Dec 092017
Break out of a locked system. Only works on DOS 2.1 systems.
File ALTBRK.ZIP from The Programmer’s Corner in
Category Utilities for DOS and Windows Machines
Break out of a locked system. Only works on DOS 2.1 systems.
File Name File Size Zip Size Zip Type
ALTBRK.COM 2274 1915 deflated
ALTBRK.DOC 3740 1741 deflated
CHKROM.COM 233 196 deflated

Download File ALTBRK.ZIP Here

Contents of the ALTBRK.DOC file

ALTBRK - Keyboard buffer, strong break key, etc.


Under normal DOS operation, a Control-Break keystroke will not take effect
until a program either reads a character from the console or sends a character
to the screen. Even after setting BREAK ON, it will not take effect until a
program calls DOS. Thus if a program is in a tight infinite loop, Ctrl-Break
will never take effect and the only recourse is Ctrl-Alt-Del, which has drastic
effects if you have a RAMDISK.

The main feature of ALTBRK is that it allows you to break a program
wherever it is, by using the Alt-Break key combination. The only exception to
this is if the program is in DOS, in which case it beeps. But if it is in DOS,
then either Ctrl-Break should work or you can try Alt-Break again until you
catch the program outside of DOS. Unlike BREAK ON, use of ALTBRK does not slow
down the program in any appreciable way.

ALTBRK also includes a 75-keystroke keyboard buffer, support for the
Scroll-lock key, and support for the following keystrokes:

Shift-ScrollLockAllow the screen to scroll just one line.
Alt-EscClear the keyboard buffer
Alt-BackspaceErase the last keystroke from the buffer
Alt-Ctrl-LClear the screen (best used in combination with


ALTBRK is installed by including a line,


in the CONFIG.SYS file. (If you do not already have a CONFIG.SYS file, create
a file by this name, containing the above line, in the directory in which you


1. ALTBRK will not coexist with the ANSI.SYS device driver.

2. It only works with DOS 2.1.

3. One should not use the Alt-Break keys in BASIC or BASICA, since doing
so would pull you out of BASIC entirely and probably leave a few loose
ends hanging. This is not a limitation, since Ctrl-Break works
perfectly fine there. Also, the keyboard buffer works perfectly fine

4. Since ALTBRK traps the keyboard interrupt and performs its own
processing, it is unlikely to work with any other program that does
the same thing; for example programs like, alas, SideKick,
ProKey, etc. You'll just have to try out these combinations and
see if they work. It will work with PRINT, however, and using
Alt-Break will not disrupt any printing that is going on.

5. Alt-Ctrl-L will sometimes clear the screen and leave the cursor in its
original position. This happens if you clear the screen while the
computer is in the middle of its routines which print to the screen.
Not much can be done about this, but it is merely a cosmetic bug.


This software will work nicely with DEBUG, but it works better if you fix
the following bug. The problem is that DEBUG always starts programs with all
flags cleared, which unfortunately means that they run with interrupts DISABLED.
That is why, even without ALTBRK, if a program gets into an infinite loop not

involving calls to DOS, you have to turn the machine off and on again to
recover. To fix this, you must DEBUG DEBUG as follows:

-e 2afd 2

This alters the initial setting of the flag bits so that interrupts are
initially enabled. This is a good change to make regardless of whether you
adopt ALTBRK.


This package also includes CHKROM, a command that checks most of the
internal pointers of ALTBRK to make sure that things are running smoothly. If
things are acting funny, and you want to check to see if some program has
tripped up ALTBRK, run this program and it will tell you.

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