A process with a sensitivity list, as shown in the process on the
left, is implemented as a wait on "sensitivity list" at the
bottom of the process (as shown in the process on the right). This
allows every process with a sensitivity list to execute once at the
beginning of a simulation and suspend at the bottom waiting for a a
relevant signal event to occur. Note that the VHDL standard prohibits
the use of both process sensitivity lists and wait statements within
the same process.
This should help clarify how a process works. When the simulation
begins at time=0, all processes execute until a wait statement is
reached. If the wait statement is at the end of the process, the
behavior is exactly as if a sensitivity list were used. The wait
statement prevents the process from immediately executing from the
beginning again. Rather, it waits until one or more of the signals in
the wait statement change value.
