The debugger displays the source code of your program in the source
  view of the main window.

  Whenever execution is suspended (for example, at a breakpoint), the
  debugger updates the source view, which displays the code surrounding
  the point at which execution is paused.  The current-location pointer,
  to the left of the source code, marks the line whose code will execute
  next.  (A source line corresponds to one or more programming language
  statements, depending on the language and coding style.)

  By default, the debugger displays compiler-generated line numbers to
  the left of the source code.  These numbers help you identify
  breakpoints that are listed in the breakpoint view.  You can choose not
  to display line numbers so that more of the source code can show in the
  window.  To hide or display line numbers, choose Show Line Numbers from
  the File menu on the main window.

  The Call Stack menu, at the bottom right of the source window, shows
  the name of the routine whose source code is displayed.

  The current-location pointer is normally filled in.  It is cleared if
  the displayed code is not that of the routine in which execution is
  paused (see the Making Source Code Available for Display and the
  Setting the Current Scope Relative to the Call Stack topics).

  You can use the scroll bars to show more of the source code.  However,
  you can scroll vertically through only one module of your program at a
  time.  (A module corresponds generally to a compilation unit.  With
  many programming languages, a module corresponds to the contents of a
  source file.  With some languages, such as Ada, a source file might
  contain one or more modules.)

  The Displaying Source Code in Another Module topic explains how to
  display source code for other parts of your program so that you can set
  breakpoints in various modules, and so on.

  The Making Source Code Available for Display topic explains what to do
  if the debugger cannot find source code for display.

  The Setting the Current Scope Relative to the Call Stack topic explains
  how to display source code for routines that are currently active on
  the call stack.

  After navigating through the source view, you can redisplay the
  location at which execution is paused by clicking on the Call Stack
  menu.

  If your program was optimized during compilation, the source code
  displayed might not reflect the actual contents of some program
  locations (see the Compiling and Linking Your Program for Debugging
  topic).