A null transaction is used to deactivate a signal driver. This is
analogous to putting a tri-state driver in a high-impedance state. In
such a case, the value of the signal is determined by any active
drivers. Of course, if there is more than one active driver at any
one time, a Bus Resolution Function would be needed.
There are two actions that can take place if all drivers of a signal
are disconnected:
- Use the last known value
- Require that a bus resolution function specify a value
The keyword register is used if the last known value action is
desired, and the keyword bus if a bus resolution function must
specify a value. These special keywords are used only in the
declaration of a signal.
