In certain cases, the debugger cannot display source code.  Possible
  causes are:

   -  Execution might be paused within a module of your program that was
      compiled without the debug option.

   -  Execution might be paused within a system or library routine for
      which no symbolic information is intended to be available.  In such
      cases you can quickly return execution to the calling routine by
      clicking one or more times on the Step-Return push button on the
      push-button view of the main window.

   -  The source file might have been moved to a different directory
      after it was compiled.  The Specifying the Location of Source Files
      topic explains how to tell the debugger where to look for source
      files.

  If the debugger cannot find source code for display, it tries to
  display the source code for the next routine down on the call stack for
  which source code is available.  If the debugger can display source
  code for such a routine, the current-location pointer is cleared and
  marks the source line to which execution returns in the calling
  routine.