Q59088: _setactivepage() and _setvisualpage() May Corrupt Display

Article: Q59088
Product(s): See article
Version(s): 2.00
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickasm | mspl13_c
Last Modified: 15-MAR-1990

Interrupting execution of the following code, then attempting to step
through the program with F10, may cause the mouse to be temporarily
frozen and the video display to be corrupted. The problem is caused by
an unresolved swapping between _setactivepage() and _setvisualpage().
The solution is to either press F4 twice or allow the program to
terminate normally, thereby resetting the default video mode.

Sample Code
-----------

#include <stdio.h>
#include <graph.h>

#define DELAY 1

void main(void)
{
   int x=0, y=0, time=DELAY, loop=0, pages;
   struct videoconfig config;

   _setvideomode(_MRES16COLOR);
   _getvideoconfig( &config );
   pages = config.numvideopages;

   while ( !kbhit() )
   {
      _setactivepage( loop % pages );
      _setcolor( loop % 16 );
      _rectangle( _GFILLINTERIOR, ++x, ++y, x+160, y+100 );
      _setvisualpage( loop++ % pages );
      while( time-- );
      time = DELAY;
   }
   _setvideomode (_DEFAULTMODE);
}

The sequence of events causing the situation to occur is listed below:

1. Open the above source code in QuickC 2.00 or 2.01.

2. GO (using F5 or a mouse click).

3. CTRL+BREAK to interrupt execution in the first few seconds of the
   run. The Source window appears.

4. Press F10. The cursor may disappear, and the mouse will be locked
   out.

5. CTRL+BREAK again. The mouse is now active, but the screen display is
   corrupted. Dragging the mouse around the screen leaves a trail of
   blocks.

6. Toggle F4 to restore the source window and refresh the screen.

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