Subject: comp.os.msdos.programmer FAQ part 2 of 4

From: [email-masked] (Stan Brown)
Date: Sat, 25 Jul 1992 16:11:05 GMT

Archive-name: msdos-programmer-faq/part2
Last-modified: 19 July 1992

(continued from part 1)         (no warranty on the code or information)

If the posting date is more than six weeks in the past, see instructions
in the last part of this list for how to get an updated copy.

            Copyright (C) 1992  Stan Brown, Oak Road Systems

section 1. General questions
============================

Q101. 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 MSDOS and your PC
    hardware.  (Your manual should tell you which features are standard
    and which are vendor- or MSDOS- 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 list first, to make sure
    your question isn't already answered.)

    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 this group 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.

    When you do post a question, it's good manners to say "email please;
    I'll post a summary."  Then everybody else in the group doesn't have
    to read ten virtually identical responses.  Of course, then you have
    to follow through.  A summary is not simply pasting together all the
    email you received.  Instead, write your own (brief) description of
    the solution:  this is the best way to make sure you really
    understand it.  Definitely don't repost people's cute signatures.

Q102. What is this newsgroup about?

    comp.os.msdos.programmer (comp.sys.ibm.pc.programmer until September
    1990) concerns programming for MS-DOS systems.  The article "USENET
    Readership report for Feb 92" in news.lists shows 62,000 readers of
    this 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.

    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.

Q103. What's the difference from comp.sys.ibm.pc.programmer?

    c.s.i.p.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 c.s.i.p.programmer even though they're actually posting to
    c.o.m.programmer.

    You can easily verify the non-existence of c.s.i.p.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 "Where are FAQ lists archived?" in section C,
    "More information").

Q104. What other newsgroups should I know about?

    Windows mavens are found in comp.os.ms-windows.programmer.tools and
    comp.os.ms-windows.programmer.misc.  According to the FAQ list in
    the latter, comp.windows.ms.programmer has been replaced by those
    two plus comp.os.ms-windows.programmer.win32.

    Your best bet is to read the periodic information postings in the
    comp.binaries.ibm.pc newsgroup.  Specially helpful articles:
        Using the comp.binaries.ibm.pc.d groups
        Beginner's guide to binaries
        Starter kit
        About archives and archivers
    Please wait for these articles to come around; don't post a request.

    Also check out news.announce.newusers, even if you're not a new
    user.  You may be surprised how much useful information is in the
    monthly postings there.  Lots of old-timers also get useful stuff
    from news.newusers.questions, especially the periodic postings.

    Remember that it's good manners to subscribe to any newsgroup and
    read it for a while before you post a question.  When you post, it's
    also good manners to ask for replies to be emailed and then to post
    a summary, which you've edited down to the absolute minimum size.

    You may also be interested in the following newsgroups.  Caution:
    Some of them have specialized charters; you'll probably get (and
    deserve) some flames if you post to an inappropriate group.

    - comp.os.ms-windows.programmer.tools and ...misc (formerly part of
      comp.windows.ms.programmer):  Similar to this group, but focus
      on programming for the MS-Windows platform.

    - 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.binaries.ibm.pc.archives explains how to use the archive
      sites, especially garbo and Simtel.  Look for the excellent
      periodic postings, such as "Recent msdos uploads to SIMTEL20".

    - 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).  cbip.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.

section 2. Compile and link
===========================

Q201. What the heck is "DGROUP > 64K"?

    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.

    See the next Q for possible remedies.

    For more information, see topics like "memory models" and "memory
    management" in the index of your compiler manual.

Q202. 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 3.0.  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.

Q203. Will Borland C code and Microsoft C code link together?

    Not in general.  The usual context for this question is that you've
    bought a package of application .OBJs or .LIBs that were meant for
    use with a compiler that you don't have.

    One problem is that each compiler uses calls to undocumented
    functions to do things like long integer arithmetic, floating point
    arithmetic, and stack checking.  And the undocumented functions from
    Microsoft are not the same as those from Borland.  So a link will
    generally fail with "not found" messages for oddball function names
    that you can't find in the manuals.  Although the link fails, it
    usually creates an .EXE file.  But if you try to run the program, it
    crashes as soon as it tries to call one of those functions.

    Another problem is that 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.

    A third-party vendor of compiled object libraries can take some
    steps to minimize such problems, but it's usually far more cost
    effective for the vendor to issue different versions of the software
    to be used with different compilers.  I looked at this problem a
    year or so ago for a client.  The choice was between severely
    restricting functions of the libraries the client would distribute,
    or distributing them in companion disks for use with Microsoft and
    Borland respectively.  The client chose the latter solution, which
    still seems to me the right one.

Q204. Why did my program bomb at run time with "floating point formats
      not linked"?

    You're probably using a Borland compiler for C or C++ (including
    Turbo C and Turbo 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/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.

    A new solution for Borland C++ 3.0 was posted, but I don't own the
    product and have not been able to verify it.  Insert these
    statements in your program:

        extern unsigned _floatconvert;
        #pragma extref _floatconvert

Q205. 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 "extern" in this context isn't ANSI C and ought not
    to be required, but the above statement is a direct quote from the
    Library Reference manual of Borland C++ 2.0.)  The linker may give
    you a duplicate symbol warning, which you can ignore.

Q206. 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.

    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.  Ralf Brown's interrupt list also documents the
    .EXE header (including extensions by Borland's TLINK) at INT 21
    function 4B, including New Executable and Linear Executable formats.

section 3. Keyboard
===================

Q301. 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, load 8 in register AH and execute
    INT 21; 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 with
    AH=10; this will return the scan code in AH and ASCII code (if
    possible) in AL, except that AL=E0 with AH nonzero indicates one of
    the grey "extended" keys was pressed.  (If your BIOS doesn't
    support the extended keyboard, use INT 16 function 0 not 10.)

Q302. 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 function 01 or 11, to check
    whether an actual keystroke is waiting; or the DOS call, INT 21
    function 0B, to check for a keystroke from stdin (subject to
    redirection).  See Ralf Brown's interrupt list.

Q303. How can I disable Ctrl-C/Ctrl-Break and/or Ctrl-Alt-Del?

    You can download the file PD1:<MSDOS.KEYBOARD>CADEL.ZIP, 2534 bytes,
    from Simtel.  It contains a TSR to do disable those three keys, with
    source code in ASM.

    To disable only Ctrl-Alt-Del (actually, to change the boot keys to
    leftShift-Alt-Del), use DEBOOT.COM.  Along with KEYKILL.COM, which
    lets you disable up to three keys of your choice, it is at Simtel in
    the file PD1:<MSDOS.KEYBOARD>KEYKILL.ARC, 3395 bytes.

    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, the
    characters ^C will appear on the screen when the user presses Ctrl-C
    or Ctrl-Break.  There are many ways to work around that, including:
    use INT 21 function 7, which allows redirection but doesn't display
    the ^C (or echo any other character, for that matter); or use INT 16
    function 0 or 10; or call _bios_keybrd( ) in MSC or bioskey( ) in
    BC++; 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.

Q304. 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 BIOSes dated
    December 1982 or later, uses a byte at 0040:0100 (alias 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 (tested under MSC 5.0 and BC++ 2.0):

        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):

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

    Change <27> in the above to an Escape character, ASCII 27.

    If you haven't installed ANSI.SYS, I can't offer an easy way to
    disable the echo-screen-to-printer function.  Please send any tested
    solutions to [email-masked] and I'll add them to this list.

    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
    I.  As an alternative, install one of the many print spoolers that
    intercept printer-status queries and always return "Printer ready".

Q305. How can my program turn NumLock (CapsLock, ScrollLock) on or off?

    You need to twiddle bit 5, 6, or 4 of location 0040:0017.  Here's
    some code (tested under MSC 5.0 and BC++ 2.0):  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
    function 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;
        }

Q306. 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 BIOSes since 1986 let software change
    the delay and typematic rate by calling INT 16 function 3, "set
    typematic rate and delay"; see Ralf Brown's interrupt list.  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 the {PC Magazine} of 15 Jan 1991,
    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 I'm told that the QUICKEYS utility that
    {PC} published in 1986 does this sort of watching; you can download
    source and object code in PD1:<MSDOS.PCMAG>VOL5N05.ARC from Simtel.

Q307. 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
    function 4F to translate the scan code.  If the translated scan code
    is 54 hex (for the SysRq key) then INT 9 calls INT 15 function 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 function 4F is not called.  Early ATs have 84-key keyboards,
    so their BIOS calls INT 15 function 85 but nor 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 function 85, which is called when
    SysRq is pressed or released.

Q308. How can my program tell what kind of keyboard is on the system?

    Ralf Brown's Interrupt List has a detailed breakdown of the contents
    of the BIOS system block that starts at 0040:0000.  Bit 4 of byte
    0040:0096 is "1=enhanced keyboard installed".

    This code is tested under MSC 5.0 and BC++ 2.0:
        char far *kbd_stat_byte3 = (char far *)0x00400096UL;
        if (0x10 & *kbd_stat_byte3)
            /* 101- or 102-key keyboard is installed */

    {PC Magazine}'s 15 Jan 1991 issue 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 {PC Magazine} article suggests
    a different approach to determining the type of keyboard.

Q309. How can I tell if input or output 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, download the /pc/ts/tspa*.zip
    collection of Turbo Pascal units from garbo; or from Simtel,
    PD1:<MSDOS.TURBOPAS>TSPA*.ZIP.  Source code is not included.  Also
    see the information in garbo.uwasa.fi:/pc/ts/tsfaq*.zip Frequently
    Asked Questions, the Turbo Pascal section.

    If you program in C, use isatty( ) if your implementation has it.
    Otherwise, you can download PD1:<MSDOS.SYSUTL>IS_CON10.ZIP, 4267
    bytes (contains source code) from Simtel.

    Good references for the principles are {PC Magazine} 16 Apr 1991
    (vol 10 nr 7) pg 374; Ray Duncan's {Advanced MS-DOS}, ISBN
    1-55615-157-8, or Ralf Brown's interrupt list for INT 21 function
    4400; and Terry Dettman and Jim Kyle's {DOS Programmer's Reference:
    2d edition}, ISBN 0-88022-458-4, pp 602-603.

(continued in part 3)
-- 
Stan Brown, Oak Road Systems                      [email-masked]