Factory Overrides

      从construction这个观点来说，UVM factory允许一个UVM 类被另一个继承类替换。此工具可用于更改或更新组件行为或扩展配置对象。必须在构造目标对象之前指定factory重载，因此在构建过程开始时这样做很方便。

      Sub-Component Configuration Objects

      每个集合组件（如agent或env）都应具有定义其结构和行为的配置对象。这些配置对象应该在test build method里被创建，并根据测试用例的要求进行配置。如果子组件的配置要么复杂，要么可能会发生变化，那么值得添加一个虚函数调用来处理配置，因为这可能会在从基本测试类扩展来的测试用例中过载。

      //
      // Class Description:
      //
      // class spi_test_base extends uvm_test;

      // UVM Factory Registration Macro
      //
      `uvm_component_utils(spi_test_base)

      //------------------------------------------
      // Data Members
      //------------------------------------------

      //------------------------------------------
      // Component Members
      //------------------------------------------
      // The environment class
      spi_env m_env;
      // Configuration objects
      spi_env_config m_env_cfg;
      apb_agent_config m_apb_cfg;
      spi_agent_config m_spi_cfg;    
     

       //------------------------------------------
       // Methods
       //------------------------------------------

       // Standard UVM Methods:
       extern function new(string name = "spi_test_base", uvm_component parent = null);
       extern function void build_phase( uvm_phase phase );
       extern virtual function void configure_env(spi_env_config cfg);
       extern virtual function void configure_apb_agent(apb_agent_config cfg);

       endclass: spi_test_base

       function spi_test_base::new(string name = "spi_test_base", uvm_component parent = null);
            super.new(name, parent);
       endfunction

       // Build the env, create the env configuration
       // including any sub configurations and assigning virtual interfaces
       function void spi_test_base::build_phase( uvm_phase phase );
        // Create env configuration object
       m_env_cfg = spi_env_config::type_id::create("m_env_cfg");
       // Call function to configure the env
        m_apb_cfg = apb_agent_config::type_id::create("m_apb_cfg");
       // Call function to configure the apb_agent
       configure_apb_agent(m_apb_cfg);
       // More to follow
       endfunction: build_phase

       //
       // Convenience function to configure the env
       //
       // This can be overloaded by extensions to this base class
       function void spi_test_base::configure_env(spi_env_config cfg);
           cfg.has_functional_coverage = 1;
           cfg.has_reg_scoreboard = 0;
           cfg.has_spi_scoreboard = 1;
       endfunction: configure_env

       //
       // Convenience function to configure the apb agent
       //
       // This can be overloaded by extensions to this base class
       function void spi_test_base::configure_apb_agent(apb_agent_config cfg);
           cfg.active = UVM_ACTIVE;
           cfg.has_functional_coverage = 0;
           cfg.has_scoreboard = 0;
       endfunction: configure_apb_agent