uvm_config_db类派生自uvm_resource_db类。它是uvm_resource_db顶部的另一层便利层，简化了用于uvm_component实例的基本接口（资源库的访问方法）。
下面uvm_config_db类的代码段取自uvm源代码。
class uvm_config_db#(type T=int) extends uvm_resource_db#(T);static uvm_pool#(string,uvm_resource#(T)) m_rsc[uvm_component];static local uvm_queue#(m_uvm_waiter) m_waiters[string];static function bit get(uvm_component cntxt,string inst_name,string field_name,inout T value);...endfunctionstatic function void set(uvm_component cntxt,string inst_name,string field_name,T value);...endfunction...
...
endclass1. uvm_config_db methods
下表中除了wait_modified是静态任务外，其余都是的静态函数。
其中，
uvm_component cntxt：该context是可访问数据库条目的起点（hierarchical starting point）。
string inst_name：实例名称是限制数据库条目可访问性的路径。
string field_name：field_name是用于查找数据库条目的标记或标签。
T value: 是从数据库中set或者get的值。default是整数类型。
简单地说，完整path取决于 cntxt 和 inst_name拼接 {cntxt， “.” ，inst_name} .
例子：uvm_test_top.env.agent_o的层次路径可以从测试用例中提到为：
cntxt = null
inst_name = env.agent_oORcntxt = this
inst_name = "env.agent_o"Syntax for set method：
void uvm_config_db#(type T = int)::set(uvm_component cntxt,string inst_name,string field_name,T value);Syntax for get method：
bit uvm_config_db#(type T=int)::get(uvm_component cntxt,string inst_name,string field_name,ref T value);2. uvm_config_db example
在下面的示例中，control位存储在数据库中，作为在component_B中为my_component创建对象的使能条件(enable condition)。
名为“control”、类型为bit的新资源将从测试用例添加到资源池中。
uvm_config_db #(bit)::set(null, "*", "control", 1);//where,
//uvm_component cntxt= null;
//string inst_name = "*"
//String field_name = "control";
//T value = 1;// In component_B,
if(!uvm_config_db #(bit)::get(this, "*", "control", ctrl))`uvm_fatal(get_type_name(), "get failed for resource in this scope");使用get()static函数检索资源，该函数在数据库中以field_name作为“control”进行查找。如果get()在数据库中找不到“control”字符串field_name，将报告致命错误。尽管致命检查不是强制性的，但建议将其用于调试目的。一旦表中的查找成功，存储在资源数据库中的值就会在局部变量ctrl中更新。此控制位用于控制my_component对象的创建。
`include "uvm_macros.svh"
import uvm_pkg::*;class component_A #(parameter ID_WIDTH = 8) extends uvm_component;bit [ID_WIDTH-1:0] id;`uvm_component_param_utils(component_A #(ID_WIDTH))function new(string name = "component_A", uvm_component parent = null);super.new(name, parent);id = 1;endfunctionfunction display();`uvm_info(get_type_name(), $sformatf("inside component_A: id = %0d", id), UVM_LOW);endfunction
endclassclass mycomponent #(parameter ID_WIDTH = 8) extends uvm_component;bit [ID_WIDTH-1:0] id;`uvm_component_param_utils(mycomponent #(ID_WIDTH))function new(string name = "mycomponent", uvm_component parent = null);super.new(name, parent);id = 2;endfunctionfunction display();`uvm_info(get_type_name(), $sformatf("inside mycomponent: id = %0d", id), UVM_LOW);endfunction
endclassclass component_B #(int ID_WIDTH = 8) extends component_A #(ID_WIDTH);bit ctrl;bit [ID_WIDTH-1:0] id;mycomponent #(8) my_comp;`uvm_component_param_utils(component_B #(ID_WIDTH))function new(string name = "component_B", uvm_component parent = null);super.new(name, parent);id = 3;endfunctionfunction void build_phase(uvm_phase phase);super.build_phase(phase);if(!uvm_config_db #(bit)::get(this, "*", "control", ctrl))`uvm_fatal(get_type_name(), "get failed for resource in this scope");if(ctrl) my_comp = mycomponent #(8)::type_id::create("my_comp", this);endfunctionfunction display();`uvm_info(get_type_name(), $sformatf("inside component_B: id = %0d, ctrl = %0d", id, ctrl), UVM_LOW);if(ctrl) void'(my_comp.display());endfunction
endclassclass my_test extends uvm_test;bit control;`uvm_component_utils(my_test)component_A #(32) comp_A;component_B #(16) comp_B;function new(string name = "my_test", uvm_component parent = null);super.new(name, parent);endfunctionfunction void build_phase(uvm_phase phase);super.build_phase(phase);comp_A = component_A #(32)::type_id::create("comp_A", this);comp_B = component_B #(16)::type_id::create("comp_B", this);uvm_config_db #(bit)::set(null, "*", "control", 1);//or//uvm_config_db #(bit)::set(this, "*", "control", 1);endfunctionfunction void end_of_elaboration_phase(uvm_phase phase);super.end_of_elaboration_phase(phase);uvm_top.print_topology();endfunctiontask run_phase(uvm_phase phase);super.run_phase(phase);void'(comp_A.display());void'(comp_B.display());endtask
endclassmodule tb_top;initial beginrun_test("my_test");end
endmoduleOutput:
UVM_INFO /xcelium20.09/tools//methodology/UVM/CDNS-1.2/sv/src/base/uvm_root.svh(605) @ 0: reporter [UVMTOP] UVM testbench topology:
----------------------------------------
Name Type Size Value
----------------------------------------
uvm_test_top my_test - @1812comp_A uvm_component - @1879comp_B uvm_component - @1910my_comp uvm_component - @1958
----------------------------------------UVM_INFO testbench.sv(14) @ 0: uvm_test_top.comp_A [uvm_component] inside component_A: id = 1
UVM_INFO testbench.sv(52) @ 0: uvm_test_top.comp_B [uvm_component] inside component_B: id = 3, ctrl = 1
UVM_INFO testbench.sv(28) @ 0: uvm_test_top.comp_B.my_comp [uvm_component] inside mycomponent: id = 23. precedence rule
有两个优先规则适用于uvm_config_db。在build_phase中，
1.在组件层次结构较高的上下文中的set()调用优先于层次结构路径较低的set()调用。
例子：
`include "uvm_macros.svh"
import uvm_pkg::*;class component_A extends uvm_component;int id;`uvm_component_utils(component_A)function new(string name = "component_A", uvm_component parent = null);super.new(name, parent);id = 1;endfunctionfunction display();`uvm_info(get_type_name(), $sformatf("inside component_A: id = %0d", id), UVM_LOW);endfunction
endclassclass component_B extends component_A;int receive_value;int id;`uvm_component_utils(component_B)function new(string name = "component_B", uvm_component parent = null);super.new(name, parent);id = 2;endfunctionfunction void build_phase(uvm_phase phase);super.build_phase(phase);if(!uvm_config_db #(int)::get(this, "*", "value", receive_value))`uvm_fatal(get_type_name(), "get failed for resource in this scope"); endfunctionfunction display();`uvm_info(get_type_name(), $sformatf("inside component_B: id = %0d, receive_value = %0d", id, receive_value), UVM_LOW);endfunction
endclassclass env extends uvm_env;`uvm_component_utils(env)component_A comp_A;component_B comp_B;function new(string name = "env", uvm_component parent = null);super.new(name, parent);endfunctionfunction void build_phase(uvm_phase phase);super.build_phase(phase);comp_A = component_A ::type_id::create("comp_A", this);comp_B = component_B ::type_id::create("comp_B", this);uvm_config_db #(int)::set(this, "*", "value", 200);endfunctiontask run_phase(uvm_phase phase);super.run_phase(phase);void'(comp_A.display());void'(comp_B.display());endtask
endclassclass my_test extends uvm_test;bit control;`uvm_component_utils(my_test)env env_o;function new(string name = "my_test", uvm_component parent = null);super.new(name, parent);endfunctionfunction void build_phase(uvm_phase phase);super.build_phase(phase);env_o = env::type_id::create("env_o", this);uvm_config_db #(int)::set(null, "*", "value", 100);endfunctionfunction void end_of_elaboration_phase(uvm_phase phase);super.end_of_elaboration_phase(phase);uvm_top.print_topology();endfunction
endclassmodule tb_top;initial beginrun_test("my_test");end
endmoduleOutput:
UVM_INFO /xcelium20.09/tools//methodology/UVM/CDNS-1.2/sv/src/base/uvm_root.svh(605) @ 0: reporter [UVMTOP] UVM testbench topology:
--------------------------------------
Name Type Size Value
--------------------------------------
uvm_test_top my_test - @1810env_o env - @1877comp_A component_A - @1922comp_B component_B - @1953
--------------------------------------UVM_INFO testbench.sv(14) @ 0: uvm_test_top.env_o.comp_A [component_A] inside component_A: id = 1
UVM_INFO testbench.sv(36) @ 0: uvm_test_top.env_o.comp_B [component_B] inside component_B: id = 2, receive_value = 1002.在具有相同的上下文字段时，最后一个set()调用优先于前面的set()。
Example:
`include "uvm_macros.svh"
import uvm_pkg::*;class component_A extends uvm_component;int id;`uvm_component_utils(component_A)function new(string name = "component_A", uvm_component parent = null);super.new(name, parent);id = 1;endfunctionfunction display();`uvm_info(get_type_name(), $sformatf("inside component_A: id = %0d", id), UVM_LOW);endfunction
endclassclass component_B extends component_A;int receive_value;int id;`uvm_component_utils(component_B)function new(string name = "component_B", uvm_component parent = null);super.new(name, parent);id = 2;endfunctionfunction void build_phase(uvm_phase phase);super.build_phase(phase);if(!uvm_config_db #(int)::get(this, "*", "value", receive_value))`uvm_fatal(get_type_name(), "get failed for resource in this scope"); endfunctionfunction display();`uvm_info(get_type_name(), $sformatf("inside component_B: id = %0d, receive_value = %0d", id, receive_value), UVM_LOW);endfunction
endclassclass env extends uvm_env;`uvm_component_utils(env)component_A comp_A;component_B comp_B;function new(string name = "env", uvm_component parent = null);super.new(name, parent);endfunctionfunction void build_phase(uvm_phase phase);super.build_phase(phase);comp_A = component_A ::type_id::create("comp_A", this);comp_B = component_B ::type_id::create("comp_B", this);endfunctiontask run_phase(uvm_phase phase);super.run_phase(phase);void'(comp_A.display());void'(comp_B.display());endtask
endclassclass my_test extends uvm_test;bit control;`uvm_component_utils(my_test)env env_o;function new(string name = "my_test", uvm_component parent = null);super.new(name, parent);endfunctionfunction void build_phase(uvm_phase phase);super.build_phase(phase);env_o = env::type_id::create("env_o", this);uvm_config_db #(int)::set(null, "*", "value", 100);uvm_config_db #(int)::set(null, "*", "value", 200);endfunctionfunction void end_of_elaboration_phase(uvm_phase phase);super.end_of_elaboration_phase(phase);uvm_top.print_topology();endfunction
endclassmodule tb_top;initial beginrun_test("my_test");end
endmoduleOutput:
UVM_INFO /xcelium20.09/tools//methodology/UVM/CDNS-1.2/sv/src/base/uvm_root.svh(605) @ 0: reporter [UVMTOP] UVM testbench topology:
--------------------------------------
Name Type Size Value
--------------------------------------
uvm_test_top my_test - @1810env_o env - @1877comp_A component_A - @1924comp_B component_B - @1955
--------------------------------------UVM_INFO testbench.sv(14) @ 0: uvm_test_top.env_o.comp_A [component_A] inside component_A: id = 1
UVM_INFO testbench.sv(36) @ 0: uvm_test_top.env_o.comp_B [component_B] inside component_B: id = 2, receive_value = 2004. uvm_config_db usage
- 将配置变量/类向下传递到testbench的层次结构中（如上面的示例所示）。
- 将虚拟接口从顶部层次结构传递到测试台层次结构中的组件。
5. Difference between uvm_config_db and uvm_resource_db
- 虽然这两个类都在uvm资源facility上提供了一个方便层(便利)，但uvm_config_db是从uvm_resource_db派生而来的。这将在uvm_resource_db方法之上添加其他方法。
- 关于组件层次结构，与具有两个字符串scope和name的uvm_resource相比，uvm_config_db具有一个参数作为上下文，其类型为uvm_component，实例名称为字符串，提供了更大的灵活性。由于上下文中提到了组件层次结构，因此它提供了对层次路径的更多控制。这个context = this参数提供组件的相对路径。这个context = null提供了一个绝对路径，这意味着上面没有层次路径。
With respect to component hierarchy, the uvm_config_db has an argument as a context that has the type of uvm_component and instance name as a string that provides more flexibility as compared to uvm_resource that has two strings scope and name. Since component hierarchy is mentioned in the context, it provides more control over the hierarchical path.
The context = this argument provides a relative path from the component. The context = null provides an absolute path which means there is no hierarchical path above.
因此，建议总是使用uvm_config_db。
