SPECIAL TOPIC

        Built-in Variables

        SCL supports a set of built-in variables.  These variables always
        have a value.  Some you can assign a new value to and some you
        cannot.  Here's the list:

                STEP - The built-in variable STEP replaces the old STEP
                        command from ValidSIM.  The unit of STEP is ticks.
                        Typical usage of this variable is as part (or all)
                        of an expression that forms an argument to the
                        SIMULATE command.       

                C - This variable contains the clock period.  Like STEP, C
                        is expressed in ticks and typically is used as a
                        parameter for the SIMULATE command.  You can assign
                        a new value to C.  The PERIOD command also alters
                        the value of C.  Note that the argument to PERIOD
                        has the units of nanoseconds.  The argument is
                        converted to ticks before being assigned to C.

                CURR_OPEN_SIG - This is a string variable that contains the
                        name of the most recently opened signal.  If no
                        signal has been opened then CURR_OPEN_SIG will
                        contain an empty string.  You cannot directly
                        assign to this variable.

                TIME - This is an integer value that contains the current
                        simulator time in ticks.  You cannot assign a new
                        value to this variable directly.

                TIME_SEC - This is a floating point variable that contains
                        the current simulation time in seconds.  You cannot
                        assign a new value directly to this variable.
