The OpenVMS Debugger helps you locate run-time programming or logic
  errors, also known as bugs.  You use the debugger with a program that
  has been compiled and linked successfully but does not run correctly.
  For example, the program might give incorrect output, go into an
  infinite loop, or terminate prematurely.  You locate errors with the
  debugger by observing and manipulating your program interactively as it
  executes.  The debugger lets you:

   -  Display and edit the source code for your program, and browse
      through other source code files.

   -  Monitor, examine, and change program variable and data structure
      values and examine their type.

   -  Examine and manipulate the currently active functions on the call
      stack.

   -  Set breakpoints that suspend program execution or issue debugger
      command sequences.

   -  Step through execution one line of source code or machine
      instruction at a time.

   -  Disassemble and examine machine code; examine and modify
      machine-register values.

   -  Customize the debugging environment.

  These are the basic debugging techniques.  After you are satisfied that
  you have found the error in the program, you can edit the source code
  and compile, link, and execute the corrected version.

  As you use the debugger and its documentation, you will discover
  variations on the basic techniques.  You can also customize the
  debugger to meet your own needs.

  The debugger is a symbolic debugger.  You can specify variable names,
  routine names, and so on, precisely as they appear in your source code.
  You do not need to specify memory addresses or registers when referring
  to program locations, but you can if you want.

  You can also use the debugger with programs written in any of the
  languages identified in the topic Debugger Support for Languages.