The event data type is an enhancement over Verilog named events. SystemVerilog events provide a handle to a synchronization object. Like Verilog, event variables can be explicitly triggered and waited for. Furthermore, SystemVerilog events have a persistent triggered state that lasts for the duration of the entire time step. In addition, an event variable can be assigned another event variable or the special value null. When assigned another event variable, both event variables refer to the same synchronization object. When assigned null, the association between the synchronization object and the event variable is broken. Events can be passed as arguments to tasks.
The syntax to declare an event is:
event variable_name [= initial_value];
Where variable_name is a valid identifier and the optional initial_value can be another event variable or the special value null.
If an initial value is not specified then the variable is initialized to a new synchronization object.
If the event is assigned null, the event becomes nonblocking, as if it were permanently triggered.
Examples:
event E1; // declare a new event called E1
event E2 = E1; // declare E2 as alias to E1
event empty = null; // event variable with no synchronization object