4.4.3 Suspending Program Execution
  The SET BREAK command lets you select breakpoints ,
  which are locations at which the program will stop running.
  When you reach a breakpoint, you can enter commands to
  check the call stack, examine the current values of variables,
  and so on.

  A typical use of the SET BREAK command is shown in the
  following example:
  DBG> SET BREAK COUNTER
  DBG> GO
        .
        .
        .
  break at TEST\COUNTER
          34: SUB COUNTER(LONG X,Y)
  DBG>

  In this example, the SET BREAK command sets a breakpoint
  on the subprogram COUNTER; the GO command starts ex-
  ecution. When the subprogram COUNTER is encountered,
  execution is suspended, the debugger announces that the
  breakpoint at COUNTER has been reached (break at . . . ),
  displays the source line (34) where execution is suspended,
  and prompts you for another command. At this breakpoint,
  you can step through the subprogram COUNTER, using
  the STEP command, and use the EXAMINE command (see
  Section 4.5.1) to check on the current values of X and Y.

  When using the SET BREAK command, you can specify pro-
  gram locations using various kinds of address expressions (for
  example, line numbers, routine names, instructions, virtual
  memory addresses). With high-level languages, you typi-
  cally use routine names, labels, or line numbers, possibly with
  directory specifications to ensure uniqueness.

  Routine names and labels should be specified as they appear
  in the source code. Line numbers may be derived from ei-
  ther a source code display or a listing file. When specifying
  a line number, use the prefix %LINE. (Otherwise, the de-
  bugger will interpret the line number as a memory location.)
  For example, the next command sets a breakpoint at line 41
  of the currently executing module; the debugger will suspend
  execution when the PC is at the start of line 41:
  DBG> SET BREAK %LINE 41

  Note that you can set breakpoints only on lines that resulted
  in machine code instructions. The debugger warns you if you
  try to do otherwise (for example, on a comment line). If you
  want to pick a line number in a module other than the one
  currently executing, you need to specify the module's name
  in a directory specification. For example:
  DBG> SET BREAK SCREEN_IO\%LINE 58

  You do not always have to specify a particular program lo-
  cation, such as line 58 or COUNTER, to set a breakpoint.
  You can set breakpoints on events, such as exceptions. You
  can use the SET BREAK command with a qualifier, but
  no parameter, to break on every line, or on every CALL
  instruction, and so on. For example:
  DBG> SET BREAK/LINE
  DBG> SET BREAK/CALL

  You can conditionalize a breakpoint (with a WHEN clause)
  or specify that a list of commands be executed at the break-
  point (with a DO clause on the debugger command). For
  example, the next command sets a breakpoint on the la-
  bel LOOP3. The DO (EXAMINE TEMP) clause causes the
  value of the variable TEMP to be displayed whenever the
  breakpoint is triggered.
  DBG> SET BREAK LOOP3 DO (EXAMINE TEMP)
  DBG> GO
        .
        .
        .
  break at COUNTER\LOOP3
          37: LOOP3: FOR I = 1 TO 10
  COUNTER\TEMP: 284.19
  DBG>

  To display the currently active breakpoints, enter the SHOW
  BREAK command:
  DBG> SHOW BREAK
  breakpoint at SCREEN_IO\%LINE 58
  breakpoint at COUNTER\LOOP3
      do (EXAMINE TEMP)
        .
        .
        .
  DBG>

  To cancel a breakpoint, enter the CANCEL BREAK com-
  mand, specifying the program location exactly as you did
  when setting the breakpoint. The CANCEL BREAK/ALL
  command cancels all breakpoints.