Internet Info 1997 December/Internet_Info_CD-ROM_Walnut_Creek_December_1997.iso/faqs/comp/answers/msdos-programmer-faq/part3

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Path: senator-bedfellow.mit.edu!faqserv
From: [e-mail] (Jeffrey Carlyle)
Newsgroups: comp.os.msdos.programmer,alt.msdos.programmer,comp.answers,alt.answers,news.answers
Subject: comp.os.msdos.programmer FAQ part 3/5
Supersedes: <msdos-programmer-faq/[e-mail]>
Followup-To: comp.os.msdos.programmer
Date: 30 Sep 1997 09:50:04 GMT
Organization: The COMP-FAQ Project
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Sender: [e-mail] (Jeffrey Carlyle)
Approved: [e-mail]
Expires: 3 Nov 1997 09:49:05 GMT
Message-ID: <msdos-programmer-faq/[e-mail]>
References: <msdos-programmer-faq/[e-mail]>
Reply-To: [e-mail] (Jeffrey Carlyle)
NNTP-Posting-Host: penguin-lust.mit.edu
Summary: Frequently asked questions by DOS programmers with tested answers.
X-Last-Updated: 1997/08/04
Originator: [e-mail]
Xref: senator-bedfellow.mit.edu comp.os.msdos.programmer:92882 alt.msdos.programmer:39079 comp.answers:28270 alt.answers:29277 news.answers:113415

Archive-name: msdos-programmer-faq/part3
Comp-os-msdos-programmer-archive-name: dos-faq-pt3.txt
Posting-frequency: 28 days
Last-modified: 04 Aug 97

------------------------------

Subject: comp.os.msdos.programmer FAQ part 3/5


 For more information about this FAQ list please see Part 1.


 FAQ        updates       can       be       found        at
 <http://www.premiernet.net/~carlyle>.


 This  is  part 3 of the frequently asked question list  for
 the newsgroup comp.os.msdos.programmer.

Part 3:
  Section 5.  Disks and files
  Section 6.  Serial ports (COM ports)

------------------------------

Subject: comp.os.msdos.programmer FAQ


 Comp.os.msdos.programmer FAQ Version 1997.08


 Copyright  1997  by Jeffrey Carlyle. 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 to the FAQ would be polite. This
 FAQ  is not to be included in any static archive (e.g.  CD-
 ROM  or  book);  however,  a pointer  to  the  FAQ  may  be
 included.

=============================
TABLE OF CONTENTS
=============================
Part 1:
  Section 1.  General FAQ and Newsgroup Information
  Section 2.  General Reference
Part 2:
  Section 3.  Compile and Link
  Section 4.  Keyboard
Part 3:
  Section 5.  Disks and files
  Section 6.  Serial ports (COM ports)
Part 4:
  Section 7.  Other hardware questions and problems
  Section 8.  Other software questions and problems
Part 5:
  Section 9.  Downloading
  Section 10. Vendors and products

------------------------------

Subject: Section 5.  Disks and files

<Q: 5.01> - What drive was the PC booted from?
<Q: 5.02> - How can I boot from drive B:?
<Q: 5.03> - Which real and virtual disk drives are valid?
<Q: 5.04>  -  How can I make my single floppy drive both  B:
     and B:?
<Q: 5.05> - How can I disable access to a drive?
<Q: 5.06>  -  How  can  a  batch file test  existence  of  a
     directory?
<Q: 5.07> - Why won't my C program open a file with a path?
<Q: 5.08> - How can I redirect printer output to a file?
<Q: 5.09> - How can I redirect the output of a batch file?
<Q: 5.10> - How can I redirect stderr?
<Q: 5.11>  -  How can my program open more files than  DOS's
     limit of 20?
<Q: 5.12>  -  How can I read, create, change, or delete  the
     volume label?
<Q: 5.13> - How can I get the disk serial number?
<Q: 5.14> - What's the format of .OBJ, .EXE, .COM files?
<Q: 5.15> - How can I flush the software disk cache?
<Q: 5.16> - How can I see if a drive is a RAM drive?

------------------------------

Subject: <Q: 5.01> - 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.).


------------------------------

Subject: <Q: 5.02> - How can I boot from drive B:?
Date: Sun, 03 Aug 97 20:07:24 CST


 Downloadable shareware:

  <ftp://ftp.simtel.net/pub/simtelnet/msdos/diskutil/boot_b.
  zip>
  <ftp://garbo.uwasa.fi/pc/bootutil/boot_b.zip>

 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.


------------------------------

Subject: <Q: 5.03> - Which real and virtual disk drives are
     valid?
Date: Wed, 11 Jan 95 15:34:00 CDT


 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: 2.03>.


 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++ 4.5 (in the large model):

  #include <dos.h>
  #include <stdio.h>
  void drvlist(void)
  {
    char s[3] = "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[3] = "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");
     }
  }

------------------------------

Subject: <Q: 5.04> - How can I make my single floppy drive
     both a: and b:?


 Under  any  DOS  since DOS 2.0, you can put  the  following
 command into your AUTOEXEC.BAT file:

  assign b=a

 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.


------------------------------

Subject: <Q: 5.05> - How can I disable access to a drive?
Date: Sun, 03 Aug 97 20:08:47 CST


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

 <ftp://ftp.simtel.net/pub/simtelnet/msdos/diskutil/rmdriv30
 .zip>

 Reader  Igor  Karp  reports that  MS-DOS  version  5.0  and
 greater provides two interrupts to do this.

  --------D-215F07-----------------------------
  INT 21 - DOS 5+ - ENABLE DRIVE
     AX = 5F07h
     DL = drive number (0=A:)
  Return: CF clear if successful
     CF set on error
     AX = error code (0Fh) (see #0885 at AH=59h)
  Notes:   simply  sets the "valid" bit in the  drive's  CDS
  this function is not supported by Novell DOS 7
  See Also: AH=52h,AX=5F08h"DOS"

  --------D-215F08-----------------------------
  INT 21 - DOS 5+ - DISABLE DRIVE
     AX = 5F08h
     DL = drive number (0=A:)
  Return: CF clear if successful
     CF set on error
     AX = error code (0Fh) (see #0885 at AH=59h)
  Notes:   simply clears the "valid" bit in the drive's  CDS
  this function is not supported by Novell DOS 7

------------------------------

Subject: <Q: 5.06> - How can a batch file test existence of
     a directory?
Date: Tue, 7 Mar 95 12:00:00 CDT


 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.) As a legacy from earlier versions
 of DOS it always succeeds if the path is DEV.


 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:

 <ftp://garbo.uwasa.fi/pc/ts/tsfaqp*.zip>

------------------------------

Subject: <Q: 5.07> - 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.


------------------------------

Subject: <Q: 5.08> - How can I redirect printer output to a
     file?
Date: Sun, 03 Aug 97 20:16:07 CST


 Recommended: PRN2FILE from PC Magazine, downloadable as:

 <ftp://ftp.simtel.net/pub/simtelnet/msdos/printer/prn2fil3.
 zip>

 PRN2FILE  contains ASM source code. 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.


------------------------------

Subject: <Q: 5.09> - How can I redirect the output of a
     batch file?
Date: Sun, 03 Aug 97 20:18:26 CST


 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:

 <ftp://ftp.simtel.net/pub/simtelnet/msdos/batchutil/tsbat*.
 zip>
 <ftp://garbo.uwasa.fi/pc/ts/tsbat*.zip>

------------------------------

Subject: <Q: 5.10> - How can I redirect stderr?
Date: Sun, 03 Aug 97 20:20:33 CST


 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:

 <ftp://ftp.simtel.net/pub/simtelnet/msdos/sysutl/rdstderr.z
 ip>

 A C example is downloadable as:

 <ftp://ftp.simtel.net/pub/simtelnet/msdos/c/redirect.c>

 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.


------------------------------

Subject: <Q: 5.11> - 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.


------------------------------

Subject: <Q: 5.12> - 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 "<Q: 5.13> -
   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

------------------------------

Subject: <Q: 5.13> - How can I get the disk serial number?


 If  the disk was formatted 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: 2.03>, 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 <Q: 5.12> - "How can  I  read,
 create, change, or delete the volume label?"


------------------------------

Subject: <Q: 5.14> - What's the format of .OBJ, .EXE., .COM
     files?


 Please  see <Q: 3.06> "What's the format of an .OBJ file?";
 <Q:  3.07> "What's the format of an .EXE header?"; and  <Q:
 3.08>   "What's  the  difference  between  .COM  and   .EXE
 formats?"


------------------------------

Subject: <Q: 5.15> - How can I flush the software disk
     cache?


 Please  see "<Q: 8.01> - How can a program reboot  my  PC?"
 (Trust me.)


------------------------------

Subject: <Q: 5.16> - How can I see if a drive is a RAM
     drive?
Date: Tue, 7 Mar 95 12:00:00 CDT


 Use  INT 21 AX=4409h. See Ralph Brown's interrupt list  <Q:
 2.03> for more information.


------------------------------

Subject: Section 6.  Serial ports (COM ports)

<Q: 6.01> - How do I set my machine up to use COM3 and COM4?
<Q: 6.02> - How do I find the I/O address of a COM port?
<Q: 6.03> - But aren't the COM ports always at I/O addresses
     3F8, 2F8, 3E8, and 2E8?
<Q: 6.04>  -  How do I configure a COM port and  use  it  to
     transmit data?

------------------------------

Subject: <Q: 6.01> - 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

------------------------------

Subject: <Q: 6.02> - How do I find the I/O address of a COM
     port?
Date: Sun, 03 Aug 97 20:27:12 CST


 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:

 <ftp://ftp.simtel.net/pub/msdos/io_util/pf271.zip>

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


------------------------------

Subject: <Q: 6.03> - 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.


------------------------------

Subject: <Q: 6.04> - How do I configure a COM port and use
     it to transmit data?
Date: Sun, 03 Aug 97 20:27:31 CST


 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,
 <ftp://oak.oakland.edu/pub/msdos/c/pcl4c34.zip>      (March
 1993)  is described as "Asynchronous communications library
 for  C";  Garbo  has a whole <ftp://garbo.uwasa.fi/pc/comm>
 directory.   Also,  an  extended example  is  in  Borland's
 TechFax TI445, downloadable as part of:

 <ftp://ftp.simtel.net/pub/msdos/turbo-c/bchelp10.zip>
 <ftp://garbo.uwasa.fi/pc/turbopas/bchelp10.zip>

 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 1-55828-198-3),
 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:

 <ftp://ftp.phil.uni-sb.de/pub/people/chris/*Serial*>

 For rtfm.mit.edu instructions, see <Q: 1.13> "Where are FAQ
 lists archived?"


------------------------------

Subject: End

 (FAQ       updates       can       be       found        at
 <http://www.premiernet.net/~carlyle>.)
 (End  of comp.os.msdos.programmer FAQ Version 1997.08  Part
 3/5)
 (This text is copyright 1997 by Jeffrey Carlyle. All rights
 reserved.)



// Jeffrey Carlyle, Bowling Green, Kentucy USA
//
// comp.os.msdos.programmer FAQ maintainer
// <http://www.premiernet.net/~carlyle>