4.4.4 Tracing Program Execution
  The SET TRACE command lets you select tracepoints ,
  which are locations for tracing the execution of your program
  without stopping its execution. After setting a tracepoint, you
  can start execution with the GO command and then monitor
  the PC's path, checking for unexpected behavior. By setting a
  tracepoint on a routine, you can also monitor the number of
  times the routine is called.

  As with breakpoints, every time a tracepoint is reached, the
  debugger issues a message and displays the source line. It
  can also display other information that you have specified (as
  shown in the last example in this section, in which the value
  of a specified variable is displayed). However, at tracepoints,
  unlike breakpoints, the program continues executing, and the
  debugger prompt is not displayed. For example:
  DBG> SET TRACE COUNTER
  DBG> GO
        .
        .
        .
  trace at TEST\COUNTER
          34: SUB COUNTER(LONG X,Y)
        .
        .
        .

  When using the SET TRACE command, you specify address
  expressions, qualifiers, and optional clauses exactly as with
  the SET BREAK command.

  The /LINE qualifier instructs the SET TRACE command to
  trace every line and is a convenient means of checking the
  execution path. By default, lines are traced within all called
  routines as well as the currently executing routine. If you
  do not want to trace system routines or routines in shareable
  images, use the /NOSYSTEM or /NOSHARE qualifiers. For
  example:
  DBG> SET TRACE/LINE/NOSYSTEM/NOSHARE

  The /SILENT qualifier suppresses the trace message and
  source code display. This is useful when you want to use the
  SET TRACE command to execute a debugger command at
  the tracepoint. For example:
  DBG> SET TRACE\SILENT %LINE 83 DO (EXAMINE STATUS)
  DBG> GO
        .
        .
        .
  SCREEN_IO\CLEAR\STATUS: 'OFF'
        .
        .
        .