Q57220: Inline Assembly May Cause Debugger to Skip Lines

Article: Q57220
Product(s): See article
Version(s): 2.00 2.01
Operating System(s): MS-DOS
Keyword(s): ENDUSER | buglist2.00 buglist2.01 | mspl13_c
Last Modified: 17-JAN-1990

The following code causes the debugger to skip (or jump) over a line
of source code. The statement that causes this action is the "_asm in
ax,21h;" inline assembly statement. This problem happens only when you
are debugging with the integrated debugger and loading the 21h port
using the "in" instruction. If you run the program from the DOS
prompt, the program will execute properly.

Code Example
------------

 1: void main (void)
 2:    {
 3:    _asm in ax, 21h;
 4:    printf ("Test1\n");
 5:    printf ("Test2\n");
 6:    }

Compile the above program and trace through it using F8. Line #4 will
be skipped. The "_asm in ax,21h;" inline assembly statement is what
causes the line to be skipped; however, this program will run as
expected when executed outside the integrated environment. It will
also run as expected from within the environment as long as the trace
(F8) command is not used.

Microsoft has confirmed this to be a problem in Versions 2.00 and
2.01. We are researching this problem and will post new information
here as it becomes available.

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