[UVM foundation] UVM_ Is in agent_ Active variable definition

The sample code ：

function void my_agent::build_phase(uvm_phase phase);
   super.build_phase(phase);
   if (is_active == UVM_ACTIVE) begin
       drv = my_driver::type_id::create("drv", this);
   end
   mon = my_monitor::type_id::create("mon", this);
endfunction 

Perhaps the most puzzling thing here is build_phase in Why according to is_active The value of this variable determines whether to create driver Example . is_active yes uvm_agent A member variable of , from UVM Its prototype can be found in the source code of ：

 source ： UVM  Source code 
uvm_active_passive_enum is_active = UVM_ACTIVE;

and uvm_active_passive_enum Is an enum type variable , The definition for ：

 source ： UVM  Source code 
typedef enum bit {
     UVM_PASSIVE=0, UVM_ACTIVE=1 } uvm_active_passive_enum;

This enumeration variable has only two values ： UVM_PASSIVE and UVM_ACTIVE. stay uvm_agent in , is_active The default value of is UVM_ACTIVE, In this mode , You need to instantiate driver Of . So what is UVM_PASSIVE How about the model ？ In this chapter DUT For example , There is no need to drive any signal on the output port , Just monitor the signal . under these circumstances , On the port, you only need monitor Of , therefore driver You don't have to instantiate .