When a tracepoint is triggered, the debugger takes the following
    actions:

    1. Suspends program execution at the tracepoint location.

    2. If you specified /AFTER when you set the tracepoint, checks
       the AFTER count. If the specified number of counts has not
       been reached, execution is resumed and the debugger does not
       perform the remaining steps.

    3. Evaluates the expression in a WHEN clause, if you specified
       one when you set the tracepoint. If the value of the
       expression is false, execution is resumed and the debugger
       does not perform the remaining steps.

    4. Reports that execution has reached the tracepoint location by
       issuing a "trace . . . " message, unless you specified /SILENT.

    5. Displays the line of source code corresponding to the
       tracepoint, unless you specified /NOSOURCE or /SILENT when
       you set the tracepoint or entered a previous SET STEP NOSOURCE
       command.

    6. Executes the commands in a DO clause, if you specified one
       when you set the tracepoint.

    7. Resumes execution.

    You set a tracepoint at a particular location in your program
    by specifying an address expression with the SET TRACE command.
    You set a tracepoint on consecutive source lines, classes of
    instructions, or events by specifying a qualifier with the SET
    TRACE command. Generally, you must specify either an address
    expression or a qualifier, but not both. Exceptions are /EVENT
    and /RETURN.

    The /LINE qualifier sets a tracepoint on each line of source
    code.

    The following qualifiers set tracepoints on classes of
    instructions. Using these qualifiers and /LINE causes the
    debugger to trace every instruction of your program as it
    executes and thus significantly slows down execution.

       /BRANCH
       /CALL
       /INSTRUCTION
       /RETURN
       /SYSEMULATE (Alpha only)

    The following qualifiers set tracepoints on classes of events:

       /ACTIVATING
       /EVENT=event-name
       /EXCEPTION
       /TERMINATING