1. The UVM automatically creates a single instance of uvm_root that users can access via the global (uvm_pkg-scope) variable, uvm_top.
2. uvm_top实例的关键作用：
1. Implicit top-level（隐式的顶层） The uvm_top serves as an implicit top-level component. Any component whose parent is specified as NULL becomes a child of uvm_top. Thus, all UVM components in simulation are descendants of uvm_top.所有组建的parent被指定为NULL的，就会成为uvm_top的child，uvm_top是uvm组建树的顶层。
2. Phase control uvm_top manages the phasing for all components.
3. Search Use uvm_top to search for components based on their hierarchical name. See find and find_all.（使用uvm_top基于层次名搜索组件）
4. Report configuration Use uvm_top to globally configure report verbosity, log files, and actions. For example, uvm_top.set_report_verbosity_level_hier(UVM_FULL) would set full verbosity for all components in simulation.
5. Global reporter Because uvm_top is globally accessible (in uvm_pkg scope), UVM’s reporting mechanism is accessible from anywhere outside uvm_component, such as in modules and sequences. See uvm_report_error, uvm_report_warning, and other global methods.
1.属性
uvm_component top_levels[$];//各个top注册在这 i.e test，It includes the uvm_test_top component that is created by run_test as well as any other top level components that have been instantiated anywhere in the hierarchy.
bit enable_print_topology = 0;//如果被设置，在end_of_elaboration phase结束的时候打印拓扑结构
bit finish_on_completion = 1;//If set, then run_test will call $finish after all phases are executed.
time phase_timeout = `UVM_DEFAULT_TIMEOUT;
static local uvm_root m_inst;
bit m_phase_all_done;//所有阶段执行完毕
2. 主要的API方法：
1. virtual task run_test ( string test_name = "" )：//解析命令行的"+UVM_TESTNAME="，从工厂中取出uvm_test_top并构造一个实例，调用相应的实例。
1. Phases all components through all registered phases
2. If the optional test_name argument is provided, or if a command-line plusarg, +UVM_TESTNAME=TEST_NAME, is found, then the specified component is created just prior to phasing.
2. find/find_all:
1. function void uvm_root::find_all(string comp_match, ref uvm_component comps[$],
input uvm_component comp=null); //查找整个uvm_top，调用m_find_all_recurse实现
2.function uvm_component uvm_root::find (string comp_match);//调用uvm_root::find_all实现
3. uvm_root私有函数：
1. function uvm_root::new();//设置reporter, command_processor,调用report_header
1. 调用super.new("__top__",null)将uvm_root设置成树的顶层
2. 设置reporter，拿到command_processor，调用report_header
3. 调用m_check_verbosity通过命令行获取并设置verbosity
2. function uvm_root uvm_root::get();
1. 构成uvm_root的单态类
2. 调用了uvm_domain::get_common_domain()；m_inst.m_domain = uvm_domain::get_uvm_domain()即生成了两个domain 一个common 一个uvm_domain
3. task uvm_root::run_phase (uvm_phase phase);//该函数主要检查$time是否为0，如果不为0就报告错误，因为run必须从0时刻开始
4. function void uvm_root::m_do_dump_args();//如果命令行有+UVM_DUMP_CMDLINE_ARGS，就把命令行的所有参数输出
5. function void uvm_root::build_phase(uvm_phase phase);//调用一些内建函数完成对uvm仿真环境的配置：
super.build_phase(phase);
m_set_cl_msg_args();
m_do_verbosity_settings();//解析命令行输入+uvm_set_verbosity=，主要进行一些语法检查，如果发现问题就报错，并不对设置做处理
m_do_timeout_settings();//解析命令行的"+UVM_TIMEOUT="，并把该设置应用于uvm_top
m_do_factory_settings();//解析命令行的+(UVM_SET_INST_OVERRIDE|uvm_set_inst_override)=，调用m_process_inst_override或者调用m_process_type_override
m_do_config_settings();从命令行中获取+(UVM_SET_CONFIG_INT|uvm_set_config_int)=设置，并解析该设置，最后调用m_process_config记录这些设置到resource数据库里边
m_do_max_quit_settings();+UVM_MAX_QUIT_COUNT=解析命令行的这个选项，并把该选项的设置设置入全局的report_server
m_do_dump_args(); //如果命令行有+UVM_DUMP_CMDLINE_ARGS，就把命令行的所有参数输出