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comp.os.msdos.programmer FAQ part 4/5

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Archive-name: msdos-programmer-faq/part4
Comp-os-msdos-programmer-archive-name: dos-faq-pt4.txt
Posting-frequency: 28 days
Last-modified: 14 Aug 2003

See reader questions & answers on this topic! - Help others by sharing your knowledge
comp.os.msdos.programmer FAQ Version 2003.08.14

This is the Frequently Asked Questions list for the newsgroup
comp.os.msdos.programmer.

COPYRIGHT

Copyright 2003 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. See <Q:01.14> [Where can I get the
latest copy of this FAQ list?] for a link to the latest version of the
FAQ.)

This is part 4 of 5 parts.

TABLE OF CONTENTS

PART 1: 
Section 1.  General FAQ and Newsgroup Information
  <Q:01.01> - Is MS-DOS Dead?
  <Q:01.02> - What is this article for?
  <Q:01.03> - Who has contributed to this article?
  <Q:01.04> - How can I search this article for a particular topic?
  <Q:01.05> - Are the answers guaranteed to be correct and complete?
  <Q:01.06> - What is comp.os.msdos.programmer about?
  <Q:01.07> - Is comp.os.msdos.programmer just for C programmers?
  <Q:01.08> - What is comp.sys.ibm.pc.programmer?
  <Q:01.09> - Is comp.os.msdos.programmer available as a mailing list?
  <Q:01.10> - What's this netiquette?
  <Q:01.11> - How can I learn more about Usenet?
  <Q:01.12> - What other technical newsgroups should I know about?
  <Q:01.13> - Where are FAQ lists archived?
  <Q:01.14> - Where can I get the latest copy of this FAQ list?

Section 2.  General Reference
  <Q:02.01> - Are there any good on-line references for PC hardware
              components?
  <Q:02.02> - Are there any good on-line references for PC interrupts?
  <Q:02.03> - What and where is Ralf Brown's interrupt list?
  <Q:02.04> - Where can I find lex, yacc, and language grammars?
  <Q:02.05> - What's the best book to learn programming?
  <Q:02.06> - Why won't my code work?
  <Q:02.07> - Are there any good sources of example code?
  <Q:02.08> - What and where is SNIPPETS?
  <Q:02.09> - Is the source code MS-DOS available?
  <Q:02.10> - What are my alternatives for MS-DOS compatible OSes?
  <Q:02.11> - What and where is FreeDOS?
  <Q:02.12> - Where can I find out about batch files?

PART 2: 
Section 3.  Compile and link
  <Q:03.01> - What the heck is DGROUP > 64K?
  <Q:03.02> - How do I fix 'automatic data segment exceeds 64K' or 'stack
              plus data exceed 64K'?
  <Q:03.03> - Will Borland C code and Microsoft C code link together?
  <Q:03.04> - Why did my program bomb at run time with 'floating point
              formats not linked' or 'floating point not loaded'?
  <Q:03.05> - How can I change the stack size in Borland's C compilers?
  <Q:03.06> - What's the format of an .OBJ file?
  <Q:03.07> - What's the format of an .EXE header?
  <Q:03.08> - What's the difference between .COM and .EXE formats?
  <Q:03.09> - How do I create a .COM file?
  <Q:03.10> - Where is EXE2BIN located?
  <Q:03.11> - What does this message mean: 'A20 already enabled so test
              is meaning less?'

Section 4.  Keyboard
  <Q:04.01> - How can I read a character without echoing it to the
              screen, and without waiting for the user to press the Enter
              key?
  <Q:04.02> - How can I find out whether a character has been typed,
              without waiting for one?
  <Q:04.03> - How can I disable Ctrl-C/Ctrl-Break and/or Ctrl-Alt-Del?
  <Q:04.04> - How can I disable the print screen function?
  <Q:04.05> - How can my program turn NumLock (CapsLock, ScrollLock) on
              or off?
  <Q:04.06> - How can I speed up the keyboard's auto-repeat?
  <Q:04.07> - What is the SysRq key for?
  <Q:04.08> - How can my program tell what kind of keyboard is on the
              system?
  <Q:04.09> - How can I tell if input, output, or stderr has been
              redirected?
  <Q:04.10> - How can I increase the size of the keyboard buffer?
  <Q:04.11> - How can I stuff characters into the keyboard buffer?

PART 3: 
Section 5.  Disks and files
  <Q:05.01> - What drive was the PC booted from?
  <Q:05.02> - How can I boot from drive B:?
  <Q:05.03> - Which real and virtual disk drives are valid?
  <Q:05.04> - How can I make my single floppy drive both a: and b:?
  <Q:05.05> - How can I disable access to a drive?
  <Q:05.06> - How can a batch file test existence of a directory?
  <Q:05.07> - Why won't my C program open a file with a path?
  <Q:05.08> - How can I redirect printer output to a file?
  <Q:05.09> - How can I redirect the output of a batch file?
  <Q:05.10> - How can I redirect stderr?
  <Q:05.11> - How can my program open more files than DOS's limit of 20?
  <Q:05.12> - How can I read, create, change, or delete the volume label?
  <Q:05.13> - How can I get the disk serial number?
  <Q:05.14> - What's the format of .OBJ, .EXE., .COM files?
  <Q:05.15> - How can I flush the software disk cache?
  <Q:05.16> - How can I see if a drive is a RAM drive?
  <Q:05.17> - How can I determine a hard drive's manufacturer?
  <Q:05.18> - Where can I find information about the ATA/ATAPI
              specification?
  <Q:05.19> - How can I copy files to or from filenames containing date
              information?

Section 6.  Serial ports (COM ports)
  <Q:06.01> - How do I set my machine up to use COM3 and COM4?
  <Q:06.02> - How do I find the I/O address of a COM port?
  <Q:06.03> - But aren't the COM ports always at I/O addresses 3F8, 2F8,
              3E8, and 2E8?
  <Q:06.04> - How do I configure a COM port and use it to transmit data?

PART 4: (this part)
Section 7.  Other hardware questions and problems
  <Q:07.01> - Which 80x86 CPU is running my program?
  <Q:07.02> - How can a C program send control codes to my printer?
  <Q:07.03> - How can I redirect printer output?
  <Q:07.04> - Which video adapter is installed?
  <Q:07.05> - How do I switch to 43- or 50-line mode?
  <Q:07.06> - How can I find the Microsoft mouse position and button
              status?
  <Q:07.07> - How can I access a specific address in the PC's memory?
  <Q:07.08> - How can I read or write my PC's CMOS memory?
  <Q:07.09> - How can I access memory beyond 640K?
  <Q:07.10> - How can I use the protected mode?
  <Q:07.11> - How can I tell if my program is running on a PS/2-style
              machine.
  <Q:07.12> - Is there a 80x87 math unit installed?
  <Q:07.13> - How can I power off the computer from a batch file?

Section 8.  Other software questions and problems
  <Q:08.01> - How can a program reboot my PC?
  <Q:08.02> - How can I time events with finer resolution than the system
              clock's 55 ms (about 18 ticks a second)?
  <Q:08.03> - How can I find the error level of the previous program?
  <Q:08.04> - How can a program set DOS environment variables?
  <Q:08.05> - How can I change the switch character to - from /?
  <Q:08.06> - How can I write a TSR (terminate-stay-resident utility)?
  <Q:08.07> - Why does my interrupt function behave strangely?
  <Q:08.08> - How can I write a device driver?
  <Q:08.09> - What can I use to manage versions of software?
  <Q:08.10> - What's this 'null pointer assignment' after my C program
              executes?
  <Q:08.11> - How can a batch file tell whether it's being run in a DOS
              box under Windows?
  <Q:08.12> - How can my program tell if it's running under Windows?
  <Q:08.13> - How can a program tell whether ANSI.SYS is installed?
  <Q:08.14> - How do I copyright software that I write?
  <Q:08.15> - How can I place date and time information into environment
              variables?

PART 5: 
Section 9.  Downloading
  <Q:09.01> - What are SimTel and Garbo?
  <Q:09.02> - Can I get archives on CD-ROM?
  <Q:09.03> - Where do I find program <mumble>?

Section 10.  Vendors and products
  <Q:10.01> - How can I contact Borland?
  <Q:10.02> - How can I contact Microsoft?
  <Q:10.03> - What is the current version of DJGPP?
  <Q:10.04> - What and where is DJGPP?
  <Q:10.05> - Are there any good shareware/freeware compilers?
  <Q:10.06> - Where is QBASIC?
  <Q:10.07> - What is a vendor's web site address?


Subject: Section 7. Other hardware questions and problems Date: 5 Feb 2002 22:03:03 -0400 The subject says it all. Bv)
Subject: <Q:07.01> - Which 80x86 CPU is running my program? Date: 5 Feb 2002 22:03:03 -0400 SNIPPETS (see <Q:02.08> [What and where is SNIPPETS?]) contains C-callable x86 assembly language code for determining the type of CPU in CPUCHECK.ASM.
Subject: <Q:07.02> - How can a C program send control codes to my printer? Date: 5 Feb 2002 22:03:03 -0400 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, in DOS 5.0 a colon must not follow PRN or LPT1. There's one special case, Ctrl-Z (ASCII 26), the DOS end-of- file character. If you try to send an ASCII 26 to your printer, DOS simply ignores it. To get around this, you need to reset the printer from "cooked" to "raw" mode. Microsoft C users must use INT 21 AH=44, "get/set device information". Turbo C and Borland C++ users can use ioctl to accomplish the same thing: ioctl(fileno(prn), 1, ioctl(fileno(prn),0) & 0xFF | 0x20, 0); An alternative approach is simply to write the printer output into a disk file, then copy the file to the printer with the /B switch. A third approach is to bypass DOS functions entirely and use the BIOS printer functions at INT 17. If you also fprintf(stdprn,...) in the same program, you'll need to use fflush() to synchronize fprintf()'s buffered output with the BIOS's unbuffered. By the way, if you've opened the printer in binary mode from a C program, remember that outgoing \n won't be translated to carriage return/line feed. Depending on your printer, you may need to send explicit \n\r sequences.
Subject: <Q:07.03> - How can I redirect printer output? Date: 5 Feb 2002 22:03:03 -0400 Please see <Q:05.08> [How can I redirect printer output to a file?]
Subject: <Q:07.04> - Which video adapter is installed? Date: 5 Feb 2002 22:03:03 -0400 The technique below should work if your BIOS is not too old. It uses three functions from INT 10, the BIOS video interrupt. (If you're using a Borland language, you may not have to do this the hard way. Look for a function called DetectGraph or something similar.) Set AX=1200, BL=32 and call INT 10. If AL returns 12, you have a VGA. If not, set AH=12, BL=10 and call INT 10 again. If BL returns 0,1,2,3, you have an EGA with 64,128,192,256K memory. If not, set AH=0F and call INT 10 a third time. If AL is 7, you have an MDA (original monochrome adapter) or Hercules; if not, you have a CGA. This worked when tested with a VGA, but I had no other adapter types to test it with.
Subject: <Q:07.05> - How do I switch to 43- or 50-line mode? Date: 8 Feb 2002 20:05:49 -0400 The following file contains .COM utilities and .ASM source code: <http://www.simtel.net/pub/pd/49657.html>
Subject: <Q:07.06> - How can I find the Microsoft mouse position and button status? Date: 8 Feb 2002 20:07:14 -0400 Use INT 33 AX=3, described in Ralf Brown's interrupt list (<Q:02.03> [What and where is Ralf Brown's interrupt list?]). The Windows manual says that the Logitech mouse is compatible with the Microsoft one, so the interrupt will probably work the same. Also, many files are downloadable from: <http://www.simtel.net/pub/msdos/mouse/>
Subject: <Q:07.07> - How can I access a specific address in the PC's memory? Date: 7 Feb 2002 14:50:10 -0400 First check the library that came with your compiler. Many vendors have some variant of peek and poke functions. For example: * In Turbo Pascal, use the pseudo-arrays Mem, MemW, and MemL. Be sure you use the correct array for the size of data you want to access: byte, word, or double word. Alternatively, use pointers. * In Turbo C/Borland C, and in recent versions of Microsoft C, use MK_FP; in older versions of Microsoft C, use FP_OFF and FP_SEG. (Caution: Turbo C and Turbo C++ also have FP_OFF and FP_SEG macros, but they can't be used to construct a pointer.) Be sure to pick the right data type: probably "unsigned char far *" if you're planning to access bytes and "unsigned short far *" for words. (The "far" isn't needed if your memory model uses 32-bit data pointers, but including it does no harm.) By the way, it's not useful to talk about "portable" ways to do this. Any operation that is tied to a specific memory address is not likely to work on another kind of machine.
Subject: <Q:07.08> - How can I read or write my PC's CMOS memory? Date: 8 Feb 2002 20:11:37 -0400 There are a great many public-domain utilities that do this. The following files can be downloaded from <http://www.simtel.net/pub/msdos/sysutl/>: cmos14.zip 5965 920817 Saves/restores CMOS to/from file cmoser11.zip 28323 910721 386/286 enhanced CMOS setup program cmosram.zip 76096 920214 Save AT/386/486 CMOS data to file and restore rom2.zip 15692 900131 Save AT and 386 CMOS data to file and restore setup21.zip 18172 880613 Setup program which modifies CMOS RAM viewcmos.zip 11068 900225 Display contents of AT CMOS RAM, w/C source A program to check and display CMOS memory (but not write to it) is downloadable as part of: <http://www.simtel.net/pub/pd/50522.html> <ftp://garbo.uwasa.fi/pc/ts/tsutle23.zip> Good reports of CMOS299.ZIP have been posted, but it no longer appears to be online. Stan Brown, the former list maintainer, reports that he personally tested CMOSRAM and that it worked 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.
Subject: <Q:07.09> - How can I access memory beyond 640K? Date: 8 Feb 2002 20:50:34 -0400 Part of this involves switching into protected mode. An article entitled "How DOS Programs Can Use Over 1MB of RAM" appeared in PC Magazine 29 June 1993 (xii: 12) pages 302-304. I also suggest John English's XMS classes for C++: <ftp://ftp.brighton.ac.uk/pub/je/xms200je.zip>. See also <Q:07.10> [How can I use the protected mode?]
Subject: <Q:07.10> - How can I use the protected mode? Date: 8 Feb 2002 20:15:02 -0400 DJ Delorie has produced DJGPP, a protected mode programming environment which supports a port of the GNU C/C++/Ada. For more informat see <Q:10.04> [What and where is DJGPP?] Users of Borland C++ Version 4.xx could once purchase the Borland PowerPack for DOS Version 1.00; however, it appears that its sell has been discontinued. This package includes Borland C++ 4.02 Service Update, 16-bit DPMI libraries and extenders, 32-bit DPMI libraries and extenders, TurboVision 2.0 (16-bit DOS, 16-bit DPMI, 32-bit DPMI), SuperVGA BGI Drivers (16-bit DOS, 16-bit DPMI, 32- bit DPMI). A reader of comp.os.msdos.programmer has the following updateu about the PowerPack and additional information about DPMI programming: [begin quote] About protected-mode programming (Question 7.10), Borland no longer supports the PowerPack (from what I've read in this newsgroup), so I doubt that it can still be purchased. However, the FAQ could mention that the extender is integrated into Borland C++ version 5.x (and probably the Builder product as well). Borland doesn't fully support that either, since the documentation isn't very clear about compiling DPMI programs and a stub for the executable has to be extracted from one of Borland's executables. The necessary steps were posted a long time ago (by someone else), and I can provide the information if necessary. Even if the FAQ didn't describe those steps, it could at least indicate that version 5.x can compile DPMI programs (as well as clarifying the PowerPack info). [end quote] There are more extenders out there. One notable DOS extender is Adam Seychell's DOS32 Version 3.5 beta. It can be found at <http://www.programmersheaven.com/zone5/cat19/1363.htm> Also check out Adam's page at <http://www.geocities.com/SiliconValley/2151/pmode.html>
Subject: <Q:07.11> - How can I tell if my program is running on a PS/2-style machine. Date: 5 Feb 2002 22:03:03 -0400 Use INT 15 AX=C0, described in Ralf Brown's interrupt list (<Q:02.03> [What and where is Ralf Brown's interrupt list?]).
Subject: <Q:07.12> - Is there a 80x87 math unit installed? Date: 5 Feb 2002 22:03:03 -0400 SNIPPETS (see <Q:02.08> [What and where is SNIPPETS?]) contains C-callable assembly code to determine presence of coprocessor in NDPCHECK.ASM.
Subject: <Q:07.13> - How can I power off the computer from a batch file? Date: 6 Feb 2002 20:36:38 -0400 A utility known as ATXOFF.COM is available at <http://www.informatik.fh-muenchen.de/~ifw98223/dostools.htm>. It uses APM 1.2 to power off the system but doesn't attempt to flush the disk cache.
Subject: Section 8. Other software questions and problems Date: 5 Feb 2002 22:03:03 -0400 This section is noteworthy for having the longest code snippets.
Subject: <Q:08.01> - How can a program reboot my PC? Date: 8 Feb 2002 20:16:09 -0400 You can generate a "cold" boot or a "warm" boot. A cold boot is the same as turning the power off and on; a warm boot is the same as Ctrl-Alt-Del and skips the power-on self 'test. For a warm boot, store the hex value 1234 in the word at 0040:0072. For a cold boot, store 0 in that word. Then, if you want to live dangerously, jump to address FFFF:0000. Here's C code to do it: /* WARNING: data loss possible */ void bootme(int want_warm) /* arg 0 = cold boot, 1 = warm */ { void (far* boot)(void) = (void (far*)(void))0xFFFF0000UL; unsigned far* type = (unsigned far*)0x00400072UL; *type = (want_warm ? 0x1234 : 0); (*boot)( ); } What's wrong with that method? It will boot right away, without closing files, flushing disk caches, etc. If you boot without flushing a write-behind disk cache (if one is running), you could lose data or trash the file allocation table in your hard drive. There are two methods of signaling the cache to flush its buffers: (1) Simulate a keyboard Ctrl-Alt-Del in the keystroke translation function of the BIOS (INT 15 AH=4F; but see notes below) (2) Issue a disk reset (DOS function 0D). Most disk-cache programs hook one or both of those interrupts, so if you use both methods you'll probably be safe. When user code simulates a Ctrl-Alt-Del, one or more of the programs that have hooked INT 15 AH=4F can ask that the key be ignored by clearing the carry flag. For example, HyperDisk does this when it has started but not finished a cache flush. So if the carry flag comes back cleared, the boot code has to wait a couple of clock ticks and then try again. (None of this matters on older machines whose BIOS can't support 101- or 102-key keyboards; see the discussion of INT 21 AH=4F in <Q:04.07> [What is the SysRq key for?]) C code that tries to signal the disk cache (if any) to flush is given below. Turbo Pascal code by Timo Salmi that does more or less the same job may be found at question 49 (as of this writing) in the Turbo Pascal FAQ in comp.lang.pascal, and is downloadable as file FAQPAS2.TXT, which is part of: <ftp://garbo.uwasa.fi/pc/link/> Here's C code that reboots after trying to signal the disk cache: #include <dos.h> void bootme(int want_warm) /* arg 0 = cold boot, 1 = warm */ { union REGS reg; void (far* boot)(void) = (void (far*)(void))0xFFFF0000UL; unsigned far* boottype = (unsigned far*)0x00400072UL; char far* shiftstate = (char far*)0x00400017UL; unsigned ticks; int time_to_waste; /* Simulate reception of Ctrl-Alt-Del: */ for (;;) { *shiftstate |= 0x0C; /* turn on Ctrl & Alt */ reg.h.ah = 0x4F; /* see notes below */ reg.h.al = 0x53; /* 0x53 = Del's scan code */ reg.x.cflag = 1; /* sentinel for ignoring key */ int86(0x15, &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)( ); } Reader Timo Salmi reported (26 July 1993) that the INT 15 AH=4F call may not work on older PCs (below AT, XT2, XT286), according to Ralf Brown's interrupt list (<Q:02.03> [What and where is Ralf Brown's interrupt list?]). Reader Roger Fulton reported (1 July 1993) that INT 15 AH=4F hangs even a modern PC "ONLY when ANSI.SYS [is] loaded high using EMM386.EXE. (Other things loaded high with EMM386.EXE were OK; ANSI.SYS loaded high with QEMM386.SYS was OK; ANSI.SYS loaded low with EMM386.EXE installed was OK.)" His solution was to use only the disk reset, INT 21 AH=0D, which does flush SMARTDRV, then wait five seconds in hopes that any other disk-caching software would have time to flush its queue. Reader Per Bergland reported (10 Sep 1993) that the jump to FFFF:0000 will not work in Windows or other protected-mode programs. (For example, when the above reboot code ran in a DOS session under Windows, a box with "waiting for system shutdown" appeared. The PC hung and had to be reset by cycling power.) His solution, which does a cold boot not a warm boot, is to pulse pin 0 of the 8042 keyboard controller, which is connected to the CPU's "reset" line. He has tested the following code on various Compaqs, and expects it will work for any AT-class machine; he cautions that you must first flush the disk cache as indicated above. cli @@WaitOutReady: ; Busy-wait until 8042 ready for new command in al,64h ; read 8042 status byte test al,00000010b ; this bit indicates input buffer full jnz @@WaitOutReady mov al,0FEh ; Pulse "reset" = 8042 pin 0 out 64h,al ; The PC will reboot now
Subject: <Q:08.02> - How can I time events with finer resolution than the system clock's 55 ms (about 18 ticks a second)? Date: 8 Feb 2002 20:19:05 -0400 The PC Timing FAQ / Application Note, maintained by Kris Heidenstrom (kheidenstrom@actrix.gen.nz), contains information relating to timing with PC hardware and software. It can be found on SimTel: <http://www.simtel.net/pub/pd/46935.html> The following files, among others, are downloadable from SimTel in the <http://www.simtel.net/pub/msdos/sysutl/> directory: atim.zip 4783 881126 Precision program timing for AT In the <http://www.simtel.net/pub/msdos/c/> directory: millisec.zip 37734 911205 MSC/asm src for millisecond timing mschrt3.zip 53708 910605 High-res timer toolbox for MSC 5.1 msec_12.zip 8484 920320 High-def timer v1.2 (C,ASM) ztimer11.zip 77625 920428 Microsecond timer for C, C++, ASM For Turbo Pascal users, source and object code are downloadable as: <ftp://garbo.uwasa.fi/pc/turbopas/> Also see "Q: How is millisecond timing done?" in FAQPAS.TXT, downloadable as part of: <ftp://garbo.uwasa.fi/pc/link/>
Subject: <Q:08.03> - How can I find the error level of the previous program? Date: 8 Feb 2002 20:21:49 -0400 First, which previous program are you talking about? If your current program ran another one, when the child program ends its error level is available to the program that spawned it. Most high-level languages provide a way to do this; for instance, in Turbo Pascal it's Lo(DosExitCode) and the high byte gives the way in which the child terminated. In Microsoft C, the exit code of a synchronous child process is the return value of the spawn- type function that creates the process. If your language doesn't have a function to return the error code of a child process, you can use INT 21 AH=4D (get return code). By the way, this will tell you the child's exit code and the manner of its ending (normal, Ctrl-C, critical error, or TSR). It's much trickier if the current program wants to get the error level of the program that ran and finished before this one started. G.A. Theall has published source and compiled code to do this; the code is downloadable as: <http://www.simtel.net/pub/pd/40610.html> (The code uses undocumented features in DOS 3.3 through 5.0. In the .DOC file Theall says that the values returned under 4DOS or other replacements won't be right.)
Subject: <Q:08.04> - How can a program set DOS environment variables? Date: 8 Feb 2002 20:22:43 -0400 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. Stan Brown, the former list maintainer recommends the following: <ftp://garbo.uwasa.fi/pc/envutil/> It includes some utilities to manipulate the environment, with source code in C. A newer program from PC Magazine 22 Dec 1992 (XI: 22) is: <ftp://garbo.uwasa.fi/pc/pcmagvol/> You can also use a call to INT 2E, Pass Command to Interpreter for Execution; see Ralf Brown's interrupt list (<Q:02.03> [What and where is Ralf Brown's interrupt list?]) for details and cautions. Reader Dr. John Stockton has written a unit for Turbo Pascal known as jrs_envu.pas to facilitate writing to the environment. It is for DOS (not DPMI) mode programs running under DOS to Win98, but not WinNT. It can be downloaded from here: <http://www.merlyn.demon.co.uk/programs/jrs_envu.pas>. For more information, see <http://www.merlyn.demon.co.uk/batprogs.htm>.
Subject: <Q:08.05> - How can I change the switch character to - from /? Date: 5 Feb 2002 22:03:03 -0400 Under DOS 5.0 and above you can not. INT 21 AX=3700, get switch character, always returns a '/' (hex 2F). But the DOS commands don't even call that function: they simply hard code '/' as the switch character. Some history: DOS used to let you change the switch character by using SWITCHAR= in CONFIG.SYS or by calling DOS function 3701. DOS commands and other programs called DOS function 3700 to find out the switch character. If you changed the switch character to '-' (the usual choice), you could then type "dir c:/c700 -p" rather than "dir c:\c700 /p". Under DOS 4.0, the DOS commands ignored the switch character but functions 3700 and 3701 still worked and could be used by other programs. Under DOS 5.0, even those functions no longer work, though all DOS functions still accept '/' or '\' in file specs. You can reactivate the functions to get and set switchar by using programs like SLASH.ZIP or the sample TSR called SWITCHAR in amisl091.zip (see <Q:08.06> [How can I write a TSR (terminate-stay-resident utility)?]). DOS commands will still use the slash, but non-DOS programs that call DOS function 3700 will use your desired switch character. (DOS replacements like 4DOS may honor the switch character for internal commands.) Some readers may wonder why this is even an issue. Making '-' the switch character frees up the front slash to separate names in the path part of a file spec. This is easier for the ten-fingered to type, and it's one less difference to remember for commuters between DOS and Unix. The switch character is the only issue, since all the INT 21 functions accept '/' or '\' to separate directory names.
Subject: <Q:08.06> - How can I write a TSR (terminate-stay-resident utility)? Date: 8 Feb 2002 20:31:08 -0400 There are books, and there's code to download. First, the books: * Ray Duncan's {Advanced MS-DOS}, ISBN 1-55615-157-8, gives a brief checklist intended for experienced programmers. The ISBN is for the second edition, through DOS 4; but check to see whether the DOS 6 version is available yet. * {DOS 5: A Developer's Guide} by Al Williams, ISBN 1-55851-177-6, goes into a little more detail, 90 pages worth! * Pascal programmers might look at {The Ultimate DOS Programmer's Manual} by John Mueller and Wallace Wang, ISBN 0-8306-3534-3, for an extended example in mixed Pascal and assembler. * For a pure assembler treatment, check Steven Holzner's {Advanced Assembly Language}, ISBN 0-13-663014-6. He has a book with the same title out from Brady Press, but it's about half as long as this one. Next, the code. Some of it is companion code to published articles, which are also listed below: * John English (<http://www.it.bton.ac.uk/staff/je/>) has created a wonderful C++ class for creating TSRs which includes several examples. This one is my personal favorite. It is downloadable as: <ftp://ftp.brighton.ac.uk/pub/je/tsr100je.zip> <http://www.simtel.net/pub/pd/42697.html> * The Alternate Multiplex Interrupt Specification (AMIS),downloadable as: <http://www.simtel.net/pub/pd/46788.html> <ftp://garbo.uwasa.fi/pc/programming/altmpx35.zip> * Ralf Brown's assembly-language implementation of the AMIS spec, with utilities in C, is downloadable as: <ftp://garbo.uwasa.fi/pc/c-lang/amisl092.zip> * Douglas Boling's MASM template for a TSR is downloadable as: <http://www.simtel.net/pub/pd/40314.html> * 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: <http://www.simtel.net/pub/pd/41812.html> Finally, there are (were?) commercial products, of which TesSeRact (for C-language TSRs) is one of the best known.
Subject: <Q:08.07> - Why does my interrupt function behave strangely? Date: 8 Feb 2002 20:32:03 -0400 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 <Q:08.06> [How can I write a TSR (terminate-stay-resident utility)?] for general techniques that may prevent a problem.) First, did you fail to set up the registers at the start of your routine? When your routine begins executing, you can count on having CS point to your code segment and SS:SP point to some valid stack (of unknown length), and that's it. In particular, an interrupt service routine must set DS to DGROUP before accessing any data in its data segments. (If you're writing in a high-level language, the compiler may generate this code for you automatically; check your compiler manual. For instance, in Borland and Microsoft C, give your function the "interrupt" attribute.) Did you remember to turn off stack checking when compiling your interrupt server and any functions it calls? The stack during the interrupt is not where the stack-checking code expects it to be. (Caution: Some third-party libraries have stack checking compiled in, so you can't call them from your interrupt service routine.) Next, are you calling any DOS functions (INT 21, 25, or 26) in your routine? DOS is not re-entrant. This means that if your interrupt happens to be triggered while the CPU is executing a DOS function, calling another DOS function will wreak havoc. (Some DOS functions are fully re-entrant, as noted in Ralf Brown's interrupt list (<Q:02.03> [What and where is 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, INT 21 AH=34, and INT 21 AX=5D06 or 5D0B; and consult {Undocumented DOS} by Andrew Schulman. Your program must read both the "InDOS flag" and the "critical error flag".) Is a function in your language library causing trouble? Does it depend on some initializations done at program startup that is no longer available when the interrupt executes? Does it call DOS (see preceding paragraph)? For example, in both Borland and Microsoft C the memory- allocation functions (malloc(), etc..) call DOS functions and also depend on setups that they can't get at from inside an interrupt; so do the standard I/O functions like scanf() and printf(). Many other library functions have the same problem, so you can't use them inside an interrupt function without special precautions. Is your routine simply taking too long? This can be a problem if you're hooking on to the timer interrupt, INT 1C or INT 8. That interrupt expects to be called about every 55 ms, which is 18.2 times a second. Therefore your routine, plus any others hooked to the same interrupts, must execute in less than 55 ms. If they use even a substantial fraction of that time, you'll see significant slowdowns of your foreground program. A good discussion is downloadable as: <http://www.simtel.net/pub/pd/46893.html> Did you forget to restore all registers at the end of your routine? Reader, Morten Welinder, notes that programmers of interrupt procedures in Borland/Turbo Pascal 7.0 should be aware that the high words or the 32-bit registers are not saved automatically and that the run-time library may trash them if, e.g., you use longint operations. The easy way around this is to do "Test8086 := 0;" before installing the interrupt handler. 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.)
Subject: <Q:08.08> - How can I write a device driver? Date: 5 Feb 2002 22:03:03 -0400 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.
Subject: <Q:08.09> - What can I use to manage versions of software? Date: 8 Feb 2002 20:34:22 -0400 A port of the Unix RCS utility is downloadable as: <http://www.simtel.net/gnudlpage.php?product=/gnu/gnuish/rcs57pc2.zip&name=rcs57pc2.zip> (executable) <http://www.simtel.net/gnudlpage.php?product=/gnu/gnuish/rcs57pc1.zip&name=rcs57pc1.zip> (source) This version of RCS is no longer limited to one-character extensions on filenames (for example, .CPP and .BAS are now OK). As for commercial software, I posted a question asking for readers' experiences in July 1993 and seven readers responded. PVCS from Intersolv (formerly Polymake) got five positive reviews, though several readers commented that it's expensive; RCS from MKS got one positive and one negative review; Burton TLIB got one negative review; DRTS from ILSI got one positive review.
Subject: <Q:08.10> - What's this 'null pointer assignment' after my C program executes? Date: 8 Feb 2002 20:35:32 -0400 Somewhere in your program, you assigned a value through a pointer without first assigning a value to the pointer. (This might have been something like a strcpy() or memcpy() with a pointer as its first argument, not necessarily an actual assignment statement.) Your program may look like it ran correctly, but if you get this message you can be certain that there's a bug somewhere. Microsoft and Borland C, as part of their exit code (after a call to exit() or a return from your main function), check whether the location 0000 in your data segment contains a different value from what you started with. If so, they infer that you must have used an uninitialized pointer. This implies that the message will appear at the end of execution of your program regardless of where the error actually occurred. To track down the problem, you can put exit() calls at various spots in the program and narrow down where the uninitialized pointer is being used by seeing which added exit() makes the null-pointer message disappear. Or, if your program was compiled with small or medium models, which use 16-bit data pointers, tell the debugger to set a watch at location 0000 in your data segment. (If data pointers are 32 bits, as in the compact and large models, a null pointer will overwrite the interrupt vectors at 0000:0000 and probably lock up your machine.) Under MSC/C++ 7.0, you can declare the undocumented library function: extern _cdecl _nullcheck(void); 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: <http://www.simtel.net/pub/pd/50843.html> <ftp://garbo.uwasa.fi/pc/turbopas/>
Subject: <Q:08.11> - How can a batch file tell whether it's being run in a DOS box under Windows? Date: 5 Feb 2002 22:03:03 -0400 When Windows 3.0 or 3.1 is running, the DOS environment will contain a definition of the string windir, in lower case. That's not really useful, however, because the batch statement if "%windir%" == "" ... will test for an environment variable WINDIR in upper case. Your only real option is to write a program as suggested by the following question, and have it return a value which your batch file can test via "if errorlevel".
Subject: <Q:08.12> - How can my program tell if it's running under Windows? Date: 8 Feb 2002 20:37:06 -0400 Execute INT 2F AX=4680. If AX returns 0, you're in Windows real mode or standard mode (or under the DOS shell). Otherwise, call INT 2F AX=1600. If AL returns something other than 0 or 80, you're in Windows 386 enhanced mode. See PC Magazine 24 Nov 1992 (xi:20) pages 492-493. For more information, see PC Magazine 26 May 1992 (xi:10) pages 345-346. A program, WINMODE, is available as part of: <http://www.simtel.net/pub/pd/48500.html> <ftp://garbo.uwasa.fi/pc/pcmagvol/> PC Magazine 29 March 1994 (xiii: 6) pages 312 and 320 published a new program, WINVER. This would be in: <http://www.simtel.net/pub/pd/48540.html> <ftp://garbo.uwasa.fi/pc/pcmagvol/>
Subject: <Q:08.13> - How can a program tell whether ANSI.SYS is installed? Date: 5 Feb 2002 22:03:03 -0400 In DOS 4.0 and above, call INT 2F AX=1A00. If the value FF is returned in AL, ANSI.SYS is installed. For more information, see Ralf Brown's interrupt list (<Q:02.03> [What and where is Ralf Brown's interrupt list?]).
Subject: <Q:08.14> - How do I copyright software that I write? Date: 5 Feb 2002 22:03:03 -0400 You can download a very comprehensive answer from the Internet. Terry Carroll posts a six-part Copyright FAQ to misc.legal, news.answers and other groups. A short answer follows, not based on that article. Disclaimer: I am not a lawyer, and this is not legal advice. Also, there are very likely to be differences in copyright law among nations. No matter where you live, if significant money may be involved, get legal advice. The following is adapted (and greatly condensed) from chapter 4 of the Chicago Manual of Style (13th edition, ISBN 0-226- 10390-0). In the U.S. (at least), when you write something, you own the copyright. (The exception that matters most to programmers is "works made for hire", i.e., code you write because your employer or client pays you to. A contract, agreed in advance, can vest the copyright in the programmer even if an employee; otherwise the employer owns the copyright.) You don't have to register the work with the Copyright Office unless (until) the copyright is infringed and you intend to bring suit; however, it is easier to recover damages in court if you did register the work within three months of publication. From paragraph 4.16 of the Chicago Manual: "... the [copyright] notice consists of three parts: (1) the symbol [C-in-a-circle] (preferred because it also suits the requirements of the Universal Copyright Convention), the word 'Copyright', or the abbreviation 'Copr.', (2) the year of first publication, and (3) the name of the copyright owner. Most publishers also add the phrase 'All rights reserved' because it affords some protection in Central and South American countries...." Surprise: "(C)" is legally not the same as the C-in-a- circle, so those of us who are ASCII-bound must use the word or the abbreviation.
Subject: <Q:08.15> - How can I place date and time information into environment variables? Date: 6 Feb 2002 10:16:12 -0400 You can use a program known as NOWMINUS. More information about NOWMINUS can be found at <http://www.merlyn.demon.co.uk/programs/nowminus.txt>. Pascal source code is included.
Subject: Conclusion This is the end of part 4 of 5 parts. This text is copyright 2003 by Jeffrey Carlyle. All rights reserved. Please see the top of this article for additional copyright information.

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