Q58672: Using INT with _asm Does Not Output Text in Debugger

Article: Q58672
Product(s): See article
Version(s): 2.00 2.01
Operating System(s): MS-DOS
Keyword(s): ENDUSER | s_quickasm buglist2.00 | mspl13_c
Last Modified: 26-FEB-1990

Question:

If I trace (using F8) through the DOS or BIOS interrupt functions
designed to output information to the screen, I do not receive any
output from the program. I can run the program (using F5) and my
output is displayed properly. C run-time screen output functions also
properly display my output. How can I correct this problem?

Response:

To work around this problem, turn on Screen Swap. This option is found
under Options.Run/Debug. When you press F5 or use C run-time
function, the screen is swapped to the output window before executing
the statement. In the case of an interrupt call through _asm, QuickC
does not switch to the output window when Screen Swap is set to Auto
or Off.

Sample Code:

/*
DEBUGGER.C:
To see the problem, use .Option.Run/Debug.Screen Swap set to either
Auto or Off. Setting this to On will cause output to appear properly.
*/
#include <string.h>

char *str1 = "Print using INT 21H, Function 09H$" ;
char *str2 = "Print using INT 21H, Function 40H" ;

void main ( void )

{
  int len ;

  len = strlen(str2) ;

/*  Display String - outputs a string to standard out (must be */
/*                   terminated with '$').                     */
  _asm {
         push dx
         mov  dx, word ptr str1
         mov  ah, 09h
         int  21h
         pop  dx
  }

/*  Write File or Device - outputs a string to a file, use bx = 1 */
/*                         to write to standard out.              */
  _asm {
         push dx
         push cx
         push bx
         mov  ah, 40h
         mov  bx, 01h
         mov  cx, len
         mov  dx, word ptr str2
         int  21h
         pop  bx
         pop  cx
         pop  dx
  }

/*  Write Character and Attribute at Cursor - Displays 40h X's */
/*                                            at the cursor.   */
  _asm {
         push cx
         push bx
         mov  bh, 0
         mov  bl, 0f0h
         mov  cx, 40h
         mov  ah, 0ah
         mov  al, 'X'
         int  10h
         pop  bx
         pop  cx
  }

The code is generated properly, but the debugger does not recognize
the need to swap to the output screen on these interrupts.

Microsoft has confirmed this to be a problem with QuickC Version 2.00.
We are researching this problem and will post new information here as
it becomes available.

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