are
    announced every few months in comp.archives.msdos.announce.

    1.18 - Where can I find lex, yacc, and language grammars?
    ---------------------------------------------------------

    The FAQ list of the comp.compilers newsgroup answers this for Basic,
    C, Pascal, and other languages.  See "1.13 - Where are FAQ lists
    archived?"

    1.19 - What's the best book to learn programming?
    -------------------------------------------------

    Sorry, this FAQ list cannot settle religious arguments.

    Much of the heat over this topic arises because each person believes
    that the book that he or she learned from is the best book.  But
    different people have very different experiences of the same book.
    The only person who can tell you which is the best book for learning
    a given topic is you.

    Your best bet is to go to a fairly well-stocked bookstore when you
    have a couple of hours to spare.  Start at one end of the shelf and
    work your way methodically through every book that looks like it
    might cover what you want to learn.  Look at the tables of contents;
    read a page or two from each book.  Then make your decision.  If
    money is a problem, or if you're not sure of your choice, check out
    your top two or three from your library.

    1.20 - How do I use ftp?
    ------------------------
    I was much impressed with {The Whole Internet User's Guide and
    Catalog} by Ed Krol (O'Reilly & Associates, second edition ISBN
    1-56592-063-5).  It gives lots of information on using ftp, email,
    Telnet, Archie, etc.  As a new user of ftp (January 1993), I found
    the information made me productive quickly.  (Disclosure:  Though I
    don't believe it has biased my judgment, you should know that
    O'Reilly sent me a free evaluation copy.)

    You may also want to look for the "Beginner's Guide to FTP", posted
    periodically in comp.binaries.ibm.pc.  You can retrieve it from
    rtfm.mit.edu (see "1.13 - Where are FAQ lists archived?"), in
    /pub/usenet/comp.binaries.ibm.pc, as *ftp.man*.

    When downloading a file by ftp, make sure you have write access to
    your working directory on your local machine, and that your disk has
    enough space to hold the files.  Also make sure that if it's a ZIP
    file or other binary file type, you set the protocol to binary
    before downloading the file.

    1.21 - Why won't my code work?
    ------------------------------

    First you need to try to determine whether the problem is in your
    use of the programming language or in your use of MS-DOS and your PC
    hardware.  (Your manual should tell you which features are standard
    and which are vendor- or MS-DOS- or PC-specific.  You _have_ read
    your manual carefully, haven't you?)

    If the feature that seems to be working wrong is something related
    to your PC hardware or to the internals of MS-DOS, this group is the
    right place to ask.  (Please check this FAQ list first, to make sure
    your question isn't already answered here.)

    On the other hand, if your problem is with the programming language,
    the comp.lang hierarchy (including comp.lang.pascal and comp.lang.c)
    is probably a better resource.  Please read the other group's FAQ
    list thoroughly before posting.  (These exist in comp.lang.c,
    comp.lang.c++, comp.lang.modula3, comp.lang.lisp, comp.lang.perl;
    they may exist in other groups as well.)  It's almost never a good
    idea to crosspost between comp.os.msdos.programmer and a language
    group.

    Before posting in either place, try to make your program as small as
    possible while still exhibiting the bad behavior.  Sometimes this
    alone is enough to show you where the trouble is.  Also edit your
    description of the problem to be as short as possible.  This makes
    it look more like you tried to solve the problem on your own, and
    makes people more inclined to try to help you.  See also "1.10 -
    What's this "netiquette"?"

Section 2.  Compile and link
----------------------------

    2.01 - What the heck is "DGROUP > 64K"?
    ---------------------------------------

    This Q explains the problem; the next Q gives some remedies.

    DGROUP is a link-time group of data segments, and the compiler
    typically generates code that expects DS to be pointing to DGROUP.
    (Exception:  Borland's huge model has no DGROUP.)

    Here's what goes into DGROUP:

    - tiny model (all pointers near):  DGROUP holds the entire program.

    - small and medium models (data pointers near):  DGROUP holds all
      globals and static variables including string literals, plus the
      stack and the heap.

    - large, compact, and huge models in Microsoft (data pointers far):
      DGROUP holds only initialized globals and static variables
      including string literals, plus the stack and the near heap.

    - large and compact models in Borland (data pointers far): DGROUP
      holds initialized and uninitialized globals and static variables
      including string literals, but not the stack or heap.

    - huge model in Borland (data pointers far): there is no DGROUP, so
      the 64K limit doesn't apply.

    In all of the above, which is to say all six models in Microsoft C
    and all but huge in Borland C, DGROUP is limited to 64K including
    string literals (which are treated as static data).  This limitation
    is due to the Intel CPU's segmented architecture.

    For more information, see topics like "memory models" and "memory
    management" in the index of your compiler manual.  Also see
    TI738.asc, downloadable as part of

        /pub/msdos/turbo-c/bchelp10.zip at SimTel
        /pc/turbopas/bchelp10.zip at Garbo,

    for an extended general discussion of memory usage in Borland C
    programs, of which much applies to any C compiler in DOS.

    2.02 - How do I fix "automatic data segment exceeds 64K" or
               "stack plus data exceed 64K"?
    -----------------------------------------------------------

    These messages are a variation of "DGROUP > 64K".  For causes,
    please see the preceding Q.

    If you get this error in tiny model, your program is simply too big
    and you must use a different memory model.  If you get this link
    error in models S, C, M, L, or Microsoft's H, there are some things
    you can do.  (This error can't occur in Borland's huge model.)

    If you have one or two big global arrays, simply declare them far.
    The compiler takes this to mean that any references to them will use
    32-bit pointers, so they'll be in separate segments and no longer
    part of DGROUP.

    Or you can use the /Gt[number] option with Microsoft or -Ff[=size]
    with Borland C++ 2.0 and up.  This will automatically put variables
    above a certain size into their own segments outside of DGROUP.

    Yet another option is to change global arrays to far pointers.  Then
    at the beginning of your program, allocate them from the far heap
    (_fmalloc() in Microsoft, farmalloc() in Borland).

    Finally, you can change to huge model (with Borland compilers, not
    Microsoft).  Borland's H model still uses far pointers by default,
    but "sets aside the [64K] limit" and has no DGROUP group, according
    to the BC++ 2.0 Programmer's Guide.  Microsoft's H model does use
    huge data pointers by default but retains DGROUP and its 64K limit,
    so switching to the H model doesn't buy you anything if you have
    DGROUP problems.

    2.03 - Will Borland C code and Microsoft C code link together?
    ------------------------------------------------------------

    Typically this question is asked by someone who owns compiler A and
    is trying to write code to link with a third-party library that was
    compiled under compiler B.

    The answer to the question is, Not in general.  Here are some of the
    reasons:

    - "Helper" functions (undocumented functions for stack checking,
      floating-point arithmetic, and operations on longs) differ between
      the two compilers.

    - The compilers may embed instructions in the object code that tell
      the linker to look for their own run-time libraries.  You can use
      the linker option that says to ignore such instructions: /n in
      TLINK, /NOD in the Microsoft linker (the one that comes with the C
      compiler, not the one that used to come with DOS).  But getting
      around this problem will very likely just reveal other problems,
      like different helper functions, that have no easy solution.

    Those problems will generate link-time errors.  Others may not show
    up until run time:

    - Borland's compact, large, and huge models don't assume DS=SS, but
      Microsoft's do.  The -Fs option on the Borland compiler, or one of
      the /A options on Microsoft, should take care of this problem--
      once you know that's what's going on.

    - Check conventions for ordering and packing structure members, and
      for alignment of various types on byte, word, paragraph, or other
      boundaries.  Again, you can generally adjust your code to match if
      you know what conventions were used in compiling the "foreign"
      libraries.

    - Check the obvious and make sure that your code was compiled under
      the same memory model as the code you're trying to link with.
      (That's necessary, but no guarantee.  Microsoft and Borland don't
      use exactly the same conventions for segments and groups,
      particularly in the larger memory models.)

    That said, there are some circumstances where you can link hybrids.
    Your best chance of success comes if you compile in large model with
    the compiler switch that says to reload DS on entry to each
    function, avoid longs and floating point, use only 16-bit pointers,
    suppress stack checking, and specify all libraries used in the link.

    2.04 - Why did my program bomb at run time with "floating point
               formats not linked" or "floating point not loaded"?
    ---------------------------------------------------------------

    These messages look similar but have very different causes.

    "floating point not loaded" is Microsoft C's run-time message when
    the code requires a numeric coprocessor but your computer doesn't
    have one installed.  If the program is yours, relink it using the
    xLIBCE or xLIBCA library (where x is the memory model).

    "floating point formats not linked" is a Borland run-time error
    (Borland C or C++, Turbo C or C++).  Borland's compilers try to be
    smart and not link in the floating-point (f-p) library unless you
    need it.  Alas, they all get the decision wrong.  One common case is
    where you don't call any f-p functions, but you have %f or other f-p
    formats in scanf() or printf() calls.  The cure is to call an f-p
    function, or at least force one to be present in the link.

    To do that, define this function somewhere in a source file but
    don't call it:

        static void forcefloat(float *p) {
            float f = *p;
            forcefloat(&f);
        }

    It doesn't have to be in the module with the main program, as long
    as it's in a module that will be included in the link.

    If you have Borland C++ 3.0, the README file documents a slightly
    less ugly work-around.  Insert these statements in your program:

        extern unsigned _floatconvert;
        #pragma extref _floatconvert

    2.05 - How can I change the stack size in Borland's C compilers?
    ----------------------------------------------------------------

    In Turbo C, Turbo C++, and Borland C++, you may not find "stack
    size" in the index but the global variable _stklen should be there.
    The manual will instruct you to put a statement like

        extern unsigned _stklen = 54321U;

    in your code, outside of any function.  You must assign the value
    right in the extern statement; it won't work to assign a value at
    run time.  The linker may give you a duplicate symbol warning, which
    you can ignore.

    2.06 - What's the format of an .OBJ file?
    -----------------------------------------

    - base .OBJ format:  Intel's document number #121748-001, {8086
      Relocatable Object Module Formats}.  (not verified)

    Both Microsoft and Borland have extended the .OBJ format, as has IBM
    for OS/2; and according to the MS-DOS encyclopedia, Microsoft
    doesn't actually use all the listed formats.

    - Microsoft-specific .OBJ formats:

      * The .OBJ format document dated 14 Dec 1992, a WinHelp file from
        the Microsoft developer's CD-ROM (266K after unzipping), is
        downloadable as

        /vendor/microsoft/developer-network/ctech/11-9.zip from
            ftp.uu.net.

      * A 45-page article can be found in the {MS-DOS Encyclopedia},
        ISBN 1-55615-049-0, now out of print.

      * "Microsoft Object Module Format (OMF)" Specification, 22 Nov
        1991, was published by the Microsoft Languages Group.  (not
        verified)

    - Borland-specific .OBJ formats:  Open Architecture Handbook.  The
      Borland Developer's Technical Guide, 1991, no ISBN.  Chapter 2,
      "Object file contents", (pages 27-50) covers the comment records
      sent to the object file by Borland C++ version 3.0 and other
      Borland compilers.  The comment records mostly contain information
      for the Borland debugger. (not verified)

    - A "tutorial on the .OBJ format" comes with the VAL experimental
      linker, downloadable as

        /pub/msdos/pgmutil/val-link.zip at SimTel
        /pc/assembler/linker.zoo at Garbo.

      Despite such different names, those files have the same contents,
      but their contents are dated 18 Feb 1989.  You'd be better off
      with one of the more recent references listed above.

    2.07 - What's the format of an .EXE header?
    -------------------------------------------

    See PC Magazine 30 June 1992 (xi:12) pages 349-350 for the old and
    new formats.  For a more detailed layout, look under INT 21 AH=4B in
    Ralf Brown's interrupt list (Q 1.17).  That list includes extensions
    for Borland's TLINK and Borland debugger info.

    Among the books that detail formats of executable files are {DOS
    Programmer's Reference: 2d Edition} by Terry Dettman and Jim Kyle,
    ISBN 0-88022-458-4; and {Microsoft MS-DOS Programmer's Reference},
    ISBN 1-55615-329-5.

    2.08 - What's the difference between .COM and .EXE formats?
    -----------------------------------------------------------

    To oversimplify:  a .COM file is a direct image of core, and an .EXE
    file will undergo some further relocation when it is run (and so it
    begins with a relocation header).  A .COM file is limited to 64K for
    all segments combined, but an .EXE file can have as many segments as
    your linker will handle and be as large as RAM can take.

    The actual file extension doesn't matter.  DOS knows that a file
    being loaded is in .EXE format if its first two bytes are MZ or ZM;
    otherwise it is assumed to be in .COM format.  For instance, DR-DOS
    6.0's COMMAND.COM is in .EXE format.

Section 3.  Keyboard
--------------------

    3.01 - How can I read a character without echoing it to the
           screen, and without waiting for the user to press the
           Enter key?
    ------------------------------------------------------------

    The C compilers from Microsoft and Borland offer getch() (or
    getche() to echo the character); Turbo Pascal has ReadKey.

    In other programming languages, execute INT 21 AH=8; AL is returned
    with the character from standard input (possibly redirected).  If
    you don't want to allow redirection, or you want to capture Ctrl-C
    and other special keys, use INT 16 AH=10; this will return the scan
    code in AH and ASCII code (if possible) in AL, but  AL=E0 with AH
    nonzero indicates that one of the grey "extended" keys was pressed.
    (If your BIOS doesn't support the extended keyboard, use INT 16 AH=0
    not 10.)

    3.02 - How can I find out whether a character has been typed,
           without waiting for one?
    -------------------------------------------------------------

    In Turbo Pascal, use KeyPressed.  Both Microsoft C and Turbo C offer
    the kbhit() function.  All of these tell you whether a key has been
    pressed.  If no key has been pressed, they return that information
    to your program.  If a keystroke is waiting, they tell your program
    that but leave the key in the input buffer.

    You can use the BIOS call, INT 16 AH=01 or 11, to check whether an
    actual keystroke is waiting; or the DOS call, INT 21 AH=0B, to check
    for a keystroke from stdin (subject to redirection).  See Ralf
    Brown's interrupt list (Q 1.17).

    3.03 - How can I disable Ctrl-C/Ctrl-Break and/or Ctrl-Alt-Del?
    ---------------------------------------------------------------

    Several utilities are downloadable from /pub/msdos/keyboard at
    SimTel.  In that directory, cadel.zip contains a TSR (with source
    code) to disable those keys.  Also, keykill.arc contains two
    utilities:  keykill.com lets you disable up to three keys of your
    choice, and deboot.com changes the boot key to leftShift-Alt-Del.

    C programmers who simply want to make sure that the user can't Ctrl-
    Break out of their program can use the ANSI-standard signal()
    function; the Borland compilers also offer ctrlbrk() for handling
    Ctrl-Break.  However, if your program uses normal DOS input such as
    getch(), ^C will appear on the screen when the user presses Ctrl-C
    or Ctrl-Break.  You can avoid the ^C echo for Ctrl-C by using
    _bios_keybrd() in MSC or bioskey() in BC++; however, Ctrl-Break will
    still terminate the program.

    An alternative approach involves programming input at a lower level.
    You can use INT 21 AH=7, which allows redirection but doesn't echo
    the ^C (or any other character, for that matter); or use INT 16 AH=0
    or 10; or hook INT 9 to discard Ctrl-C and Ctrl-Break before the
    regular BIOS keyboard handler sees them; etc., etc.

    You should be aware that Ctrl-C and Ctrl-Break are processed quite
    differently internally.  Ctrl-Break, like all keystrokes, is
    processed by the BIOS code at INT 9 as soon as the user presses the
    keys, even if earlier keys are still in the keyboard buffer:  by
    default the handler at INT 1B is called.  Ctrl-C is not special to
    the BIOS, nor is it special to DOS functions 6 and 7; it _is_
    special to DOS functions 1 and 8 when at the head of the keyboard
    buffer.  You will need to make sure BREAK is OFF to prevent DOS
    polling the keyboard for Ctrl-C during non-keyboard operations.

    Some good general references are {Advanced MS-DOS} by Ray Duncan,
    ISBN 1-55615-157-8; {8088 Assembler Language Programming:  The IBM
    PC}, ISBN 0-672-22024-5, by Willen & Krantz; and {COMPUTE!'s Mapping
    the IBM PC}, ISBN 0-942386-92-2.

    3.04 - How can I disable the print screen function?
    ---------------------------------------------------

    There are really two print screen functions:  1) print current
    screen snapshot, triggered by PrintScreen or Shift-PrtSc or Shift-
    grey*, and 2) turn on continuous screen echo, started and stopped by
    Ctrl-P or Ctrl-PrtSc.

    1) Screen snapshot to printer

    The BIOS uses INT 5 for this.  Fortunately, you don't need to mess
    with that interrupt handler.  The standard handler, in BIOS versions
    dated December 1982 or later, uses a byte at 0040:0100 (= 0000:0500)
    to determine whether a print screen is currently in progress.  If it
    is, pressing PrintScreen again is ignored.  So to disable the screen
    snapshot, all you have to do is write a 1 to that byte.  When the
    user presses PrintScreen, the BIOS will think that a print screen is
    already in progress and will ignore the user's keypress.  You can re-
    enable PrintScreen by zeroing the same byte.

    Here's some simple code:

        void prtsc_allow(int allow) /* 0=disable, nonzero=enable */ {
            unsigned char far* flag = (unsigned char far*)0x00400100UL;
            *flag = (unsigned char)!allow;
        }

    2) Continuous echo of screen to printer

    If ANSI.SYS is loaded, you can easily disable the continuous echo of
    screen to printer (Ctrl-P or Ctrl-PrtSc).  Just redefine the keys by
    "printing" strings like these to the screen (BASIC print, C
    printf(), Pascal Write statements, or ECHO command in batch files),
    where <27> stands for the Escape character, ASCII 27:

        <27>[0;114;"Ctrl-PrtSc disabled"p
        <27>[16;"^P"p

    If you haven't installed ANSI.SYS, I can't offer an easy way to
    disable the echo-screen-to-printer function.

    Actually, you might not need to disable Ctrl-P and Ctrl-PrtSc.  If
    your only concern is not locking up your machine, when you see the
    "Abort, Retry, Ignore, Fail" prompt just press Ctrl-P again and then
    press I.  As an alternative, install one of the many print spoolers
    that intercept printer-status queries and always return "Printer
    ready".

    3.05 - How can my program turn NumLock (CapsLock, ScrollLock) on
           or off?
    ----------------------------------------------------------------

    First, if you just don't want NumLock turned on when you reboot,
    check your system's setups.  (Use Ctrl-Alt-Enter any time, or press
    a special key like Del at boot time, or run the setup program
    supplied with your system.)  Many systems now have an option in
    setup to turn NumLock off at boot time.

    You need to twiddle bit 5, 6, or 4 of location 0040:0017.  Here's
    some code:  lck() turns on a lock state, and unlck() turns it off.
    (The status lights on some keyboards may not reflect the change.  If
    yours is one, call INT 16 AH=2, "get shift status", and that may
    update them.  It will certainly do no harm.)

        #define NUM_LOCK  (1 << 5)
        #define CAPS_LOCK (1 << 6)
        #define SCRL_LOCK (1 << 4)
        void lck(int shiftype) {
            char far* kbdstatus = (char far*)0x00400017UL;
            *kbdstatus |= (char)shiftype;
        }
        void unlck(int shiftype) {
            char far* kbdstatus = (char far*)0x00400017UL;
            *kbdstatus &= ~(char)shiftype;
        }

    3.06 - How can I speed up the keyboard's auto-repeat?
    -----------------------------------------------------

    The keyboard speed has two components: delay (before a key that you
    hold down starts repeating) and typematic rate (the speed once the
    key starts repeating).  Most BIOS versions since 1986 let software
    change the delay and typematic rate by calling INT 16 AH=3, "set
    typematic rate and delay"; see Ralf Brown's interrupt list (Q 1.17).
    If you have DOS 4.0 or later, you can use the MODE CON command that
    you'll find in your DOS manual.

    On 83-key keyboards (mostly XTs), the delay and typematic rate can't
    easily be changed.  According to PC Magazine 15 Jan 1991 (x:1) page
    409, to adjust the typematic rate you need "a memory-resident
    program which simply '[watches]' the keyboard to see if you're
    holding down a key ... and after a certain time [starts] stuffing
    extra copies of the held-down key into the buffer."  No source code
    is given in that issue; but the QUICKEYS utility that PC Magazine
    published in 1986 does this sort of watching (not verified); source
    and object code are downloadable in

        /pub/msdos/pcmag/vol5n05.zip from SimTel.

    3.07 - What is the SysRq key for?
    ---------------------------------

    There is no standard use for the key.  The BIOS keyboard routines in
    INT 16 simply ignore it; therefore so do the DOS input routines in
    INT 21 as well as the keyboard routines in libraries supplied with
    high-level languages.

    When you press or release a key, the keyboard triggers hardware line
    IRQ1, and the CPU calls INT 9.  INT 9 reads the scan code from the
    keyboard and the shift states from the BIOS data area.

    What happens next depends on whether your PC's BIOS supports an
    enhanced keyboard (101 or 102 keys).  If so, INT 9 calls INT 15
    AH=4F to translate the scan code.  If the translated scan code is 54
    hex (for the SysRq key) then INT 9 calls INT 15 AH=85 and doesn't
    put the keystroke into the keyboard buffer.  The default handler of
    that function does nothing and simply returns.  (If your PC has an
    older BIOS that doesn't support the extended keyboards, INT 15 AH=4F
    is not called.  Early ATs have 84-key keyboards, so their BIOS calls
    INT 15 AH=85 but not 4F.)

    Thus your program is free to use SysRq for its own purposes, but at
    the cost of some programming.  You could hook INT 9, but it's
    probably easier to hook INT 15 AH=85, which is called when SysRq is
    pressed or released.

    3.08. How can my program tell what kind of keyboard is on the
               system?
    -------------------------------------------------------------

    Ralf Brown's Interrupt List (Q 1.17) includes MEMORY.LST, a detailed
    breakdown by Robin Walker of the contents of the BIOS system block
    that starts at 0040:0000.  Bit 4 of byte 0040:0096 is "1=enhanced
    keyboard installed".  C code to test the keyboard type:

        char far *kbd_stat_byte3 = (char far *)0x00400096UL;
        if (0x10 & *kbd_stat_byte3)
            /* 101- or 102-key keyboard is installed */

    PC Magazine 15 Jan 1991 (x:1) suggests on page 412 that "for some
    clones [the above test] is not foolproof".  If you use this method
    in your program you should provide the user some way to override
    this test, or at least some way to tell your program to assume a non-
    enhanced keyboard.  The article suggests a different approach to
    determining the type of keyboard.

    3.09 - How can I tell if input, output, or stderr has been redirected?
    ----------------------------------------------------------------------

    Normally, input and output are associated with the console (i.e.,
    with the keyboard and the screen, respectively).  If either is not,
    you know that it has been redirected.  Some source code to check
    this is available at the usual archive sites.

    If you program in Turbo Pascal, you'll want this downloadable
    collection of Turbo Pascal units:

        /pc/ts/tspa33*.zip at Garbo
        /pub/msdos/turbopas/tspa33*.zip at SimTel.

    (where the * is 70, 60, 55, 50, or 40 for Turbo Pascal 7.0, 6.0,
    5.5, 5.0, or 4.0 respectively.)  Source code is not included.  Also
    see the downloadable Frequently Asked Questions files by Timo Salmi:

        /pc/ts/tsfaqp18.zip at Garbo
        /pub/msdos/info/tsfaqp18.zip at SimTel.

    If you program in C, use isatty() if your implementation has it.
    Otherwise, /pub/msdos/sysutil/is_con10.zip is downloadable from
    SimTel; it includes source code.

    Good references for the principles are PC Magazine 16 Apr 1991 (x:7)
    page 374; Ray Duncan's {Advanced MS-DOS}, ISBN 1-55615-157-8, or
    Ralf Brown's interrupt list (Q 1.17) for INT 21 AX=4400; and Terry
    Dettman and Jim Kyle's {DOS Programmer's Reference: 2d edition},
    ISBN 0-88022-458-4, pages 602-603.

    3.10 - How can I increase the size of the keyboard buffer?
    ----------------------------------------------------------

    I tested only one of the many available device drivers that do this,
    namely BUF160, which extends the keyboard buffer to 160 characters.
    It performed flawlessly for two years with MS-DOS 5 and Windows 3.1.
    It's downloadable as

        /pub/msdos/keyboard/buf160_6.zip at SimTel
        /pc/keyboard/buf160_6.zip at Garbo.

    3.11 - How can I stuff characters into the keyboard buffer?
    -----------------------------------------------------------

    If your computer has an enhanced keyboard (see "3.08 - How can my
    program tell what kind of keyboard is on the system?"), put the scan
    code in CH and the ASCII character in CL, then execute INT 16 AH=5.
    The return in AL is 0 for success or 1 for buffer full.


Section 4.  Disks and files
---------------------------

    4.01 - What drive was the PC booted from?
    -----------------------------------------

    Under DOS 4.0 or later, use INT 21 AX=3305.  DL is returned with an
    integer indicating the boot drive (1=A:, etc.).

    4.02 - How can I boot from drive b:?

    Downloadable shareware:

        /pub/msdos/diskutil/boot_b.zip from SimTel
        /pc/bootutil/boot_b.zip from Garbo.

    The included documentation says it works by writing a new boot
    sector on a disk in your a: drive that redirects the boot to your b:
    drive.  (A similar utility is bboot.zip in the same directory at
    Garbo only.)

    If that doesn't work, you can always interchange your a: and b:
    drives by switching ribbon cables and changing the setup in your
    BIOS.  From an article posted 27 Jan 1993 on another newsgroup:

    Take the "ribbon" connector, as you call it, and switch them.  To
    double check, start at the end of the cable that connects to the
    motherboard or floppy controller.  Follow the cable until you get to
    the first connector.  Connect this to the drive you want to be b:.
    After this, there should be a few lines on the cable that get
    flipped left to right.  (On most cables, they just cut the lines and
    physically reverse them.  It should be quite obvious from looking at
    the cable.)  Anyway, the connector after the pins get flipped right
    to left is the connector for your a: drive.

    4.03 - Which real and virtual disk drives are valid?
    ----------------------------------------------------

    Use INT 21 AH=29 (parse filename).  Point DS:SI at a null-terminated
    ASCII string that contains the drive letter and a colon, point ES:DI
    at a 37-byte dummy FCB buffer, and call INT 21 AX=2900.  On return,
    AL is FF if the drive is invalid, something else if the drive is
    valid.  RAM disks and SUBSTed drives are considered valid.

    You can detect whether the drive is ASSIGNed by using INT 2F
    AX=0601.  To check whether the drive is SUBSTed, use INT 21 AX=4409;
    or use INT 21 AH=52 to test for both JOIN and SUBST.  See Ralf
    Brown's interrupt list (Q 1.17).

    Unfortunately, the b: drive is considered valid even on a single-
    diskette system.  You can check that special case by interrogating
    the BIOS equipment byte at 0040:0010.  Bits 7-6 contain the one less
    than the number of diskette drives, so if those bits are zero you
    know that b: is an invalid drive even though function 29 says it's
    valid.

    Following is some code originally posted by Doug Dougherty to test
    valid drives (treating SUBSTed and JOINed drives as valid), with my
    fix for the b: special case, tested in Borland C++ 2.0 (in the small
    model):

        #include <dos.h>
        #include <stdio.h>
        void drvlist(void) {
            char *s = "A:", fcb_buff[37];
            int valid;
            for (   ;  *s<='Z';  (*s)++) {
                _SI = (unsigned) s;
                _DI = (unsigned) fcb_buff;
                _ES = _DS;
                _AX = 0x2900;
                geninterrupt(0x21);
                valid = _AL != 0xFF;
                if (*s == 'B'  &&  valid) {
                    char far *equipbyte = (char far *)0x00400010UL;
                    valid = (*equipbyte & (3 << 6)) != 0;
                }
                printf("Drive '%s' is %sa valid drive.\n",
                        s, valid ? "" : "not ");
            }
        }

    This code was translated to MSC 7.0 and tested it in small model:

        #include <dos.h>
        #include <stdio.h>
        void drvlist(void) {
            char *s = "A:", fcb_buff[37], *buff=fcb_buff;
            int valid;
            for (   ;  *s<='Z';  (*s)++) {
                __asm mov si,s      __asm mov di,buff
                __asm mov ax,ds     __asm mov es,ax
                __asm mov ax,0x2900 __asm int 21h
                __asm xor ah,ah     __asm mov valid,ax
                valid = (valid != 0xFF);
                if (*s == 'B'  &&  valid) {
                    char far *equipbyte = (char far *)0x00400010UL;
                    valid = (*equipbyte & (3 << 6)) != 0;
                }
                printf("Drive '%s' is %sa valid drive.\n",
                        s, valid ? "" : "not ");
            }
        }

   4.04 - How can I make my single floppy drive both a: and b:?
   ------------------------------------------------------------

    Under any DOS since DOS 2.0, you can put the command

        assign b=a

    into your AUTOEXEC.BAT file.  Then, when you type "dir b:" you'll no
    longer get the annoying prompt to insert diskette B (and the even
    more annoying prompt to insert A the next time you type "dir a:").

    You may be wondering why anybody would want to do this.  Suppose you
    use two different machines, maybe one at home and one at work.  One
    of them has only a 3.5" diskette drive; the other machine has two
    drives, and b: is the 3.5" one.  You're bound to type "dir b:" on
    the first one, and get the nuisance message

        Insert diskette for drive B: and press any key when ready.

    But if you assign drive b: to point to a:, you avoid this problem.

    Caution:  there are a few commands, such as DISKCOPY, that will not
    work right on ASSIGNed or SUBSTed drives.  See the DOS manual for
    the full list.  Before typing one of those commands, be sure to turn
    off the mapping by typing "assign" without arguments.

    The DOS 5.0 manual says that ASSIGN is obsolete, and recommends the
    equivalent form of SUBST: "subst b: a:\".  Unfortunately, if this
    command is executed when a: doesn't hold a diskette, the command
    fails.  ASSIGN doesn't have this problem, so under DOS 5.0 you
    should disregard that particular bit of advice in the manual.

    4.05 - How can I disable access to a drive?
    -------------------------------------------

    Reader Eric DeVolder writes that he has made available a program to
    do this.  I haven't tried it, but it's downloadable as

        /pub/msdos/diskutil/rmdriv20.zip from SimTel
        /pc/sysutil/rmdriv20.zip at Garbo.

    4.06 - How can a batch file test existence of a directory?
    ----------------------------------------------------------

    The standard way, which in fact is documented in the DOS manual, is

        if exist d:\path\nul goto found

    Unfortunately, this is not entirely reliable.  I found it failed in
    Pathworks (a/k/a PCSA, DEC's network that connects PCs and VAXes),
    or on a MARS box that uses an OEM version of MS-DOS 5.0.  Readers
    have reported that it gave the wrong answer on Novell networks, on
    DR-DOS, and in a DOS window under OS/2.  By "failed" I mean that it
    "found" a directory that didn't exist, or failed to find one that
    did exist, or both.  (I'm told that IBM fixed the OS/2 bug in
    version 2.11 of OS/2.)

    There appears to be no foolproof way to use pure batch commands to
    test for existence of a directory.  The real solution is to write a
    program, which returns a value that your batch program can then test
    with an "if errorlevel".  Reader Duncan Murdoch kindly posted the
    following Turbo Pascal version:

        program existdir;
            { Confirms the existence of a directory given on the command
            line.  Returns errorlevel 2 on error, 1 if not found, 0 if
            found. }

        uses
            dos;

        var
            s : searchrec;

        begin
            if paramcount <> 1 then
            begin
                writeln('Syntax:  EXISTDIR directory');
                halt(2);
            end
            else
            begin
                findfirst(paramstr(1),Directory,S);
                while (Doserror = 0) and ((Directory and S.Attr) = 0) do
                    findnext(S);
                if Doserror <> 0 then
                begin
                    Writeln('Directory not found.');
                    halt(1);
                end
                else
                begin
                    Writeln('Directory found.');
                    halt(0);
                end;
            end;
        end.

    Timo Salmi also has a Turbo Pascal version in his Turbo Pascal FAQ,
    which is downloadable as

        /pc/ts/tsfaqp18.zip at Garbo
        /pub/msdos/info/tsfaqp18.zip at SimTel.

    4.07 - Why won't my C program open a file with a path?
    ------------------------------------------------------

    You've probably got something like the following code:

        char *filename = "c:\foo\bar\mumble.dat";
        FILE *fptr;
        . . .
        fptr = fopen(filename, "r");

    The problem is that \f is a form feed, \b is a backspace, and \m is
    m.  Whenever you want a backslash in a string constant in C, you
    must use two backslashes:

        char *filename = "c:\\foo\\bar\\mumble.dat";

    This is a feature of every C compiler, because Dennis Ritchie
    designed C this way.  It's a problem only on MS-DOS systems, because
    only DOS (and Atari ST/TT running TOS) uses the backslash in
    directory paths.  But even in DOS this backslash convention applies
    _only_ to string constants in your source code.  For file and
    keyboard input at run time, \ is just a normal character, so users
    running your program would type in file specs the same way as in DOS
    commands, with single \ characters.

    Another possibility is to code all paths in source programs with /
    rather than \ characters:

        char *filename = "c:/foo/bar/mumble.dat";

    Ralf Brown writes, "All versions of the DOS kernel accept either
    forward or backslashes as directory separators.  I tend to use this
    form more frequently than backslashes since it is easier to type and
    read."  This applies to DOS function calls (and therefore to calls
    to the file library of every programming language), but not to DOS
    commands.

    4.08 - How can I redirect printer output to a file?
    ---------------------------------------------------

    Recommended: PRN2FILE from PC Magazine, downloadable as:

        /pub/msdos/printer/prn2file.zip at SimTel
        /pc/printer/prn2file.zip at Garbo.

    PC Magazine has given copies away as part of its utilities disks, so
    you may already have a copy.

    The directories mentioned above have lots of other utilities to
    redirect printer output.

    4.09 - How can I redirect the output of a batch file?
    -----------------------------------------------------

    Assuming the batch file is called batch.bat, to send its output
    (stdout) to another file, just invoke COMMAND.COM as a secondary
    command processor:

        command /c batch parameters_if_any >outfile

    Timo Salmi's notes on this and other batch tricks are downloadable:

        /pub/msdos/batutil/tsbat44.zip at SimTel
        /pc/ts/tsbat44.zip at Garbo.

    4.10 - How can I redirect stderr?
    ---------------------------------

    Use freopen(..., stderr) and then execute the desired command via
    system().  There are downloadable versions of programs to do this.
    I recommend this file, which includes TP4 source and executable:

        /pub/msdos/sysutil/rdstderr.zip from SimTel.

    A C example is downloadable as

        /pub/msdos/c/redirect.c from SimTel.

    I compiled it with MSC 7.0, and it works fine with one exception:
    Contrary to the included comments, redirected output starts writing
    at the beginning of the output file rather than appending.  That is
    easily solved by adding "fseek(stderr, 0L, SEEK_END);" after the
    freopen() call for stderr.

    4.11 - How can my program open more files than DOS's limit of 20?
    -----------------------------------------------------------------

    This is a summary of an article Ralf Brown posted on 8 August 1992,
    with some additions from a Microsoft tech note and information from
    Chin Huang.)

    DOS imposes some limits.  Once you overcome those, which is pretty
    easy, you may have to take additional measures to overcome the
    limitations built into your compiler's run-time library.

    1) Limitations imposed by DOS

    There are separate limits on files and file handles.  For example,
    DOS opens three files but five file handles:  CON (stdin, stdout,
    and stderr), AUX (stdaux), and PRN (stdprn).

    The limit in FILES= in CONFIG.SYS is a system-wide limit on files
    opened by all programs (including the three that DOS opens and any
    opened by TSRs); each process has a limit of 20 handles (including
    the five that DOS opens).  Example:  CONFIG.SYS has FILES=40.  Then
    program #1 will be able to open 15 file handles.  Assuming that the
    program actually does open 15 handles pointing to 15 different
    files, other programs could still open a total of 22 files (40-3-15
    = 22), though no one program could open more than 15 file handles.

    If you're running DOS 3.3 or later, you can increase the per-process
    limit of 20 file handles by a call to INT 21 AH=67, Set Handle
    Count.  Your program is still limited by the system-wide limit on
    open files, so you may also need to increase the FILES= value in
    your CONFIG.SYS file (and reboot).  The run-time library that you're
    using may have a fixed-size table of file handles, so you may also
    need to get source code for the module that contains the table,
    increase the table size, and recompile it.

    2) Limitations in Microsoft C run-time library

    In Microsoft C the run-time library limits you to 20 file handles.
    To change this, you must be aware of two limits:

    - file handles used with _open(), _read(), etc.: Edit _NFILE_ in
      CRT0DAT.ASM.

    - stream files used with fopen(), fread(), etc.: Edit _NFILE_ in
      _FILE.C for DOS or FILE.ASM for Windows/QuickWin.  This must not
      exceed the value of _NFILE_ in CRT0DAT.ASM.

    (QuickWin uses the constant _WFILE_ in CRT0DAT.ASM and WFILE.ASM for
    the maximum number of child text windows.)

    After changing the limits, recompile using CSTARTUP.BAT.  Microsoft
    recommends that you first read README.TXT in the same directory.

    3) Limitations in Borland C++ run-time library

    (Reader Chin Huang provided this information on 12 Sep 1993.)

    To increase the open file limit for a program you compile with
    Borland C++ 3.1, edit the file _NFILE.H in the include directory and
    change the _NFILE_ value.  Compile and link the modules FILES.C and
    FILES2.C from the lib directory into your program.

    4.12 - How can I read, create, change, or delete the volume label?
    ------------------------------------------------------------------

    In DOS 5.0 (and possibly in 4.0 as well), there are actually two
    volume labels: the LABEL command reports only the first but changes
    both of them.

    - The traditional volume label is an entry with "volume label"
      attribute in the root directory of the disk.  The DIR, VOL, and
      LABEL commands report this volume label, and LABEL sets it.

    - There is a second volume label, which may be different, in the
      boot record along with the serial number. In DOS 4.0 and later,
      INT 21 AH=69 gets or sets the boot record's serial number and
      volume label together; see "4.13 - How can I get the disk serial
      number?"  DIR and VOL ignore this volume label; the LABEL command
      doesn't report it but does set it.

    The rest of this answer assumes that by "volume label" you mean the
    traditional one, the one that DIR and VOL display.  Though it's a
    directory entry in the root directory, you can't change it using the
    newer DOS file-access functions (INT 21 AH=3C, 41, 43); instead, use
    the old FCB-oriented directory functions.  Specifically, you need to
    allocate a 64-byte buffer and a 41- byte extended FCB (file control
    block).  Call INT 21 AH=1A to find out whether there is a volume
    label.  If there is, AL returns 0 and you can change the label using
    DOS function 17 or delete it using DOS function 13.  If there's no
    volume label, function 1A will return FF and you can create a label
    via function 16.  Important points to notice are that ? wildcards
    are allowed but * are not; the volume label must be space filled not
    null terminated.

    The following MSC 7.0 code worked for me in DOS 5.0; the functions
    it uses have been around since DOS 2.0.  The function parameter is 0
    for the current disk, 1 for a:, 2 for b:, etc.  It doesn't matter
    what your current directory is; these functions always search the
    root directory for volume labels.  (I didn't try to change the
    volume label of any networked drives.)

        // Requires DOS.H, STDIO.H, STRING.H
        void vollabel(unsigned char drivenum) {
            static unsigned char extfcb[41], dta[64], status, *newlabel;
            int chars_got = 0;
            #define DOS(buff,func) __asm { __asm mov dx,offset buff   \
                __asm mov ax,seg buff  __asm push ds  __asm mov ds,ax \
                __asm mov ah,func  __asm int 21h  __asm pop ds        \
                __asm mov status,al }
            #define getlabel(buff,prompt) newlabel = buff;  \
                memset(newlabel,' ',11);  printf(prompt);   \
                scanf("%11[^\n]%n", newlabel, &chars_got);  \
                if (chars_got < 11) newlabel[chars_got] = ' ';

            // Set up the 64-byte transfer area used by function 1A.
            DOS(dta, 1Ah)
            // Set up an extended FCB and search for the volume label.
            memset(extfcb, 0, sizeof extfcb);
            extfcb[0] = 0xFF;           // denotes extended FCB
            extfcb[6] = 8;              // volume-label attribute bit
            extfcb[7] = drivenum;       // 1=A,2=B,...; 0=current drive
            memset(&extfcb[8], '?', 11);// wildcard *.*
            DOS(extfcb,11h)
            if (status == 0) {          // DTA has volume label's FCB
                printf("volume label is %11.11s\n", &dta[8]);
                getlabel(&dta[0x18],
                    "new label (\"delete\" to delete): ");
                if (chars_got == 0)
                    printf("label not changed\n");
                else if (strncmp(newlabel,"delete  ",11) == 0) {
                    DOS(dta,13h)
                    printf(status ? "label failed\n" :
                                    "label deleted\n");
                }
                else {                  // user wants to change label
                    DOS(dta,17h)
                    printf(status ? "label failed\n" :
                                    "label changed\n");
                }
            }
            else {                      // no volume label was found
                printf("disk has no volume label.\n");
                getlabel(&extfcb[8], "new label (<Enter> for none): ");
                if (chars_got > 0) {
                    DOS(extfcb,16h)
                    printf(status ? "label failed\n" :
                                    "label created\n");
                }
            }
        }        // end function vollabel

    4.13 - How can I get the disk serial number?
    --------------------------------------------

    If the disk was formattted by DOS 4.0 or later, use INT 21: AX=6900
    gets the serial number; AX=6901 sets it.  (The disk serial number
    doesn't exist if the disk was formatted with an earlier version of
    DOS, or with some third-party formatters.)  See Ralf Brown's
    interrupt list (Q 1.17), or PC Magazine July 1992 (xi:13) page 496,
    for details.

    INT 21 AH=69 also gets and sets the volume label in the boot record,
    which is not necessarily the same as "the" volume label displayed by
    the DIR, VOL, and LABEL commands.  For that volume label, see "4.12 -
    How can I read, create, change, or delete the volume label?"

    4.14 - What's the format of .OBJ, .EXE., .COM files?
    ----------------------------------------------------

    Please see "2.06 - What's the format of an .OBJ file?"; "2.07 - What's
    the format of an .EXE header?"; and "2.08 - What's the difference
    between .COM and .EXE formats?"

    4.15 - How can I flush the software disk cache?
    -----------------------------------------------

    Please see "7.01 - How can a program reboot my PC?"


Section 5.  Serial ports (COM ports)
------------------------------------

    5.01 - How do I set my machine up to use COM3 and COM4?
    -------------------------------------------------------

    Unless your machine is fairly old, it's probably already set up.
    After installing the board that contains the extra COM port(s),
    check the I/O addresses in word 0040:0004 or 0040:0006.  (In DEBUG,
    type "D 40:4 L4" and remember that every word is displayed low byte
    first, so if you see "03 56" the word is 5603.)  If those addresses
    are nonzero, your PC is ready to use the ports and you don't need
    the rest of this answer.

    If the I/O address words in the 0040 segment are zero after you've
    installed the I/O board, you need some code to store these values
    into the BIOS data segment:

        0040:0004  word  I/O address of COM3
        0040:0006  word  I/O address of COM4
        0040:0011  byte (bits 3-1): number of serial ports installed

    The documentation with your I/O board should tell you the port
    addresses.  When you know the proper port addresses, you can add
    code to your program to store them and the number of serial ports
    into the BIOS data area before you open communications.  Or you can
    use DEBUG to create a little program to include in your AUTOEXEC.BAT
    file, using this script:

        n SET_ADDR.COM      <--- or a different name ending in .COM
        a 100
        mov  AX,0040
        mov  DS,AX
        mov  wo [0004],aaaa <--- replace aaaa with COM3 address or 0
        mov  wo [0006],ffff <--- replace ffff with COM4 address or 0
        and  by [0011],f1
        or   by [0011],8    <--- use number of serial ports times 2
        mov  AH,0
        int  21
                            <--- this line must be blank
        rCX
        1f
        rBX
        0
        w
        q

    5.02 - How do I find the I/O address of a COM port?
    ---------------------------------------------------

    Look in the four words beginning at 0040:0000 for COM1 through COM4.
    (The DEBUG command "D 40:0 L8" will do this.  Remember that words
    are stored and displayed low byte first, so a word value of 03F8
    will be displayed as F8 03.)  If the value is zero, that COM port is
    not installed (or you've got an old BIOS; see "5.01 - How do I set my
    machine up to use COM3 and COM4?").  If the value is nonzero, it is
    the I/O address of the transmit/receive register for the COM port.
    Each COM port occupies eight consecutive I/O addresses (though many
    chips use only the first seven).

    Here's some C code to find the I/O address:

        unsigned ptSel(unsigned comport) {
            unsigned io_addr;
            if (comport >= 1  &&  comport <= 4) {
                unsigned far *com_addr = (unsigned far *)0x00400000UL;
                io_addr = com_addr[comport-1];
            }
            else
                io_addr = 0;
            return io_addr;
        }

    You might also want to explore Port Finder, downloadable as

        /pub/msdos/io_util/pf271.zip at SimTel
        /pub/msdos/utilities/sysutl/pf271.zip at nic.funet.fi

    I haven't tried it myself, but a posted article reviewed it very
    favorably and said it also lets you swap ports around.

    5.03 - But aren't the COM ports always at I/O addresses 3F8,
               2F8, 3E8, and 2E8?
    ------------------------------------------------------------

    The first two are usually right (though not always); the last two
    are different on many machines.

    5.04. How do I configure a COM port and use it to transmit data?
    ------------------------------------------------------------
    (Revised: 05/09/1994)

    Do you want actual code, or do you want books that explain what's
    going on?

    1) Source code

    First, check your compiler's run-time library.  Many compilers offer
    functions similar to Microsoft C's _bios_serialcom() or Borland's
    bioscom(), which may meet your needs.

    Second, check for downloadable resources at SimTel and Garbo.  At
    SimTel, /pub/msdos/c/pcl4c34.zip (March 1993) is described as
    "Asynchronous communications library for C"; Garbo has a whole
    /pc/comm directory.  Also, an extended example is in Borland's
    TechFax TI445, downloadable as part of

        /pub/msdos/turbo-c/bchelp10.zip at SimTel
        /pc/turbopas/bchelp10.zip at Garbo.

    Though written by Borland, much of it is applicable to other forms
    of C, and it should give you ideas for other programming languages.

    2) Reference books

    Highly recommended: Joe Campbell's {C Programmer's Guide to Serial
    Communications}, ISBN 0-672-22584-0.  He gives complete details on
    how serial ports work, along with complete programs for doing polled
    or interrupt-driver I/O.  The book is quite thick, and none of it
    looks like filler.

    If Campbell's book is overkill for you, you'll find a good short
    description of serial I/O in {DOS 5: A Developer's Guide}, ISBN
    1-55851-177-6, by Al Williams.

    Finally, a reader has recommended {Serial Communications Programming
    in C/C++} by Mark Goodwin (ISBN 1558281983), with source code in the
    book and on disk.  Topics include the basics, various methods of
    serial communications on the PC (with consideration of high-speed
    modems), ANSI screen interface, file transfer protocols (Xmodem and
    Ymodem), etc.  There is code in C, and that code is extended into a
    C++ class for those who use C++.  There are also subroutines in
    Assembly.

    3) Downloadable information files

    A "Serial Port FAQ" is occasionally posted to this newsgroup, and is
    downloadable as multiple files:

        /pub/E-Technik/afd/*Serial* from pfsparc02.phil15.uni-sb.de.

    For rtfm.mit.edu instructions, see "1.13 - Where are FAQ lists
    archived?"

    (The uni-sb.de archive administrator warns that the ftp address may
    change, sometime in the future, to etcip1.ee.uni-sb.de.)


Section 6. Other hardware questions and problems
------------------------------------------------

    6.01 - Which 80x86 CPU is running my program?
    ---------------------------------------------

    According to an article posted by Michael Davidson, Intel's approved
    code for distinguishing among 8086, 80286, 80386, and 80486 and for
    detecting the presence of an 80287 or 80387 is published in Intel's
    486SX processor manual (order number 240950-001).  David
    Kirschbaum's improved version of this is downloadable as

        /pub/msdos/sysinfo/cpuid593.zip from SimTel
        /pc/sysinfo/cpuid593.zip from Garbo.

    According to an article posted by its author, WCPU knows the
    differences between DX and SX varieties of 386 and 486 chips, and
    can detect a math coprocessor and a Pentium.  It's downloadable as

        /pub/msdos/sysinfo/wcpu050.zip at SimTel
        /pc/sysinfo/wcpu050.zip at Garbo.

    6.02 - How can a C program send control codes to my printer?
    ------------------------------------------------------------

    If you just fprintf(stdprn, ...), C will translate some of your
    control codes.  The way around this is to reopen the printer in
    binary mode:

        prn = fopen("PRN", "wb");

    You must use a different file handle because stdprn isn't an lvalue.
    By the way, PRN or LPT1 must not be followed by a colon in DOS 5.0.

    There's one special case, Ctrl-Z (ASCII 26), the DOS end-of-file
    character.  If you try to send an ASCII 26 to your printer, DOS
    simply ignores it.  To get around this, you need to reset the
    printer from "cooked" to "raw" mode.  Microsoft C users must use INT
    21 AH=44, "get/set device information".  Turbo C and Borland C++
    users can use ioctl to accomplish the same thing:

        ioctl(fileno(prn), 1, ioctl(fileno(prn),0) & 0xFF | 0x20, 0);

    An alternative approach is simply to write the printer output into a
    disk file, then copy the file to the printer with the /B switch.

    A third approach is to bypass DOS functions entirely and use the
    BIOS printer functions at INT 17.  If you also fprintf(stdprn,...)
    in the same program, you'll need to use fflush() to synchronize
    fprintf()'s buffered output with the BIOS's unbuffered.

    By the way, if you've opened the printer in binary mode from a C
    program, remember that outgoing \n won't be translated to carriage
    return/line feed.  Depending on your printer, you may need to send
    explicit \n\r sequences.

    6.03 - How can I redirect printer output to a file?
    ---------------------------------------------------

    Please see "4.08 - How can I redirect printer output to a file?"

    6.04 - Which video adapter is installed?
    ----------------------------------------

    The technique below should work if your BIOS is not too old.  It
    uses three functions from INT 10, the BIOS video interrupt.  (If
    you're using a Borland language, you may not have to do this the
    hard way.  Look for a function called DetectGraph or something
    similar.)

    Set AX=1200, BL=32 and call INT 10.  If AL returns 12, you have a
    VGA.  If not, set AH=12, BL=10 and call INT 10 again.  If BL returns
    0,1,2,3, you have an EGA with 64,128,192,256K memory.  If not, set
    AH=0F and call INT 10 a third time.  If AL is 7, you have an MDA
    (original monochrome adapter) or Hercules; if not, you have a CGA.

    This worked when tested with a VGA, but I had no other adapter types
    to test it with.

    6.05 - How do I switch to 43- or 50-line mode?
    ----------------------------------------------

    /pub/msdos/screen/vidmode.zip, downloadable from SimTel, contains
    .COM utilities and .ASM source code.

    6.06 - How can I find the Microsoft mouse position and button status?
    ---------------------------------------------------------------------

    Use INT 33 AH=3, described in Ralf Brown's interrupt list (Q 1.17).

    The Windows manual says that the Logitech mouse is compatible with
    the Microsoft one, so the interrupt will probably work the same.

    Also, many files are downloadable from

        /pub/msdos/mouse/*.* at SimTel.

    6.07 - How can I access a specific address in the PC's memory?
    --------------------------------------------------------------

    First check the library that came with your compiler.  Many vendors
    have some variant of peek and poke functions. For example:

    - In Turbo Pascal, use the pseudo-arrays Mem, MemW, and MemL.  Be
      sure you use the correct array for the size of data you want to
      access: byte, word, or double word.

    - In Turbo C/Borland C, and in recent versions of Microsoft C, use
      MK_FP; in older versions of Microsoft C, use FP_OFF and FP_SEG.
      (Caution:  Turbo C and Turbo C++ also have FP_OFF and FP_SEG
      macros, but they can't be used to construct a pointer.)  Be sure
      to pick the right data type: probably "unsigned char far *" if
      you're planning to access bytes and "unsigned short far *" for
      words.  (The "far" isn't needed if your memory model uses 32-bit
      data pointers, but including it does no harm.)

    By the way, it's not useful to talk about "portable" ways to do
    this.  Any operation that is tied to a specific memory address is
    not likely to work on another kind of machine.

    6.08 - How can I read or write my PC's CMOS memory?
    ---------------------------------------------------

    There are a great many public-domain utilities that do this.  These
    are downloadable from SimTel in directory /pub/msdos/at:

        cmos14.zip     5965  920817  Saves/restores CMOS to/from file
        cmoser11.zip  28323  910721  386/286 enhanced CMOS setup program
        cmosram.zip   76096  920214  Save AT/386/486 CMOS data to file
            and restore
        rom2.zip      15692  900131  Save AT and 386 CMOS data to file
            and restore
        setup21.zip   18172  880613  Setup program which modifies CMOS
            RAM
        viewcmos.zip  11068  900225  Display contents of AT CMOS RAM,
            w/C source

    A program to check and display CMOS memory (but not write to it) is
    downloadable as part of

        /pc/ts/tsutle22.zip at Garbo
        /pub/msdos/sysutil/tsutle22.zip at SimTel.

    Good reports of CMOS299.ZIP, available in the pc.dir directory of
    cantva.canterbury.ac.nz [132.181.30.3], have been posted.

    Of the above, my only experience is with CMOSRAM, which seems to
    work fine.  It contains an excellent (and witty) .DOC file that
    explains the hardware involved and gives specific recommendations
    for preventing disaster or recovering from it.  It's $5 shareware.

    Robert Jourdain's {Programmer's Problem Solver for the IBM PC, XT,
    and AT} has code for accessing the CMOS RAM, according to an article
    posted in this newsgroup.

    6.09 - How can I access memory beyond 640K?
    -------------------------------------------

    PC Magazine 29 June 1993 (xii:12) pages 302-304 carry an article,
    "How DOS Programs Can Use Over 1MB of RAM".

    6.10 - How can I use the protected mode?
    ----------------------------------------
    (New: 05/09/1994)

    If you are using Borland C++ Version 4.0, you can purchase the
    Borland PowerPack for DOS Version 1.00. This package includes:
    Borland C++ 4.02 Service Update, 16-bit DPMI libraries and
    extenders, 32-bit DPMI libraries and extenders, TurboVision 2.0
    (16-bit DOS, 16-bit DPMI, 32-bit DPMI), SuperVGA BGI Drivers
    (16-bit DOS, 16-bit DPMI, 32-bit DPMI).

    Thomas Pytel has written his own DOS extender for Borland C++
    4.0.

    Here is a exert from Pytel's documentation about PMC Version 1.01:

       "PMC contains a small library of the most commonly used and needed
    system functions. Some ANSI C compliant functions are provided, but
    overall, PMC is not ANSI compatible. Malloc functions are available.
    As are many low level string and memory block functions. PMC also
    provides some DPMI functions, some non-ANSI file functions, and low
    level IRQ functions. There are low level functions for fast mode
    switching using the raw mode switching services of whatever DPMI host
    it is running under.

       "I designed the PMC interface and library to serve as a small,
    tight, kernel for other C code to be built around. My primary use
    for it will be games, demos, and many other things which do not really
    use standard C functions. But the low level functions provided in
    PMC.LIB will find much use in almost any type of program that is done.

       "PMC allows flat access to low and extended memory. You need not
    deal with segments or near and far pointers. In fact, BCC32 does
    not recognize the near or far keywords. Memory is split into two pools,
    a low memory pool and an extended memory pool. This is transparent to
    your code though, as the malloc routines check both pools for any
    memory requests. You can deal explicitly with the low or high memory
    heaps if you wish. You may need a low memory DMA buffer for example.

       "The extender used is PMODE 3.0. It is about 9k of code and it
    is internal to the EXE file which is created. For more information
    on it, you should read the PMODE documentation. The programs created
    with PMC will run on any 386+ system which is running as a clean system,
    under XMS, VCPI, or DPMI. This covers all memory managers and Windows
    and OS/2. In addition, the PMODE extender is very fast. I should know,
    I wrote for that purpose."

    PMC is freeware and includes the source code which is in C
    and assembly language. PMC Version 1.01 can be found in

        /pub/msdos/demos/programming/source/pmc101.zip at ftp.eng.ufl.edu

    Adam Seychell has written a DOS extender for ASM programmers. His
    shareware library DOS32 Version 2.4 can be found in

        /pub/msdos/demos/programming/source/dos32v24.zip at ftp.eng.ufl.edu

Section 7. Other software questions and problems
------------------------------------------------

    7.01 - How can a program reboot my PC?
    --------------------------------------

    You can generate a "cold" boot or a "warm" boot.  A cold boot is the
    same as turning the power off and on; a warm boot is the same as
    Ctrl-Alt-Del and skips the power-on self 'test.

    For a warm boot, store the hex value 1234 in the word at 0040:0072.
    For a cold boot, store 0 in that word.  Then, if you want to live
    dangerously, jump to address FFFF:0000.  Here's C code to do it:

        /* WARNING:  data loss possible */
        void bootme(int want_warm)  /* arg 0 = cold boot, 1 = warm */ {
            void (far* boot)(void) = (void (far*)(void))0xFFFF0000UL;
            unsigned far* type = (unsigned far*)0x00400072UL;
            *type = (want_warm ? 0x1234 : 0);
            (*boot)( );
        }

    What's wrong with that method?  It will boot right away, without
    closing files, flushing disk caches, etc.  If you boot without
    flushing a write-behind disk cache (if one is running), you could
    lose data or trash the file allocation table in your hard drive.

    There are two methods of signaling the cache to flush its buffers:
    (1) simulate a keyboard Ctrl-Alt-Del in the keystroke translation
    function of the BIOS (INT 15 AH=4F; but see notes below), and (2)
    issue a disk reset (DOS function 0D).  Most disk-cache programs hook
    one or both of those interrupts, so if you use both methods you'll
    probably be safe.

    When user code simulates a Ctrl-Alt-Del, one or more of the programs
    that have hooked INT 15 AH=4F can ask that the key be ignored by
    clearing the carry flag.  For example, HyperDisk does this when it
    has started but not finished a cache flush.  So if the carry flag
    comes back cleared, the boot code has to wait a couple of clock
    ticks and then try again.  (None of this matters on older machines
    whose BIOS can't support 101- or 102-key keyboards; see the
    discussion of INT 21 AH=4F in "307. What is the SysRq key for?")

    C code that tries to signal the disk cache (if any) to flush is
    given below.  Turbo Pascal code by Timo Salmi that does more or less
    the same job may be found at question 49 (as of this writing) in the
    Turbo Pascal FAQ in comp.lang.pascal, and is downloadable as file
    FAQPAS2.TXT, which is part of

        /pc/ts/tsfaqp18.zip at Garbo
        /pub/msdos/info/tsfaqp18.zip at SimTel.

    Here's C code that reboots after trying to signal the disk cache:

        #include <dos.h>
        void bootme(int want_warm)  /* arg 0 = cold boot, 1 = warm */ {
            union REGS reg;
            void    (far* boot)(void) = (void (far*)(void))0xFFFF0000UL;
            unsigned far* boottype    =     (unsigned far*)0x00400072UL;
            char     far* shiftstate  =         (char far*)0x00400017UL;
            unsigned      ticks;
            int           time_to_waste;
            /* Simulate reception of Ctrl-Alt-Del: */
            for (;;) {
                *shiftstate |= 0x0C;    /* turn on Ctrl & Alt */
                reg.h.ah = 0x4F;        /* see notes below */
                reg.h.al = 0x53;        /* 0x53 = Del's scan code */
                reg.x.cflag = 1;        /* sentinel for ignoring key */
                int86(0x15, ®, ®);
                /* If carry flag is still set, we've finished. */
                if (reg.x.cflag)
                    break;
                /* Else waste some time before trying again: */
                reg.h.ah = 0;
                int86(0x1A, ®, ®);/* system time into CX:DX */
                ticks = reg.x.dx;
                for (time_to_waste = 3;  time_to_waste > 0;  ) {
                    reg.h.ah = 0;
                    int86(0x1A, ®, ®);
                    if (ticks != reg.x.dx)
                        ticks = reg.x.dx , --time_to_waste;
                }
            }
            /* Issue a DOS disk reset request: */
            reg.h.ah = 0x0D;
            int86(0x21, ®, ®);
            /* Set boot type and boot: */
            *boottype = (want_warm ? 0x1234 : 0);
            (*boot)( );
        }

    Reader Timo Salmi reported (26 July 1993) that the INT 15 AH=4F call
    may not work on older PCs (below AT, XT2, XT286), according to Ralf
    Brown's interrupt list (Q 1.17).

    Reader Roger Fulton reported (1 July 1993) that INT 15 AH=4F hangs
    even a modern PC "ONLY when ANSI.SYS [is] loaded high using
    EMM386.EXE.  (Other things loaded high with EMM386.EXE were OK;
    ANSI.SYS loaded high with QEMM386.SYS was OK; ANSI.SYS loaded low
    with EMM386.EXE installed was OK.)"  His solution was to use only
    the disk reset, INT 21 AH=0D, which does flush SMARTDRV, then wait
    five seconds in hopes that any other disk-caching software would
    have time to flush its queue.

    Reader Per Bergland reported (10 Sep 1993) that the jump to
    FFFF:0000 will not work in Windows or other protected-mode programs.
    (For example, when the above reboot code ran in a DOS session under
    Windows, a box with "waiting for system shutdown" appeared.  The PC
    hung and had to be reset by cycling power.)  His solution, which
    does a cold boot not a warm boot, is to pulse pin 0 of the 8042
    keyboard controller, which is connected to the CPU's "reset" line.
    He has tested the following code on various Compaqs, and expects it
    will work for any AT-class machine; he cautions that you must first
    flush the disk cache as indicated above.

            cli
        @@WaitOutReady:   ; Busy-wait until 8042 ready for new command
            in al,64h         ; read 8042 status byte
            test al,00000010b ; this bit indicates input buffer full
            jnz @@WaitOutReady
            mov al,0FEh       ; Pulse "reset" = 8042 pin 0
            out 64h,al
            ; The PC will reboot now

    7.02 - How can I time events with finer resolution than the
           system clock's 55 ms (about 18 ticks a second)?
    -----------------------------------------------------------

    The following files, among others, are downloadable from SimTel:

        /pub/msdos/at :
        atim.zip       4783  881126  Precision program timing for AT

        /pub/msdos/c :
        millisec.zip  37734  911205  MSC/asm src for millisecond timing
        mschrt3.zip   53708  910605  High-res timer toolbox for MSC 5.1
        msec_12.zip    8484  920320  High-def timer v1.2 (C,ASM)
        ztimer11.zip  77625  920428  Microsecond timer for C, C++, ASM
            (also at Garbo as /pc/c/ztimer11.zip)

        /pub/msdos/turbo_c :
        tchrt3.zip    53436  910606  High-res timer for Turbo C 2.0
        tctimer.zip   15609  891030  High-res timing for Turbo C
            (same as /pc/c/tctimer.zoo at Garbo; both are version 1.0)

    For Turbo Pascal users, source and object code are downloadable in

        /pub/msdos/turbopas/bonus507.zip at SimTel
        /pc/turbopas/bonus507.zip at Garbo.

    Also see "Q: How is millisecond timing done?" in FAQPAS.TXT,
    downloadable as part of

        /pc/ts/tsfaqp18.zip at Garbo
        /pub/msdos/info/tsfaqp18.zip at SimTel.

    7.03 - How can I find the error level of the previous program?
    --------------------------------------------------------------

    First, which previous program are you talking about?  If your
    current program ran another one, when the child program ends its
    error level is available to the program that spawned it.  Most high-
    level languages provide a way to do this; for instance, in Turbo
    Pascal it's Lo(DosExitCode) and the high byte gives the way in which
    the child terminated.  In Microsoft C, the exit code of a
    synchronous child process is the return value of the spawn-type
    function that creates the process.

    If your language doesn't have a function to return the error code of
    a child process, you can use INT 21 AH=4D (get return code).  By the
    way, this will tell you the child's exit code and the manner of its
    ending (normal, Ctrl-C, critical error, or TSR).

    It's much trickier if the current program wants to get the error
    level of the program that ran and finished before this one started.
    G.A.Theall has published source and compiled code to do this; the
    code is downloadable as

        /pub/msdos/batutil/errlvl13.zip at SimTel
        /pc/batchutil/errlvl12.zip (an older version) at Garbo.

    (The code uses undocumented features in DOS 3.3 through 5.0.  Theall
    says in the .DOC file that the values returned under 4DOS or other
    replacements won't be right.)

    7.04 - How can a program set DOS environment variables?
    -------------------------------------------------------

    Program functions that read or write "the environment" typically
    access only the program's copy of it.  What this Q really wants to
    do is to modify the active environment, the one that is affected by
    SET commands in batch files or at the DOS prompt.  You need to do
    some programming to find the active environment, and that depends on
    the version of DOS.

    A fairly well-written article in PC Magazine 28 Nov 1989 (viii:20),
    pages 309-314, explains how to find the active environment, and
    includes Pascal source code.  The article hints at how to change the
    environment, and suggests creating paths longer than 128 characters
    as one application.

    Now as for downloadable source code, there are many possibilities.
    I looked at some of these, and liked

        /pub/msdos/envutil/rbsetnv1.zip at SimTel
        /pc/envutil/rbsetnv1.zip at Garbo

    the best.  It includes some utilities to manipulate the environment,
    with source code in C.  A newer program is

        /pub/msdos/batutil/strings2.zip at SimTel
        part of /pc/pcmag/vol11n22.zip at Garbo,

    which is the code from PC Magazine 22 Dec 1992 (xi:22).

    You can also use a call to INT 2E, Pass Command to Interpreter for
    Execution; see Ralf Brown's interrupt list (Q 1.17) for details and
    cautions.

    7.05 - How can I change the switch character to - from /?
    ---------------------------------------------------------

    Under DOS 5.0 and above, you can't--not completely, anyway.  INT 21
    AX=3700, get switch character, always returns a '/' (hex 2F).  But
    the DOS commands don't even call that function: they simply hard
    code '/' as the switch character.

    Some history:  DOS used to let you change the switch character by
    using SWITCHAR= in CONFIG.SYS or by calling DOS function 3701.  DOS
    commands and other programs called DOS function 3700 to find out the
    switch character.  If you changed the switch character to '-' (the
    usual choice), you could then type "dir c:/c700 -p" rather than "dir
    c:\c700 /p".  Under DOS 4.0, the DOS commands ignored the switch
    character but functions 3700 and 3701 still worked and could be used
    by other programs.  Under DOS 5.0, even those functions no longer
    work, though all DOS functions still accept '/' or '\' in file
    specs.

    You can reactivate the functions to get and set switchar by using
    programs like SLASH.ZIP or the sample TSR called SWITCHAR in
    amisl091.zip (see "7.06 - How can I write a TSR (terminate-stay-
    resident utility)?").  DOS commands will still use the slash, but
    non-DOS programs that call DOS function 3700 will use your desired
    switch character.  (DOS replacements like 4DOS may honor the switch
    character for internal commands.)

    Some readers may wonder why this is even an issue.  Making '-' the
    switch character frees up the front slash to separate names in the
    path part of a file spec.  This is easier for the ten-fingered to
    type, and it's one less difference to remember for commuters between
    DOS and Unix.  The switch character is the only issue, since all the
    INT 21 functions accept '/' or '\' to separate directory names.

    7.06 - How can I write a TSR (terminate-stay-resident utility)?
    ---------------------------------------------------------------

    There are books, and there's code to download.

    First, the books:

    - Ray Duncan's {Advanced MS-DOS}, ISBN 1-55615-157-8, gives a brief
      checklist intended for experienced programmers.  The ISBN is for
      the second edition, through DOS 4; but check to see whether the
      DOS 6 version is available yet.

    - {DOS 5:  A Developer's Guide} by Al Williams, ISBN 1-55851-177-6,
      goes into a little more detail, 90 pages worth!

    - Pascal programmers might look at {The Ultimate DOS Programmer's
      Manual} by John Mueller and Wallace Wang, ISBN 0-8306-3534-3, for
      an extended example in mixed Pascal and assembler.

    - For a pure assembler treatment, check Steven Holzner's {Advanced
      Assembly Language}, ISBN 0-13-663014-6.  He has a book with the
      same title out from Brady Press, but it's about half as long as
      this one.

    Next, the code.  Some of it is companion code to published articles,
    which are also listed below:

    - The Alternate Multiplex Interrupt Specification, downloadable as

        /pub/msdos/info/altmpx35.zip at SimTel
        /pc/programming/altmpx35.zip at Garbo
        /afs/cs/user/ralf/pub/altmpx35.zip at cs.cmu.edu

    - Ralf Brown's assembly-language implementation of the spec, with
      utilities in C, is downloadable as

        /pub/msdos/asmutil/amisl091.zip at SimTel
        /pc/c/amisl091.zip at Garbo
        /afs/cs/user/ralf/pub/amisl091.zip at cs.cmu.edu

    - Douglas Boling's MASM template for a TSR is downloadable as

        /pub/msdos/asmutl/template.zip at SimTel.

    - A posted article mentions Boling's "Strategies and Techniques for
      Writing State-of-the-Art TSRs that Exploit MS-DOS 5", Microsoft
      Systems Journal, Jan-Feb 1992, Volume 7, Number 1, pages 41-59,
      with examples downloadable in

        /pub/msdos/msjournal/msjv7-1.zip at SimTel

    - code for Al Stevens's "Writing Terminate-and-Stay-Resident
      Programs", Computer Language, February 1988, pages 37-48 and March
      1988, pages 67-76 is downloadable as

        /pub/msdos/c/tsrc.zip at SimTel

    - software examples to accompany Kaare Christian's "Using Microsoft
      C Version 5.1 to Write Terminate-and-Stay-Resident Programs",
      Microsoft Systems Journal, September 1988, Volume 3, Number 5,
      pages 47-57 are downloadable as

        /pub/msdos/msjournal/msjv3-5.arc at SimTel

    Finally, there are commercial products, of which TesSeRact (for C-
    language TSRs) is one of the best known.

    7.07 - Why does my interrupt function behave strangely?
    -------------------------------------------------------

    Interrupt service routines can be tricky, because you have to do
    some things differently from "normal" programs.  If you make a
    mistake, debugging is a pain because the symptoms may not point at
    what's wrong.  Your machine may lock up or behave erratically, or
    just about anything else can happen.  Here are some things to look
    for.  (See "7.06 - How can I write a TSR (terminate-stay-resident
    utility)?" for general techniques that may prevent a problem.)

    First, did you fail to set up the registers at the start of your
    routine?  When your routine begins executing, you can count on
    having CS point to your code segment and SS:SP point to some valid
    stack (of unknown length), and that's it.  In particular, an
    interrupt service routine must set DS to DGROUP before accessing any
    data in its data segments.  (If you're writing in a high-level
    language, the compiler may generate this code for you automatically;
    check your compiler manual.  For instance, in Borland and Microsoft
    C, give your function the "interrupt" attribute.)

    Did you remember to turn off stack checking when compiling your
    interrupt server and any functions it calls?  The stack during the
    interrupt is not where the stack-checking code expects it to be.
    (Caution:  Some third-party libraries have stack checking compiled
    in, so you can't call them from your interrupt service routine.)

    Next, are you calling any DOS functions (INT 21, 25, or 26) in your
    routine?  DOS is not re-entrant.  This means that if your interrupt
    happens to be triggered while the CPU is executing a DOS function,
    calling another DOS function will wreak havoc.  (Some DOS functions
    are fully re-entrant, as noted in Ralf Brown's interrupt list (Q
    1.17).  Also, your program can test, in a way too complicated to
    present here, when it's safe to call non-re-entrant DOS functions.
    See INT 28, INT 21 AH=34, and INT 21 AX=5D06 or 5D0B; and consult
    {Undocumented DOS} by Andrew Schulman.  Your program must read both
    the "InDOS flag" and the "critical error flag".)

    Is a function in your language library causing trouble?  Does it
    depend on some initializations done at program startup that is no
    longer available when the interrupt executes?  Does it call DOS (see
    preceding paragraph)?  For example, in both Borland and Microsoft C
    the memory-allocation functions (malloc(), etc..) call DOS functions
    and also depend on setups that they can't get at from inside an
    interrupt; so do the standard I/O functions like scanf() and
    printf().  Many other library functions have the same problem, so
    you can't use them inside an interrupt function without special
    precautions.

    Is your routine simply taking too long?  This can be a problem if
    you're hooking on to the timer interrupt, INT 1C or INT 8.  That
    interrupt expects to be called about every 55 ms, which is 18.2
    times a second.  Therefore your routine, plus any others hooked to
    the same interrupts, must execute in less than 55 ms.  If they use
    even a substantial fraction of that time, you'll see significant
    slowdowns of your foreground program.  A good discussion is
    downloadable as

        pub/msdos/SIMTEL20-mirror/info/intshare.zip at ni.funet.fi
        /pub/msdos/info/intshare.zip at SimTel.

    Did you forget to restore all registers at the end of your routine?

    Did you chain improperly to the original interrupt?  You need to
    restore the stack to the way it was upon entry to your routine, then
    do a far jump (not call) to the original interrupt service routine.
    (The process is a little different in high-level languages.)

    7.08 - How can I write a device driver?
    ---------------------------------------

    Many books answer this in detail.  Among them are {Advanced MS-DOS}
    and {DOS 5: A Developer's Guide}, cited in the preceding Q.  Michael
    Tischer's {PC System Programming}, ISBN 1-55755-036-0, has an
    extensive treatment, as does Dettman and Kyle's {DOS Programmer's
    Reference: 2d Edition}, ISBN 0-88022-458-4.  For a really in-depth
    treatment, look for a specialized book like Robert Lai's {Writing MS-
    DOS Device Drivers}, ISBN 0-201-13185-4.

    7.09 - What can I use to manage versions of software?
    -----------------------------------------------------

    A port of the Unix RCS utility is downloadable as

        /pub/msdos/gnuish/rcs55ax.zip (EXE and docs) from SimTel
        /pub/msdos/gnuish/rcs55as.zip (source) from SimTel
        /pc/unix/alrcs5ex.zip (EXE and docs ?) from Garbo.

    This version of RCS is no longer limited to one-character extensions
    on filenames (for example, .CPP and .BAS are now OK).

    An RCS56 is available at a number of archive sites, but it appears
    to be unauthorized.  In response to a query, Keith Petersen, SimTel
    administrator, said that RCS56 was removed from SimTel at the
    author's request because it did not contain source code and thus was
    in violation of the GNU copyleft.

    As for commercial software, I posted a question asking for readers'
    experiences in July 1993 and seven readers responded.  PVCS from
    Intersolv (formerly Polymake) got five positive reviews, though
    several readers commented that it's expensive; RCS from MKS got one
    positive and one negative review; Burton TLIB got one negative
    review; DRTS from ILSI got one positive review.

    7.10 - What's this "null pointer assignment" after my C program
               executes?
    ---------------------------------------------------------------

    Somewhere in your program, you assigned a value _through_ a pointer
    without first assigning a value _to_ the pointer.  (This might have
    been something like a strcpy() or memcpy() with a pointer as its
    first argument, not necessarily an actual assignment statement.)
    Your program may look like it ran correctly, but if you get this
    message you can be certain that there's a bug somewhere.

    Microsoft and Borland C, as part of their exit code (after a call to
    exit() or a return from your main function), check whether the
    location 0000 in your data segment contains a different value from
    what you started with.  If so, they infer that you must have used an
    uninitialized pointer.  This implies that the message will appear at
    the end of execution of your program regardless of where the error
    actually occurred.

    To track down the problem, you can put exit() calls at various spots
    in the program and narrow down where the uninitialized pointer is
    being used by seeing which added exit() makes the null-pointer
    message disappear.  Or, if your program was compiled with small or
    medium models, which use 16-bit data pointers, tell the debugger to
    set a watch at location 0000 in your data segment.  (If data
    pointers are 32 bits, as in the compact and large models, a null
    pointer will overwrite the interrupt vectors at 0000:0000 and
    probably lock up your machine.)

    Under MSC/C++ 7.0, you can declare the undocumented library function

        extern _cdecl _nullcheck(void);

    and then sprinkle calls to _nullcheck() through your program at
    regular intervals.

    Borland's TechFax document TI726 discusses the null pointer
    assignment from a Borland point of view.  It's one of many documents
    downloadable as part of

        /pub/msdos/turbo-c/bchelp10.zip at SimTel
        /pc/turbopas/bchelp10.zip at Garbo.

    7.11 - How can a batch file tell whether it's being run in a DOS
               box under Windows?
    ----------------------------------------------------------------

    When Windows 3.0 or 3.1 is running, the DOS environment will contain
    a definition of the string windir, in lower case.  That's not really
    useful, however, because the batch statement

        if "%windir%" == "" ...

    will test for an environment variable WINDIR in upper case.

    Your only real option is to write a program as suggested by the
    following Q, and have it return a value which your batch file can
    test via "if errorlevel".

    7.12 - How can my program tell if it's running under Windows?
    -------------------------------------------------------------

    Execute INT 2F AX=4680.  If AX returns 0, you're in Windows real
    mode or standard mode (or under the DOS shell).  Otherwise, call INT
    2F AX=1600.  If AL returns something other than 0 or 80, you're in
    Windows 386 enhanced mode.  See PC Magazine 24 Nov 1992 (xi:20)
    pages 492-493.

    For more information, see PC Magazine 26 May 1992 (xi:10) pages 345-
    346.  A program, WINMODE, is available as part of

        /pub/msdos/pcmag/vol11n10.zip at SimTel
        /pc/pcmag/vol11n10.zip at Garbo.

    PC Magazine 29 March 1994 (xiii:6) pages 312 and 320 published a new
    program, WINVER.  This would be in

        /pub/msdos/pcmag/vol13n06.zip at SimTel
        /pc/pcmag/vol13n06.zip at Garbo.

    7.13 - How can a program tell whether ANSI.SYS is installed?
    ------------------------------------------------------------

    In DOS 4.0 and above, call INT 2F AX=1A00.  If the value FF is
    returned in AL, ANSI.SYS is installed.  For more information, see
    Ralf Brown's interrupt list (question 1.17).

    7.14. How do I copyright software that I write?
    -----------------------------------------------

    You can download a very comprehensive answer from the Internet.
    Terry Carroll posts a six-part Copyright FAQ to misc.legal,
    news.answers and other groups.  A short answer follows, not based on
    that article.

    Disclaimer: I am not a lawyer, and this is not legal advice.  Also,
    there are very likely to be differences in copyright law among
    nations.  No matter where you live, if significant money may be
    involved, get legal advice.  The following is adapted (and greatly
    condensed) from chapter 4 of the Chicago Manual of Style (13th
    edition, ISBN 0-226-10390-0).

    In the U.S. (at least), when you write something, you own the
    copyright.  (The exception that matters most to programmers is
    "works made for hire", i.e., code you write because your employer or
    client pays you to.  A contract, agreed in advance, can vest the
    copyright in the programmer even if an employee; otherwise the
    employer owns the copyright.)  You don't have to register the work
    with the Copyright Office unless (until) the copyright is infringed
    and you intend to bring suit; however, it is easier to recover
    damages in court if you did register the work within three months of
    publication.

    From paragraph 4.16 of the Chicago Manual:  "... the [copyright]
    notice consists of three parts: (1) the symbol [C-in-a-circle]
    (preferred because it also suits the requirements of the Universal
    Copyright Convention), the word 'Copyright', or the abbreviation
    'Copr.', (2) the year of first publication, and (3) the name of the
    copyright owner.  Most publishers also add the phrase 'All rights
    reserved' because it affords some protection in Central and South
    American countries ...."  Surprise: "(C)" is legally not the same as
    the C-in-a-circle, so those of us who are ASCII-bound must use the
    word or the abbreviation.


Section 8.  Downloading
-----------------------

    8.01 - What are SimTel, Garbo, and Wustl?
    -----------------------------------------

    These are three of the most popular archive sites, with a few
    bazillion files available for free downloading by ftp.  (For email
    access, see "8.02 - I have no ftp access.  How can I get files from
    the archives?".)  Many of the files are shareware and you're
    expected to send a payment directly to the authors if you use them
    regularly.

    In comp.archives.msdos.d, Samuel Ko posts a two-part "Useful MS-DOS
    Programs at SIMTEL20 and Garbo"; it's downloadable as

        /pub/msdos/info/useful34.zip at SimTel
        /pc/filelist/useful34.zip at Garbo
        /pub/usenet/news.answers/msdos-archives/part* at rtfm.mit.edu.

    For rtfm.mit.edu instructions, see "1.13 - Where are FAQ lists
    archived?"

    A few words about file names and versions:  Many files at the
    archive sites are updated from time to time.  I verified every
    filename in this FAQ as of 24 Mar 1994 by ftp to the named sites, or
    by consulting their index files.  If you can't find a file given in
    these articles as mumble12.zip, perhaps there's a newer version; try
    mumble13.zip or mumble14.zip, or mumble*.zip if your ftp program
    supports wildcards (most do so).  Please let me know of any out-of-
    date file names.

    This FAQ should show both Garbo and SimTel directory and file names,
    if available, for every file mentioned for downloading.  If you see
    a listing for only one of them, it means that the file was not found
    at the other site, or that the other site's catalog shows an old
    version.

    Also remember that caps and lower case filenames are not
    interchangeable at most archive sites.

    1) SimTel = oak.oakland.edu is located at the University of
    Rochester in the USA, and there are also many mirror sites including
    wuarchive.wustl.edu, archive.orst.edu, ftp.uu.net, nic.funet.fi,
    archie.au, and nctucca.edu.tw.  For instructions, see these monthly
    articles in comp.archives.msdos.announce:

        How to find files in the SIMTEL20 msdos collection
        How to order SIMTEL20 files via e-mail
        How to upload files to SIMTEL20

    These are downloadable from SimTel as

        /pub/msdos/filedocs/aaaread.me
        /pub/msdos/filedocs/mailserv.inf
        /pub/msdos/filedocs/upload.inf.

    2) Garbo = garbo.uwasa.fi [128.214.87.1] is located at the
    University of Vaasa in Finland and maintained by Timo Salmi
    ([e-mail]) and others.  Garbo and SimTel contain many of the same
    files, but there are many differences too.  Among them: the
    directory structures differ greatly, and case is significant in
    directory and file names at Garbo.

    3) wustl = wuarchive.wustl.edu [128.252.135.4] mirrors, among
    others, Garbo (in /systems/ibmpc/garbo) and SimTel (in
    /systems/ibmpc/msdos).  As with any mirror site, it may lag by a day
    or two, so you may not want to try it on the same day you see an
    upload notice posted.

    4) others:

    A comprehensive list of MS-DOS archive sites is downloadable as

        /pc/pd2/moder35.zip at Garbo
        /pub/msdos/info/moder35.zip at SimTel.

    For archie.au via Telnet (different from the Archie file-finding
    mail servers):  Oceanian users should try archie.au first.  Paul
    Brooks has written to say that it "mirrors Garbo and SimTel-20 (in
    /micros/pc/simtel-20, /micros/pc/garbo) as well as many other
    archives.  Telnetting to 'archie.au' and logging on as 'archie' (no
    password) will access the Oceanic ARCHIE server." Email Craig Warren
    ([e-mail]) for instructions if needed.

    8.02 - I have no ftp access.  How can I get files from the archives?
    --------------------------------------------------------------------

    First, be sure that you have no ftp access before trying email
    methods.  (Ask your sysadmin, or a knowledgeable user at your site.)
    ftp is better for you because it's faster, and it uses less net
    resources too.

    When using an email server, make sure the Reply-to path in your
    message is valid.  If it's not, you'll get no reply from the server.
    Do wait a few days before assuming you're not going to get a
    response; some servers have long pending queues.  After a suitable
    wait, get your sysadmin's help to correct your reply-to, and send
    your message again.

    Occasionally a machine goes down for an extended period, which may
    prevent a timely reply to your message.  If you're sure your message
    bears a good reply path and you haven't got a reply in a week or so,
    you might send your message again, once.  Don't post it as an
    article in a newsgroup.

    For files from SimTel, see "How to order SIMTEL20 files via e-mail",
    posted monthly in comp.archives.msdos.announce; or send email
    containing only the word "help" to [e-mail].  For
    instructions on using Garbo's email server, send a request to Timo
    Salmi at [e-mail].

    The DEC Western Research Labs server will get files from any ftp
    site by ftp and then email them to you.  Send email containing
    "help" to [e-mail].

    8.03 - Can I get archives on CD-ROM?
    ------------------------------------

    Copies of the SimTel MS-DOS, Macintosh and Unix-C collections (also
    of wuarchive, cica, and others) are available from Walnut Creek
    CDROM, 1547 Palos Verdes, Suite 260, Walnut Creek, CA 94596-2228,
    telephone [phone] or [phone], or FAX [phone],
    or email [e-mail].

    For a catalog of disks available, send email to [e-mail], or
    ftp the catalog as /cdrom/catalog from cdrom.com.

    8.04 Where do I find program <mumble>?
    --------------------------------------

    You _are_ asking about shareware, freeware, or public-domain
    programs, right?  Commercial software is not legally distributed
    through the net, in general.  (Occasionally vendors will make
    patches available, but these are useful only to upgrade software you
    already own.)

    That said, there are several newsgroups to help you find a program.
    comp.binaries.ibm.pc.wanted is generally the best place to ask your
    question.  Please review the guidelines in "108. What other
    technical newsgroups should I know about?"

    Download and check the indexes from SimTel and Garbo (see "8.05 - How
    can I check SimTel or Garbo before I post a request for a
    program?").  Unless what you're looking for is commercial software,
    there's a good chance it will be at one or both of those sites.  The
    Archie servers maintain directories of many (not all) ftp sites
    worldwide.  You can Telnet to a server or client to search for
    files, or perform a search by email.

    8.05 - How can I check SimTel or Garbo before I post a request
               for a program?
    --------------------------------------------------------------

    SimTel and Garbo have indexes of their contents, which you can
    download and then search off line.

    Garbo's index file, /pc/INDEX.ZIP, contains an annotated list, often
    updated, of the MS-DOS files there.  The news file /pc/pd2/news-pd2
    contains selected news on all MS-DOS directories at Garbo.

    SimTel's index files, in /pub/msdos/filedocs, are updated several
    times a month.  simlist.zip contains a list in text format.  But you
    may prefer the file simindex.zip, which is comma-delimited for easy
    use with any of these search facilities:

    - Two downloadable search programs are simtel35.zip and
      simdir22.zip.

    - dBASE III or IV users can load the index from simindex.zip using
      instructions in simibm.inf and simibm.db3.

    - PC-FILE users should get simibm.hdr, which tells how to use
      simibm.ibm.

    8.06 - How do I download and decode a program I found?
    ------------------------------------------------------

    See the "Starter kit" and "Beginner's guide to binaries" in
    comp.binaries.ibm.pc, usually posted on the first and 15th of every
    month.  Please wait for these articles to come around; don't post a
    request.  If you can't wait, they are downloadable from

        Garbo as /pc/doc-net/starter.kit and bin.man
        rtfm.mit.edu in /pub/usenet/comp.binaries.ibm.pc .  The file
            names are quite long; try look for names of the form
            *starter.kit* and *bin.man*

    For rtfm.mit.edu instructions, see "1.13 - Where are FAQ lists
    archived?"

    Most binaries are posted and sent through email in 'uuencode'
    format.  The starter kit contains a uudecode program to turn this
    file back into binary.  Since the uuencoded file is bigger than the
    binary, you'll save connect time if you can uudecode it and then
    download the binary file.  Remember to set file type to binary.

    8.07 - Where is UUDECODE?
    -------------------------

    You can find it at SimTel and Garbo, but it's easier to take it from
    the "starter kit" mentioned in the preceding Q.

    If you're logged in at a Unix site, there's almost certainly a
    uudecode there:  just type "uudecode" followed by a space and the
    file name.  The binary file is 25% smaller than the uuencoded file,
    so you'll save connect time if you can uudecode it on the Unix host
    and then download the binary file.  Remember: set file type binary.

    8.08 - Why do I get errors when extracting from a ZIP file I downloaded?
    ------------------------------------------------------------------------

    There are many possible causes, but two of them probably account for
    95% of all problems.

    1) File transmission:  You must tell the archive site to transfer
    ZIP files in binary mode.  Depending on your software, you may also
    need to set your local software to receive files in binary mode.

    2) Unzipping program:  Make sure you aren't using an obsolete
    version.  In <[e-mail]> on 29 Mar
    1993, Keith Petersen, SimTel administrator, wrote:  "SIMTEL20 has
    standardized on the Info-ZIP group's ZIP and UNZIP because they are
    freely distributable and they have no restrictions on exporting.
    The latest version of Info-ZIP's ZIP and UNZIP can always be found
    in directory" /pub/msdos/zip "and will always have the name 'Info-
    ZIP' in the description to make them easy to locate."  PKUNZIP
    version 1.10 may not unzip newer stuff from archive sites because
    site administrators have now embraced version 2.04.  Also see "9.03 -
    What's the current version of UNZIP?"


Section 9.  Vendors and products
--------------------------------

    9.01 - How can I contact Borland?
    ---------------------------------

    Patches, examples, and product info can be found at ftp.borland.com in
    the /pub directory.

    Borland has set up these email addresses.  However, none of them is
    for technical support such as help with finding your own programming
    errors and explaining compiler messages.

    - [e-mail] will reply to any message with 17K of info on
      Borland Pascal with Objects 7.0 and Turbo Pascal 7.0.

    - [e-mail] is for questions about prices and
      features of products, replacing bad or missing disks, info on
      upgrades, etc.  They do not accept emailed credit-card numbers.

    - [e-mail] will take "a well-documented bug report" and send
      an automated response, but will not give you a work around or a
      scheduled fix date or even confirm that it is or is not considered
      a bug.  "The purpose of [e-mail] is to improve future
      products sooner, not as a substitute for tech support channels."

    Email tech support is available only through Compuserve (GO BOR).

    Telephone support is provided through separate phone numbers for
    most products.  If you're calling from the U.S., dial [phone]
    for a recorded list of toll numbers to call.  Borland's BBS is at +1
    408 439 9096.  The surface-mail address is Technical Support
    Department, Borland International, P.O. Box 660001, Scotts Valley CA
    95067-0001, USA.

    You'll need to give your product's name, version, and serial number.

    Borland's TechFax documents are also available for download:

        /pub/msdos/turbo-c/bchelp10.zip at SimTel
        /pc/turbopas/bchelp10.zip at Garbo.

    These documents are detailed answers to common questions about Turbo
    C and Borland C products, and aggregate several hundred Kbytes.

    9.02. How can I contact Microsoft?
    ----------------------------------

    Individual employees of Microsoft (not MicroSoft, please!) post here
    sometimes.  Their addresses all take the form [e-mail].
    However, Microsoft as a company does not answer individual questions
    via email through the Internet.

    1) information available via anonymous ftp

    The Microsoft gowinnt anonymous ftp server (ftp.microsoft.com) offers
    a variety of information for developers. Unlike many ftp servers,
    Microsoft's is run on Windows NT and does not suppurt UNIX-like
    directory names. Instead of cd /developer/devtools, you would use
    cd \developer\devtools; the path names are not case sensitive.

    ftp.microsoft.com directories of interest to programmers:

    Directory                          Description
    ----------------------------------------------------------------
    \developer\devtools                Files relating to Microsoft C
                                       and other programming tools.
    \developer\drg                     Microsoft Developer Relations
                                       Group information. Contains
                                       Word for Windows documents on
                                       programming issues.
    \developer\msdn                    Microsoft Developer's Network
                                       information.
    \developer\msj                     Microsoft Systems Journal info.
    \softlib                           Has index file for
                                       \softlib\mslfiles.
    \softlib\mslfiles                  Many file relating to MS-DOS,
                                       Windows, etc.


    2) information via U.S. Mail

    Microsoft Product Support is at 16011 NE 36th Way, Box 97017,
    Redmond WA 98073-9717, USA.

    You can subscribe to the {Microsoft Developer News} by mailing a
    request to Microsoft Developer Network, P.O. Box 51812, Boulder CO
    80322-1812, U.S.A.  The nearest I can figure, this is free;
    certainly I've never paid for it or been asked to.

    3) tech support via modem

    On Compuserve, GO MICROSOFT; or call Microsoft's BBS at +1 206 936-
    6735 in the U.S., or [phone] in Canada.

    4) tech support via voice telephone lines

    If you want to place an order or get general pre-sales information,
    call the appropriate sales and service number:

        U.S. end-user sales                   [phone]
        U.S. corporate/gov't/reseller/
                 educational sales            [phone]
        Canadian sales                        [phone]
        International sales                  [phone]

    For tech support you must make an ordinary long-distance phone call.
    Microsoft has separate incoming phone numbers for many products.
    Since it's your nickel, first check your documentation to see if a
    phone number is listed.  Here are phone numbers (as of 1 June 1992)
    for a few products of most interest to the readers of this group:

        C/C++               [phone]
        MASM                [phone]
        DOS Upgrade Users   [phone] (for 90 days after first
            call)
        Windows Users       [phone]

    If you can't find the direct number any other way, call the "master"
    numbers below or the sales numbers a few paragraphs above.  You'll
    get the "voice mail phone tree from h-ll" but you'll eventually get
    to the right department.  They don't provide technical assistance,
    but a voice menu will help you find the current phone number for the
    department you need.

        U.S. end-user product support         [phone]
        U.S. languages support                [phone]

    9.03. What's the current version of UNZIP?
    ------------------------------------------
    (Revised: 05/09/1994)

    The current version of PKWare's PKZip is 2.04g.
    The current version of InfoZip's ZIP is 2.0.1.
    The current version of InfoZip's UNZIP is 5.12.

    Since April 1993, the administrators of Garbo and SimTel have
    accepted uploads in the ZIP 2.0 format.  You can use the free Info-
    ZIP versions, or PKZIP 2.04g (not 2.04c or 2.04e).  SimTel has
    standardized on the Info-ZIP versions for several reasons, as
    explained in an article posted 29 Mar 1993 in
    comp.archives.msdos.announce.

    The primary source of the free Info-ZIP programs is at ftp.uu.net,
    directory /pub/archiving/zip; or at quest.jpl.nasa.gov, directory
    /pub.  Files are in those directories or in subdirectories as shown
    below.  The DOS executable for UNZIP is self extracting; all others
    require UNZIP 5.0, 5.1, 5.11 or 5.12:

        MSDOS/unzip512x.exe - DOS executable and doc, UNZIP 5.12
        unzip512.zip - source code for UNZIP 5.12, all platforms
        MSDOS/zip20x.zip - DOS exe and doc, ZIP without crypt support
        zip201.zip - source code for ZIP 2.0, all platforms
        MSDOS/zcryp20x.zip - DOS exe and doc, ZIP with crypt support
        zcrypt21.zip - source code for ZIP crypt support
        WINDOWS/wunz20x.zip - Windows 3.1 WIZUNZIP 2.0 exe and help file
        WINDOWS/wunz20sr.zip - Windows 3.1 WIZUNZIP 2.0 source

    The essential files are also available at SimTel in /pub/msdos/zip
    and at Garbo in /pc/arcers.

    PKZIP and PKUNZIP are shareware products of PKWARE Inc.  The current
    version is 2.04g, which is the third official version after 1.10.
    They are downloadable

        from PKWARE's bulletin board, [phone], or
        from Garbo as /pc/arcers/pkz204g.exe
        from SimTel as /pub/msdos/zip/pkz204g.exe

    By the way, if you want to develop your own utilities, you will find
    the ZIP 2.0 data structures described in the downloadable file

        /pub/msdos/zip/appnote.zip at SimTel.

    9.04. What is the phone number for a vendor's BBS?
    --------------------------------------------------

    Robert Baker ([e-mail]) uploaded a list of 800 bulletin-
    board support numbers operated by software vendors as of August
    1993.  It is downloadable as /pub/msdos/bbslist/bbs_9308.zip from
    SimTel.

(End of comp.os.msdos.programmer FAQ Version 2.01)