The env

        SPI UVM environment中的下一个层次是spi_env。该类包含许多子组件，即SPI和APB agents, a scoreboard, a functional coverage monitor 和 a virtual sequencer。构建这些子组件中的哪一个由spi_env配置对象中的变量确定。
        在这个case里，spi_env配置对象还包含虚拟接口和用于检测中断的method。这将被在虚拟sequencer上运行的sequence使用。spi_env_config类的内容如下所示：

        //
        // Class Description:
        //
        //
        class spi_env_config extends uvm_object;

        localparam string s_my_config_id = "spi_env_config";
        localparam string s_no_config_id = "no config";
        localparam string s_my_config_type_error_id = "config type error";

        // UVM Factory Registration Macro
        //
        `uvm_object_utils(spi_env_config)

        // Interrupt Virtual Interface - used in the wait for interrupt task
        //
        virtual intr_if INTR;

        //------------------------------------------
        // Data Members
        //------------------------------------------
        // Whether env analysis components are used:
        bit has_functional_coverage = 0;
        bit has_spi_functional_coverage = 1;
        bit has_reg_scoreboard = 0;
        bit has_spi_scoreboard = 1;
        // Whether the various agents are used:
        bit has_apb_agent = 1;
        bit has_spi_agent = 1;
        // Configurations for the sub_components
        apb_agent_config m_apb_agent_cfg;
        spi_agent_config m_spi_agent_cfg;
        // SPI Register model
        uvm_register_map spi_rm;

        //------------------------------------------
        // Methods
        //------------------------------------------
        extern task wait_for_interrupt;
        extern function bit is_interrupt_cleared;
             // Standard UVM Methods:
        extern function new(string name = "spi_env_config");

        endclass: spi_env_config

        function spi_env_config::new(string name = "spi_env_config");
             super.new(name);
        endfunction

        // This task is a convenience method for sequences waiting for the interrupt
        // signal
        task spi_env_config::wait_for_interrupt;
            @(posedge INTR.IRQ);
        endtask: wait_for_interrupt

        // Check that interrupt has cleared:
        function bit spi_env_config::is_interrupt_cleared;
             if(INTR.IRQ == 0)
                 return 1;
             else
                 return 0;
        endfunction: is_interrupt_cleared

       在该示例中，存在用于每个子组件的build配置字段位。这为env提供了最大的重用灵活性。
       在spi_env的build phase期间，使用uvm_config_db get（）从配置空间中检索spi_env_config的句柄。然后，build过程测试配置对象中的各种has_ <sub_component>字段，以确定是否构建子组件。对于APB和SPI agent，还有一个步骤是从envs配置对象解每个代理的配置对象，然后在任何本地修改后在envs配置表中设置agent配置对象。
       在connect phase里，spi_env配置对象再次用于确定要进行与哪个子组件的TLM连接。

       //
       // Class Description:
       //
       //
       class spi_env extends uvm_env;

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

       //------------------------------------------
       // Data Members
       //------------------------------------------
       apb_agent m_apb_agent;
       spi_agent m_spi_agent;
       spi_env_config m_cfg;
       spi_register_coverage m_reg_cov_monitor;
       spi_reg_functional_coverage m_func_cov_monitor;
       spi_scoreboard m_scoreboard;
       //------------------------------------------
       // Constraints
       //------------------------------------------

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

       extern function new(string name = "spi_env", uvm_component parent = null);
       extern function void build_phase( uvm_phase phase );
       extern function void connect_phase( uvm_phase phase );

       endclass:spi_env

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

       function void spi_env::build_phase( uvm_phase phase);
            if(!uvm_config_db #( spi_env_config )::get( this , "", "spi_env_config" , m_cfg ) begin
                `uvm_error("build_phase", "unable to get spi_env_config")
            end
            if(m_cfg.has_apb_agent) begin
               uvm_config_db #( apb_agent_config )::set( this , "m_apb_agent*", "apb_agent_config", m_cfg.m_apb_agent_cfg );
               m_apb_agent = apb_agent::type_id::create("m_apb_agent", this);
            end
            if(m_cfg.has_spi_agent) begin
               uvm_config_db #( spi_agent_config )::set("m_spi_agent*", "spi_agent_config", m_cfg.m_spi_agent_cfg );
               m_spi_agent = spi_agent::type_id::create("m_spi_agent", this);
            end
            if(m_cfg.has_functional_coverage) begin
               m_reg_cov_monitor = spi_register_coverage::type_id::create("m_reg_cov_monitor", this);
            end
            if(m_cfg.has_spi_functional_coverage) begin
               m_func_cov_monitor = spi_reg_functional_coverage::type_id::create("m_func_cov_monitor", this);
            end
            if(m_cfg.has_spi_scoreboard) begin
               m_scoreboard = spi_scoreboard::type_id::create("m_scoreboard", this);
            end

       endfunction:build_phase

       function void spi_env::connect_phase( uvm_phase phase );
            if(m_cfg.has_functional_coverage) begin
                m_apb_agent.ap.connect(m_reg_cov_monitor.analysis_export);
            end
            if(m_cfg.has_spi_functional_coverage) begin
                m_apb_agent.ap.connect(m_func_cov_monitor.analysis_export);
            end
            if(m_cfg.has_spi_scoreboard) begin
               m_apb_agent.ap.connect(m_scoreboard.apb.analysis_export);
               m_spi_agent.ap.connect(m_scoreboard.spi.analysis_export);
               m_scoreboard.spi_rm = m_cfg.spi_rm;
            end

        endfunction: connect_phase

        The Agents

        由于UVM build过程是自顶向下的，因此接下来才构建SPI和APB agent。agent build过程中的文章描述了如何配置和构建APB agent，SPI agent遵循相同的过程。agent中的组件位于测试平台层次结构的底层，因此build过程终止于agent中的组件。
        （在http://verificationacademy.com/uvm-ovm上在线下载源代码示例）。