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Q50625: OS/2 Program Selector Fails to Show DOS Box If Using SCREEN 3

Article: Q50625
Product(s): See article
Version(s): 6.00 6.00b | 6.00 6.00b
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | SR# S891010-116 B_QuickBas | mspl13_basic
Last Modified: 15-DEC-1989

The Program Selector of OS/2 Version 1.00 or 1.10 can't correctly
switch to the screen of a running BASIC application that is currently
using SCREEN 3 (Hercules graphics) in the DOS 3.x box (real mode). To
work around this OS/2 limitation, you must stop the BASIC program with
CTRL+BREAK or switch back to SCREEN 0 (or text mode) within the
program.

This behavior occurs in Microsoft QuickBASIC Versions 4.00, 4.00b, and
4.50 for MS-DOS, in Microsoft BASIC Compiler Versions 6.00 and 6.00b
for MS-DOS and MS OS/2, and in Microsoft BASIC PDS 7.00 for MS-DOS
and MS OS/2.

The Program Selector of OS/2 Version 1.00 or 1.10 is a simple
interface that runs in character mode. When you invoke the OS/2 DOS
box when it is running a program in Hercules graphics mode, OS/2
maintains character mode and can't refresh the Hercules screen.
QuickBASIC thinks it is still running under SCREEN 3 and the output is
incorrect. Text mode must be reinvoked to restore visibility in the
DOS box.

Note that this problem does not occur for non-Hercules graphics screen
modes (CGA, EGA, VGA) or for the Task Manager of OS/2 Presentation
Manager (which can't run on Hercules graphics cards anyway).

When you choose MS-DOS Command Prompt from the Switch To A Running
Program menu in the OS/2 1.10 Program Selector screen and press ENTER,
the highlighting on the menu selection disappears, the mouse pointer
(if any) disappears, and the Program Selector appears to be hung
(frozen) on the screen. If the BASIC program in the DOS box scrolls
the screen during this time, the Program Selector will scroll upward
showing some BASIC output, but the display will be incorrect
(garbage).

To stop the apparent hang or garbage on the screen, press CTRL+BREAK
to abort the BASIC program and switch the screen over to text mode,
thus making the DOS box visible. You can also press CTRL+ESC or
ALT+ESC to go back to OS/2 at any time.

Example
-------

To duplicate the problem, invoke the OS/2 Program Selector (CTRL+ESC),
and then select MS-DOS Command Prompt from the Switch To A Running
Program menu. Run the following code in the QuickBASIC QB.EXE editor
or in a compiled .EXE program:

   SCREEN 3
   PRINT "This is a test"
   idle: GOTO idle

[You must compile the .EXE program with /D (debug) so that CTRL+BREAK
can stop the continuous loop in the program.] Invoke the OS/2 Program
Selector (CTRL+ESC), and then select MS-DOS Command Prompt again. The
menu highlight will disappear and the Program Selector screen will
remain intact. The system will appear to be hung, but you are actually
in the BASIC program. Pressing CTRL+BREAK will stop execution of the
BASIC program and switch the DOS box back to text mode. When this is
done, the Program Selector screen will be replaced with the DOS box
screen.

Note that if the BASIC application is not provided with a means to
terminate or switch to the text mode screen, then the DOS box (for all
practical purposes) will be hung. Technically, it can still be
accessed by the keyboard, but the screen is never displayed correctly
unless the program itself reinvokes SCREEN 0. The only way out of this
condition is to reboot the computer.

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