4.4.5 Monitoring Changes in Variables
  The SET WATCH command lets you set watchpoints that
  will be monitored continuously as your program executes.

  If the program modifies the value of a watched variable, the
  debugger suspends execution and displays the old and new
  values.
  DBG> SET WATCH TOTAL

  Subsequently, every time the program modifies the value of
  TOTAL, the watchpoint is triggered. The debugger monitors
  watchpoints continuously during program execution.

  The next example shows what happens when your program
  modifies the contents of a watched variable:
  DBG> SET WATCH TOTAL
  DBG> GO
        .
        .
        .
  watch of SCREEN_IO\TOTAL\%LINE 13
          13: TOTAL = TOTAL + 1
        old value: 16
        new value: 17
  break at SCREEN_IO.%LINE 14
          14: CALL Pop_rtn(TOTAL)
  DBG>

  In this example, a watchpoint is set on the variable TOTAL
  and the GO command starts execution. When the value of
  TOTAL changes, execution is suspended. The debugger an-
  nounces the event (watch of . . . ), identifying where TOTAL
  changed (line 13) and the associated source line. The de-
  bugger then displays the old and new values and announces
  that execution has been suspended at the start of the next line
  (14). (The debugger reports break at . . . , but this is not a
  breakpoint; it is still the effect of the watchpoint.) Finally, the
  debugger prompts for another command.

  When a change in a variable occurs at a point other than the
  start of a source line, the debugger gives the line number plus
  the byte offset from the start of the line.