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Archive-name: msdos-programmer-faq/part2
Comp-os-msdos-programmer-archive-name: dos-faq-pt2.txt
Posting-frequency: 28 days
Last-modified: 14 Aug 2003
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 2 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: (this part)
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:
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 3. Compile and link
Date: 5 Feb 2002 22:03:03 -0400
The Compile and Link section contains information issues involving
compiling and linking of MS-DOS code.
Subject: <Q:03.01> - What the heck is DGROUP > 64K?
Date: 8 Feb 2002 19:38:12 -0400
This question explains the problem; the next question gives some
remedies.
DGROUP is a link-time group of data segments, and the compiler typically
generates code that expects DS to be pointing to DGROUP. (Exception:
Borland's huge model has no DGROUP.)
Here's what goes into DGROUP:
* Tiny models (all pointers near): DGROUP holds the entire program.
* Small and medium models (data pointers near): DGROUP holds all globals
and static variables including string literal data, plus the stack and
the heap.
* Large, compact, and huge models in Microsoft (data pointers far):
DGROUP holds only initialized globals and static variables including
string literal data, plus the stack and the near heap.
* Large and compact models in Borland (data pointers far): DGROUP holds
initialized and uninitialized globals and static variables including
string literal data, but not the stack or heap.
* Huge model in Borland (data pointers far): there is no DGROUP, so the
64K limit doesn't apply.
In all of the above, which is to say all six models in Microsoft C and
all but huge in Borland C, DGROUP is limited to 64K including string
literal data (which are treated as static data). This limitation is due
to the Intel CPU's segmented architecture.
For more information, see topics like "memory models" and "memory
management" in the index of your compiler manual. Also for an extended
general discussion of memory usage in Borland C programs, of which much
applies to any C compiler in DOS see TI738.asc, downloadable as part of:
<ftp://garbo.uwasa.fi/pc/c-lang/bchelp10.zip>
Subject: <Q:03.02> - How do I fix 'automatic data segment exceeds 64K' or
'stack plus data exceed 64K'?
Date: 5 Feb 2002 22:03:03 -0400
These messages are a variation of "DGROUP > 64K". For causes, please see
the preceding question.
If you get this error in tiny model, your program is simply too big and
you must use a different memory model. If you get this link error in
models small, compact, medium, large, or Microsoft's huge, there are
some things you can do. (This error can't occur in Borland's huge
model.)
If you have one or two big global arrays, simply declare them far. The
compiler takes this to mean that any references to them will use 32-bit
pointers, so they'll be in separate segments and no longer part of
DGROUP.
Or you can use the /Gt[number] option with Microsoft or - Ff[=size] with
Borland C++ 2.0 and up. This will automatically put variables above a
certain size into their own segments outside of DGROUP.
Yet another option is to change global arrays to far pointers. Then at
the beginning of your program, allocate them from the far heap
(_fmalloc() in Microsoft, farmalloc() in Borland).
Finally, you can change to huge model (with Borland compilers, not
Microsoft). Borland's H model still uses far pointers by default, but
"sets aside the [64K] limit" and has no DGROUP group, according to the
BC++ 2.0 Programmer's Guide. Microsoft's H model does use huge data
pointers by default but retains DGROUP and its 64K limit, so switching
to the huge model doesn't buy you anything if you have DGROUP problems.
Subject: <Q:03.03> - Will Borland C code and Microsoft C code link
together?
Date: 5 Feb 2002 22:03:03 -0400
Typically someone who owns compiler A and is trying to write code to
link with a third-party library that was compiled under compiler B asks
this question.
The answer to the question is, Not in general. Here are some of the
reasons:
* "Helper" functions (undocumented functions for stack checking,
floating-point arithmetic, and operations on longs) differ between the
two compilers.
* Extended dictionaries are not compatible between the 2 formats.
However, the basic structure of both MS and Borland OBJ formats is
based on the OMF format so specifying that the linker ignore the
extended dictionary records (/NOE for LINK, -e for TLINK) will disable
this little hassle.
* The compilers may embed instructions in the object code that tell the
linker to look for their own run-time libraries. You can use the
linker option that says to ignore such instructions: /n in TLINK, /NOD
in the Microsoft linker (the one that comes with the C compiler, not
the one that used to come with DOS). But getting around this problem
will very likely just reveal other problems, like different helper
functions, that have no easy solution.
Those problems will generate link-time errors. Others may not show up
until run time:
* Borland's compact, large, and huge models don't assume DS=SS, but
Microsoft's do. The -Fs option on the Borland compiler, or one of the
/A options on Microsoft, should take care of this problem-once you
know that's what's going on.
* Check conventions for ordering and packing structure members, and for
alignment of various types on byte, word, paragraph, or other
boundaries. Again, you can generally adjust your code to match if you
know what conventions were used in compiling the "foreign" libraries.
* Check the obvious and make sure that your code was compiled under the
same memory model as the code you're trying to link with. (That's
necessary, but no guarantee. Microsoft and Borland don't use exactly
the same conventions for segments and groups, particularly in the
larger memory models.)
That said, there are some circumstances where you can link hybrids. Your
best chance of success comes if you compile in large model with the
compiler switch that says to reload DS on entry to each function, avoid
longs and floating point, use only 16-bit pointers, suppress stack
checking, and specify all libraries used in the link.
Subject: <Q:03.04> - Why did my program bomb at run time with 'floating
point formats not linked' or 'floating point not loaded'?
Date: 5 Feb 2002 22:03:03 -0400
These messages look similar but have very different causes.
"Floating point not loaded" is Microsoft C's run-time message when the
code requires a numeric coprocessor but your computer doesn't have one
installed. If the program is yours, relink it using the xLIBCE or xLIBCA
library (where x is the memory model).
"Floating point formats not linked" is a Borland run-time error (Borland
C or C++, Turbo C or C++). Borland's compilers try to be smart and not
link in the floating- point (f-p) library unless you need it. Alas, they
all get the decision wrong. One common case is where you don't call any
f-p functions, but you have %f or other f-p formats in scanf() or
printf() calls. The cure is to call an f-p function, or at least force
one to be present in the link.
To do that, define this function somewhere in a source file but don't
call it:
static void forcefloat(float *p)
{
float f = *p;
forcefloat(&f);
}
It doesn't have to be in the module with the main program, as long as
it's in a module that will be included in the link.
If you have Borland C++ 3.0, the README file documents a slightly less
ugly work-around. Insert these statements in your program:
extern unsigned _floatconvert;
#pragma extref _floatconvert
Subject: <Q:03.05> - How can I change the stack size in Borland's C
compilers?
Date: 5 Feb 2002 22:03:03 -0400
In Turbo C, Turbo C++, and Borland C++, you may not find "stack size" in
the index but the global variable _stklen should be there. The manual
will instruct you to put a statement like
extern unsigned _stklen = 54321U;
in your code, outside of any function. You must assign the value right
in the extern statement; it won't work to assign a value at run time.
The linker may give you a duplicate symbol warning, which you can
ignore.
If you are using the Borland PowerPack for DOS _stklen does not change
the stack size. To change the stack size you must use STACKSIZE in your
.DEF file. HEAPSIZE can be used to change the size of your program's
heap.
Subject: <Q:03.06> - What's the format of an .OBJ file?
Date: 27 Jun 2003 07:17:15 -0400
Reader Bob Smith, reports that there is a free tool at
<ftp://ftp.sudleyplace.com/sudleyplace/dispobj.zip> which displays the
contents of an .OBJ file, as well as two more free tools at
<ftp://ftp.sudleyplace.com/sudleyplace/displib.zip> which display and
search the contents of .LIB files.
Information about the base .OBJ format can be found in Intel's document
number #121748-001, {8086 Relocatable Object Module Formats} (not
verified).
Both Microsoft and Borland have extended the .OBJ format, as has IBM for
OS/2; and according to the MS-DOS encyclopedia, Microsoft doesn't
actually use all the listed formats.
Microsoft-specific .OBJ formats:
* A 45-page article can be found in the {MS-DOS Encyclopedia}, ISBN
1-55615-049-0, now out of print.
* "Microsoft Object Module Format (OMF)" Specification, 22 Nov 1991, was
published by the Microsoft Languages Group (not verified).
Borland-specific .OBJ formats:
* Open Architecture Handbook. The Borland Developer's Technical Guide,
1991, no ISBN. Chapter 2, "Object file contents", (pages 27-50) covers
the comment records sent to the object file by Borland C++ version 3.0
and other Borland compilers. The comment records mostly contain
information for the Borland debugger (not verified).
A "tutorial on the .OBJ format" comes with the VAL experimental linker,
downloadable as <ftp://garbo.uwasa.fi/pc/assembler/>.
Subject: <Q:03.07> - What's the format of an .EXE header?
Date: 5 Feb 2002 22:03:03 -0400
See PC Magazine 30 June 1992 (XI: 12) pages 349-350 for the old and new
formats. For a more detailed layout, look under INT 21 AH=4B in Ralf
Brown's interrupt list <Q:02.03> [What and where is Ralf Brown's
interrupt list?] That list includes extensions for Borland's TLINK and
Borland debugger info.
Among the books that detail formats of executable files are {DOS
Programmer's Reference: 2d Edition} by Terry Dettman and Jim Kyle, ISBN
0-88022-458-4; and {Microsoft MS-DOS Programmer's Reference}, ISBN
1-55615-329-5.
Subject: <Q:03.08> - What's the difference between .COM and .EXE formats?
Date: 7 Feb 2002 14:47:51 -0400
To oversimplify: a .COM file is a direct image of how the program will
look in main memory, and a .EXE file will undergo some further
relocation when it is run (and so it begins with a relocation header). A
.COM file is limited to 64K for all segments combined, but a .EXE file
can have as many segments as your linker will handle and be as large as
RAM can take.
The actual file extension doesn't matter. DOS knows that a file being
loaded is in .EXE format if its first two bytes are MZ or ZM; otherwise
it is assumed to be in .COM format. For instance, DR-DOS 6.0's
COMMAND.COM is in .EXE format as is COMMAND.COM in recent versions of
MS-DOS.
Reader Paul Schylter posted this description of .COM files vs. .EXE
files to the newsgroup in message <a3rpp8$a9h$1@merope.saaf.se>:
"Actually they must be less than 0xFF00 bytes long, since the PSP, which
isn't included in the COM file but is within those 64K, is 256 bytes
long.
"Then CAN use many segments, but they don't have to. In particular, any
.COM file can be converted to an .EXE file by adding an appropriate
header to it.
"There are some other differences between a .COM file and a single
segment .EXE file (both of which must be smaller than 64K):
"The entry point of the .COM file is _always_ 0x100, while the entry
point of the .EXE file can be at any address.
"The stack size of the .COM file is the remainder of those 64K which
isn't used by the code image, while the stack size if the single segment
.EXE file can be set at any size as long as it fits within those 64K.
Thus the stack can be smallere in the .EXE file.
"When a COM file is loaded, the entire TPA (= "free memory") of MS-DOS
is allocated for that COM file -- including those parts of the TPA which
are outside the 64k of the COM file. If you don't want this (e.g.
because your COM file is a TSR and you want to load other programs
later), you must explicitly free those parts of the TPA you want freed.
In the header of an .EXE file you can specify how large part of the TPA
that .EXE file should receive."
Subject: <Q:03.09> - How do I create a .COM file?
Date: 5 Feb 2002 22:03:03 -0400
There are two steps to creating a .COM file. First, your program must
not have a stack. In C, you must compile your program with the TINY
memory model. Second, use EXE2BIN or a similar program to convert an EXE
file to a COM file. To find EXE2BIN see subject: <Q:03.10> [Where is
EXE2BIN located?]
Subject: <Q:03.10> - Where is EXE2BIN located?
Date: 5 Feb 2002 22:03:03 -0400
EXE2BIN was formerly shipped with MS-DOS. If you are still using DOS 5.0
or earlier you can find EXE2BIN in your DOS directory. Users of DOS 6.x
need to get the MS-DOS Supplemental Disks. These disks are available via
FTP at ftp.microsoft.com.
<ftp://ftp.microsoft.com/peropsys/msdos/public/>
Subject: <Q:03.11> - What does this message mean: 'A20 already enabled so
test is meaning less?'
Date: 5 Feb 2002 22:03:03 -0400
The DPMIINST program included with older versions of Borland C++ and
Turbo C++ compilers generates this message. Before running DPMIINST you
must clean boot your computer.
Subject: Section 4. Keyboard
Date: 5 Feb 2002 22:03:03 -0400
The keyboards sections coontains information about how to access the
keyboard.
Subject: <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?
Date: 5 Feb 2002 22:03:03 -0400
The C compilers from Microsoft and Borland offer getch() (or getche() to
echo the character); Turbo Pascal has ReadKey.
In other programming languages, execute INT 21 AH=8; AL is returned with
the character from standard input (possibly redirected). If you don't
want to allow redirection, or you want to capture Ctrl-C and other
special keys, use INT 16 AH=10; this will return the scan code in AH and
ASCII code (if possible) in AL, but AL=E0 with AH nonzero indicates that
one of the gray "extended" keys was pressed. (If your BIOS doesn't
support the extended keyboard, use INT 16 AH=0 not 10.)
Subject: <Q:04.02> - How can I find out whether a character has been
typed, without waiting for one?
Date: 5 Feb 2002 22:03:03 -0400
In Turbo Pascal, use KeyPressed. Both Microsoft C and Turbo C offer the
kbhit() function. All of these tell you whether a key has been pressed.
If no key has been pressed, they return that information to your
program. If a keystroke is waiting, they tell your program that but
leave the key in the input buffer.
You can use the BIOS call, INT 16 AH=01 or 11, to check whether an
actual keystroke is waiting; or the DOS call, INT 21 AH=0B, to check for
a keystroke from stdin (subject to redirection). See Ralf Brown's
interrupt list <Q:02.03> [What and where is Ralf Brown's interrupt
list?].
Subject: <Q:04.03> - How can I disable Ctrl-C/Ctrl-Break and/or
Ctrl-Alt-Del?
Date: 5 Feb 2002 22:03:03 -0400
Several utilities are downloadable from:
<ftp://ftp.simtel.net/pub/simtelnet/msdos/keyboard/>
In that directory, cadel.zip contains a TSR (with source code) to
disable those keys. Also, keykill.arc contains two utilities:
keykill.com lets you disable up to three keys of your choice, and
deboot.com changes the boot key to leftShift-Alt-Del. C programmers who
simply want to make sure that the user can't Ctrl-Break out of their
program can use the ANSI-standard signal() function; the Borland
compilers also offer ctrlbrk() for handling Ctrl-Break. However, if your
program uses normal DOS input such as getch(), ^C will appear on the
screen when the user presses Ctrl-C or Ctrl-Break. You can avoid the ^C
echo for Ctrl-C by using _bios_keybrd() in MSC or bioskey() in BC++;
however, Ctrl-Break will still terminate the program.
An alternative approach involves programming input at a lower level. You
can use INT 21 AH=7, which allows redirection but doesn't echo the ^C
(or any other character, for that matter); or use INT 16 AH=0 or 10; or
hook INT 9 to discard Ctrl-C and Ctrl-Break before the regular BIOS
keyboard handler sees them; etc., etc.
You should be aware that Ctrl-C and Ctrl-Break are processed quite
differently internally. Ctrl-Break, like all keystrokes, is processed by
the BIOS code at INT 9 as soon as the user presses the keys, even if
earlier keys are still in the keyboard buffer: by default the handler at
INT 1B is called. Ctrl-C is not special to the BIOS, nor is it special
to DOS functions 6 and 7; it is special to DOS functions 1 and 8 when at
the head of the keyboard buffer. You will need to make sure BREAK is OFF
to prevent DOS polling the keyboard for Ctrl-C during non-keyboard
operations.
Some good general references are {Advanced MS-DOS} by Ray Duncan, ISBN
1-55615-157-8; {8088 Assembler Language Programming: The IBM PC}, ISBN
0-672-22024-5, by Willen & Krantz; and {COMPUTE!'s Mapping the IBM PC},
ISBN 0-942386- 92-2.
Subject: <Q:04.04> - How can I disable the print screen function?
Date: 5 Feb 2002 22:03:03 -0400
There are really two print screen functions: 1) print current screen
snapshot, triggered by PrintScreen or Shift- PrtSc or Shift-gray*, and
2) turn on continuous screen echo, started and stopped by Ctrl-P or
Ctrl-PrtSc.
1) Screen snapshot to printer:
The BIOS uses INT 5 for this. Fortunately, you don't need to mess with
that interrupt handler. The standard handler, in BIOS versions dated
December 1982 or later, uses a byte at 0040:0100 (= 0000:0500) to
determine whether a print screen is currently in progress. If it is,
pressing PrintScreen again is ignored. So to disable the screen
snapshot, all you have to do is write a 1 to that byte. When the user
presses PrintScreen, the BIOS will think that a print screen is already
in progress and will ignore the user's keypress. You can re-enable
PrintScreen by zeroing the same byte.
Here's some simple code:
void prtsc_allow(int allow) /* 0=disable, nonzero=enable */
{
unsigned char far* flag = (unsigned char far*)0x00400100UL;
*flag = (unsigned char)!allow;
}
2) Continuous echo of screen to printer:
If ANSI.SYS is loaded, you can easily disable the continuous echo of
screen to printer (Ctrl-P or Ctrl- PrtSc). Just redefine the keys by
"printing" strings like these to the screen (BASIC print, C printf(),
Pascal Write statements, or ECHO command in batch files), where <27>
stands for the Escape character, ASCII 27:
<27>[0;114;"Ctrl-PrtSc disabled"p
<27>[16;"^P"p
If you haven't installed ANSI.SYS, I can't offer an easy way to disable
the echo-screen-to-printer function.
Actually, you might not need to disable Ctrl-P and Ctrl- PrtSc. If your
only concern is not locking up your machine, when you see the "Abort,
Retry, Ignore, Fail" prompt just press Ctrl-P again and then press I. As
an alternative, install one of the many print spoolers that intercept
printer-status queries and always return "Printer ready".
Subject: <Q:04.05> - How can my program turn NumLock (CapsLock,
ScrollLock) on or off?
Date: 12 Sep 2002 17:46:15 -0400
First, if you just don't want NumLock turned on when you reboot, check
your system's setups. (Press a special key like Del at boot time, or run
the setup program supplied with your system.) Many systems may have an
option in setup to turn NumLock off at boot time.
You need to twiddle bit 5, 6, or 4 of location 0040:0017. The code
example below demonstrates changing NumLock status: lck() turns on a
lock state, and unlck() turns it off.
/* The status lights on some keyboards may not reflect the
* change. If yours is one, call INT 16 AH=2, "get shift
* status", and that may update them. It will certainly do no
* harm.)
*/
#define NUM_LOCK (1 << 5)
#define CAPS_LOCK (1 << 6)
#define SCRL_LOCK (1 << 4)
void lck(int shiftype)
{
char far* kbdstatus = (char far*)0x00400017UL;
*kbdstatus |= (char)shiftype;
}
void unlck(int shiftype)
{
char far* kbdstatus = (char far*)0x00400017UL;
*kbdstatus &= ~(char)shiftype;
}
Subject: <Q:04.06> - How can I speed up the keyboard's auto-repeat?
Date: 8 Feb 2002 19:42:39 -0400
The keyboard speed has two components: delay (before a key that you hold
down starts repeating) and typematic rate (the speed once the key starts
repeating). Most BIOS versions since 1986 let software change the delay
and typematic rate by calling INT 16 AH=3, "set typematic rate and
delay"; see Ralf Brown's interrupt list <Q:02.03> [What and where is
Ralf Brown's interrupt list?]. If you have DOS 4.0 or later, you can use
the MODE CON command that you'll find in your DOS manual.
On 83-key keyboards (mostly XTs), the delay and typematic rate can't
easily be changed. According to PC Magazine 15 Jan 1991 (x: 1) page 409,
to adjust the typematic rate you need "a memory-resident program which
simply '[watches]' the keyboard to see if you're holding down a key .
and after a certain time [starts] stuffing extra copies of the held-down
key into the buffer." No source code is given in that issue; but the
QUICKEYS utility that PC Magazine published in 1986 does this sort of
watching (not verified); source and object code are downloadable from
<http://www.simtel.net/pub/pd/48667.html>
Subject: <Q:04.07> - What is the SysRq key for?
Date: 5 Feb 2002 22:03:03 -0400
There is no standard use for the key. The BIOS keyboard routines in INT
16 simply ignore it; therefore so do the DOS input routines in INT 21 as
well as the keyboard routines in libraries supplied with high-level
languages.
When you press or release a key, the keyboard triggers hardware line
IRQ1, and the CPU calls INT 9. INT 9 reads the scan code from the
keyboard and the shift states from the BIOS data area.
What happens next depends on whether your PC's BIOS supports an enhanced
keyboard (101 or 102 keys). If so, INT 9 calls INT 15 AH=4F to translate
the scan code. If the translated scan code is 54 hex (for the SysRq key)
then INT 9 calls INT 15 AH=85 and doesn't put the keystroke into the
keyboard buffer. The default handler of that function does nothing and
simply returns. (If your PC has an older BIOS that doesn't support the
extended keyboards, INT 15 AH=4F is not called. Early ATs have 84-key
keyboards, so their BIOS calls INT 15 AH=85 but not 4F.)
Thus your program is free to use SysRq for its own purposes, but at the
cost of some programming. You could hook INT 9, but it's probably easier
to hook INT 15 AH=85, which is called when SysRq is pressed or released.
Subject: <Q:04.08> - How can my program tell what kind of keyboard is on
the system?
Date: 5 Feb 2002 22:03:03 -0400
Ralf Brown's Interrupt List <Q:02.03> [What and where is Ralf Brown's
interrupt list?] includes MEMORY.LST, a detailed breakdown by Robin
Walker of the contents of the BIOS system block that starts at
0040:0000. Bit 4 of byte 0040:0096 is "1=enhanced keyboard installed".
Here is a C code example to test the keyboard type:
char far *kbd_stat_byte3 = (char far *)0x00400096UL;
if (0x10 & *kbd_stat_byte3)
{
/* 101- or 102- keyboard is installed */
}
else
{
/* Not installed */
}
PC Magazine 15 Jan 1991 (x: 1) suggests on page 412 that "for some
clones [the above test] is not foolproof". If you use this method in
your program you should provide the user some way to override this test,
or at least some way to tell your program to assume a non-enhanced
keyboard. The article suggests a different approach to determining the
type of keyboard.
Subject: <Q:04.09> - How can I tell if input, output, or stderr has been
redirected?
Date: 8 Feb 2002 19:49:17 -0400
Normally, input and output are associated with the console (i.e., with
the keyboard and the screen, respectively). If either is not, you know
that it has been redirected. Some source code to check this is available
at the usual archive sites.
Timo Salmi has created a collection of Turbo Pascal units, one of which
can be used to detect such redirection. These can be downloaded from
<http://garbo.uwasa.fi/pc/ts.html>. The files you are looking for have
names of the format tspaVV??.zip where the VV is the current version and
?? is 70, 60, 55, 50, or 40 for Turbo Pascal 7.0, 6.0, 5.5, 5.0, or 4.0
respectively.) Source code is not included. Also see the downloadable
Frequently Asked Questions files by Timo Salmi:
<ftp://garbo.uwasa.fi/pc/link/>
If you program in C, use isatty() if your implementation has it.
Good references for the principles are PC Magazine 16 Apr 1991 (x: 7)
page 374; Ray Duncan's {Advanced MS-DOS}, ISBN 1-55615-157-8, or Ralf
Brown's interrupt list (<Q:02.03> [What and where is Ralf Brown's
interrupt list?]) for INT 21 AX=4400; and Terry Dettman and Jim Kyle's
{DOS Programmer's Reference: 2d edition}, ISBN 0-88022-458-4, pages
602-603.
Subject: <Q:04.10> - How can I increase the size of the keyboard buffer?
Date: 8 Feb 2002 19:50:41 -0400
Microsoft has its own keyboard extender available on the MS-DOS
supplemental disks for MS-DOS 6.22 which can be found at:
<ftp://ftp.microsoft.com/peropsys/msdos/public/supplmnt/>
Stan Brown, the former list maintainer, tested only one of the many
available device drivers that do this, namely BUF160, which extends the
keyboard buffer to 160 characters. It performed flawlessly for two years
with MS-DOS 5 and Windows 3.1. It's downloadable as:
<http://www.simtel.net/pub/pd/47186.html>
<ftp://garbo.uwasa.fi/pc/keyboard/>
Subject: <Q:04.11> - How can I stuff characters into the keyboard buffer?
Date: 5 Feb 2002 22:03:03 -0400
If your computer has an enhanced keyboard (see <Q:04.08> [How can my
program tell what kind of keyboard is on the system?]), put the scan
code in CH and the ASCII character in CL, then execute INT 16 AH=5. The
return in AL is 0 for success or 1 for buffer full.
Subject: Conclusion
This is the end of part 2 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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