🚀 go-pugleaf

Archive-name: msdos-programmer-faq/part3
Last-modified: 26 June 1993


(continued from part 2)         (no warranty on the code or information)

If the posting date is more than six weeks in the past, see instructions
in part 4 of this list for how to get an updated copy.

Copyright (C) 1993  Stan Brown, Oak Road Systems.  All rights reserved.


section 4.  Disks and files
===========================

Q401. What drive was the PC booted from?

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

Q402. How can I boot from drive b:?

    (rev: 18 Apr 1993)  Downloadable shareware:
        pd1:<msdos.dskutl>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.

    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.

Q403. Which real and virtual disk drives are valid?

    Use INT 21 function 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, set AX to 2900h, and do
    an INT 21.  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.

    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, with my
    fix for the b: special case, tested only in Borland C++ 2.0 (in
    the small model):

        #include <dos.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 ");
            }
        }

Q404. 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 I must advise you to
    disregard that particular bit of advice in the DOS manual.

Q405. 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";
        . . .  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, I'm told) 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 of your program would type in file specs at run time the same
    way as in DOS commands, with single backslashes.

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

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

    Ralf Brown writes that "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.

Q406. How can I redirect printer output to a file?

    (rev: 18 Apr 1993)  My personal favorite utility for this purpose is
    PRN2FILE from {PC Magazine}, downloadable as:
        pd1:<msdos.printer>prn2file.arc 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.

Q407. How can I redirect the output of a batch file?

    (new: 12 June 1993) 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:
        pd1:<msdos.batutl>tsbat43.zip at Simtel
        /pc/ts/tsbat43.zip at garbo.

Q408. How can my program open more files than DOS's limit of 20?

    (rev: 1 May 1993.  This is a summary of an article Ralf Brown posted
    on 8 August 1992, with some additions from a Microsoft tech note.)

    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 function 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.

    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.

Q409. How can I read, create, change, or delete the volume label?

    In DOS 5.0 (and, I believe, in 4.0 as well), there are actually two
    volume labels: one, the traditional one, is an entry in the root
    directory of the disk; and the other is in the boot record along
    with the serial number (see next Q).  The DIR and VOL commands
    report the traditional label; the LABEL command reports the
    traditional one but changes both of them.

    In DOS 4.0 and later, use INT 21 function 69 to access the boot
    record's serial number and volume label together; see the next Q.

    Assume 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 (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, etc.; 0=current drive
        memset(&extfcb[8], '?', 11);  // wildcard *.*
        DOS(extfcb,11h)
        if (status == 0) {            // DTA contains 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

Q410. How can I get the disk serial number?

    Use INT 21.  AX=6900 gets the serial number; AX=6901 sets it.  See
    Ralf Brown's interrupt list, or page 496 of {PC Magazine} July 1992,
    for details.

    This function also gets and sets the volume label, but it's the
    volume label in the boot record, not the volume label that a DIR
    command displays.  See the preceding Q.

Q411. What's the format of .OBJ, .EXE., .COM files?

    Please see section 2, "Compile and link".

Q412. How can I flush the software disk cache?

    Please see "How can a program reboot my PC?" in section 7, "Other
    software questions and problems".

section 5. Serial ports (COM ports)
===================================

Q501. 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

Q502. 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 the preceding Q).  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 only seven are used by many chips).

    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;
        }

Q503. 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.

Q504. How do I configure a COM port and use it to transmit data?

    (rev: 18 Apr 1993) 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, pd1:<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
        pd1:<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

    After hearing several recommendations, I looked at Joe Campbell's {C
    Programmer's Guide to Serial Communications}, ISBN 0-672-22584-0,
    and agree that it is excellent.  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.

section 6. Other hardware questions and problems
================================================

Q601. Which 80x86 CPU is running my program?

    (rev: 20 June 1993)  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
        pd1:<msdos.sysutl>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
        pd1:<msdos.sysutl>wcpu050.zip at Simtel
        /pc/sysinfo/wcpu050.zip at garbo.

Q602. 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 function 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.

Q603. How can I redirect printer output to a file?

    Please see section 4, "Disks and files", for the answer.

Q604. 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 AH=12h, AL=0, BL=32h; INT 10h.  If AL is 12h, you have a VGA.
    If not, set AH=12h, BL=10h; INT 10h.  If BL is 0,1,2,3, you have an
    EGA with 64,128,192,256K memory.  If not, set AH=0Fh; INT 10h.  If
    AL is 7, you have an MDA (original monochrome adapter) or Hercules;
    if not, you have a CGA.

    I've tested this for my VGA and got the right answer; but I can't
    test it for the other equipment types.  Please let me know by email
    at brown@ncoast.org if your results vary.

Q605. How do I switch to 43- or 50-line mode?

    pd1:<msdos.screen>vidmode.zip, downloadable from Simtel, contains
    .COM utilities and .ASM source code.

Q606. How can I find the Microsoft mouse position and button status?

    Use INT 33 function 3, described in Ralf Brown's interrupt list.

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

    Also, many files are downloadable from pd1:<msdos.mouse> at Simtel.

Q607. 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; in Turbo Pascal use
    the pseudo-arrays Mem, MemW, and MemL.  As an alternative, you can
    construct a far pointer:  use Ptr in Turbo Pascal, MK_FP in the
    Turbo C family, and FP_OFF and FP_SEG in Microsoft C.

    Caution:  Turbo C and Turbo C++ also have FP_OFF and FP_SEG macros,
    but they can't be used to construct a pointer.  In Borland C++ those
    macros work the same as in Microsoft C, but MK_FP is easier to use.

    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.

Q608. How can I read or write my PC's CMOS memory?

    (rev: 13 Jun 1993) There are a great many public-domain utilities
    that do this.  These are downloadable from Simtel:

    pd1:<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

    Downloadable from garbo, /pc/ts/tsutle22.zip contains a CMOS program
    to check and display CMOS memory, but not to write to it.

    I have heard good reports of CMOS299.ZIP, available in the pc.dir
    directory of cantva.canterbury.ac.nz [132.181.30.3].

    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.

Q609. How can I access memory beyond 640K?

    (rev: 2 May 1993) This is a legitimate FAQ, in that it is frequently
    asked.  But there is no single agreed-upon answer.  Please see the
    separate article called "How to access memory above 640K" in
    comp.os.msdos.programmer and in faqp*.zip at Simtel and garbo.

Q610. Where can I find a list of 80x86 opcodes?

    (new: 2 May 1993)  It's part of a rather long file, the 8 Dec 1992
    edition of the Info-IBMPC Digest (V92 #185), downloadable as
    pd2:<archives.ibmpc>9212.1-txt at Simtel.  (Note: pd2, not
    pd1.)  Opcodes for the 8086 through 80386 are listed.

section 7. Other software questions and problems
================================================

Q701. 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 even trash 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 function 4F), 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 function 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 "What is the
    SysRq key for?" in section 3, "Keyboard".)

    Here's C code that tries to signal the disk cache (if any) to flush:

        #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.x.ax = 0x4F53;      /* 0x53 = Del's scan code */
                reg.x.cflag = 1;        /* sentinel for ignoring key */
                int86(0x15, &reg, &reg);
                /* 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, &reg, &reg);/* system time into CX:DX */
                ticks = reg.x.dx;
                for (time_to_waste = 3;  time_to_waste > 0;  ) {
                    reg.h.ah = 0;
                    int86(0x1A, &reg, &reg);
                    if (ticks != reg.x.dx)
                        ticks = reg.x.dx , --time_to_waste;
                }
            }
            /* Issue a DOS disk reset request: */
            reg.h.ah = 0x0D;
            int86(0x21, &reg, &reg);
            /* Set boot type and boot: */
            *boottype = (want_warm ? 0x1234 : 0);
            (*boot)( );
        }

Q702. How can I time events with finer resolution than the system
      clock's 55 ms (about 18 ticks a second)?

    (rev: 20 June 1993) The following files, among others, are
    downloadable from Simtel:

    pd1:<msdos.at>
    atim.zip       4783  881126  Precision program timing for AT

    pd1:<msdos.c>
    millisec.zip  37734  911205  MSC/asm src for millisecond res timing
    mschrt3.zip   53708  910605  High-res timer toolbox for MSC 5.1
    msec_12.zip    8484  920320  High-def millisec timer v1.2 (C,ASM)
    ztimer11.zip  77625  920428  Microsecond timer for C, C++, ASM
        (also at garbo as /pc/c/ztimer11.zip)

    pd1:<msdos.turbo-c>
    tchrt3.zip    53436  910606  High-res timer toolbox for Turbo C 2.0
    tctimer.arc   20087  891030  High-res timing of events 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
        pd1:<msdos.turbopas>bonus507.zip at Simtel
        /pc/turbopas/bonus507.zip at garbo.

Q703. How can I find the error level of the previous program?

    (rev: 20 June 1993)  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 function 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
        pd1:<msdos.batutl>errlvl13.zip at Simtel
        /pc/batchutil/errlvl13.zip 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.)

Q704. How can a program set DOS environment variables?

    (rev: 13 June 1993)  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.
    Of the ones I looked at (not all of them), I liked
        pd1:<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
        pd1:<msdos.batutl>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 for details and cautions.

Q705. How can I change the switch character to - from /?

    Under DOS 5.0, you can't -- not completely, anyway.  INT 21 function
    3700, get switch character, always returns a '/' (hex 2F) -- and the
    DOS commands don't even call that function, but 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 "How can I write a TSR?", below.)  DOS commands
    will still use the slash, but non-DOS programs that call DOS func-
    tion 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.

Q706. 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 the next Q for general help before you have 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.
    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 and functions 34, 5D06, 5D0B of INT 21; 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..) and standard I/O
    functions (scanf, printf) call DOS functions and also depend on
    setups that they can't get at from inside an interrupt.  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.  Since
    that interrupt expects to be called 18.2 times a second, 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/info/INTSHARE at ni.funet.fi
        pd1:<msdos.info>intshare 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.)

Q707. How can I write a TSR (terminate-stay-resident utility)?

    (rev: 20 June 1993)  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 5 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.

    - For C programmers, there's a chapter in Herbert Schildt's {The Art
      of C:  Elegant Programming Solutions}.  I haven't seen the book,
      but a posted article recommended it.

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

    - The Alternate Multiplex Interrupt Specification, downloadable as
        pd1:<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, downloadable as
        pd1:<msdos.asmutl>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
        pd1:<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
          pd1:<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
        pd1:<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
          pd1:<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.

Q708. 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.

Q709. What can I use to manage versions of software?

    (rev: 18 Apr 1993)  A port of the Unix RCS utility is downloadable
    from Simtel as pd1:<msdos.gnuish>rcs55ax.zip (EXE and docs) and
    rcs55as.zip (source).  I haven't used it myself, but I understand
    this is no longer limited to one-character extensions on filenames
    (so .CPP and .BAS are fine).

Q710. What's this "null pointer assignment" after my C program executes?

    (rev: 18 Apr 1993)  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 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.

    To track down the problem, you can put exit( ) statements 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, in the debugger, set a watch at location
    0000 in your data segment, assuming you're in small or medium model.
    (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
        pd1:<msdos.turbo-c>bchelp10.zip at Simtel
        /pc/turbopas/bchelp10.zip at garbo.

Q711. How can my program tell if it's running under Windows?

    (rev: 18 Apr 1993)  Set AX=4680 and execute INT 2F.  If AX contains
    0, you're in Windows real mode or standard mode (or under the DOS
    5.0 shell).  Otherwise, set AX=1600 and INT 2F.  If AL does not
    contain 0 or 80, you're in Windows 386 enhanced mode.  See {PC
    Magazine} 24 Nov 1992 (xi:20), pages 492-493.

    When Windows 3.0 or 3.1 is running, the DOS environment will contain
    a definition of the string windir, in lower case.

    For more information, see {PC Magazine} 26 May 1992 (xi:10) pages
    345-346.  A program, WINMODE, is available as part of
        pd1:<msdos.pcmag>vol11n10.zip at Simtel
        /pc/pcmag/vol11n10.zip at garbo.

Q712. How do I copyright software that I write?

    (new: 5 Apr 1993) The following is adapted (and greatly condensed)
    from chapter 4 of the Chicago Manual of Style (13th edition, ISBN
    0-226-10390-0).  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.

    That said, in the U.S. (at least), when you write something, you own
    the copyright.  (The most significant exception to programmers is
    "works made for hire", i.e., something 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.)  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) a date--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.


(continued in part 4)

-- 
Stan Brown, Oak Road Systems                    brown@Ncoast.ORG

Can't find FAQ lists?  ftp to 'rtfm.mit.edu' and look in /pub/usenet
(or email me >>> with valid reply-to address <<< for instructions).