In reply to nitin273:
To understand this, I suggest you to read UVM user guide chapter 17 and Chapter 9 regarding to predefined component classes and UVM common phases. Some high level information listed here. uvm_test will be a good starting point to understand how the uvm test is specified and executed at first.
17. PREDEFINED COMpONENT CLASSES
17.2 uvm_test
This class is the virtual base class for the user-defined tests.
The uvm_test virtual class should be used as the base class for user-defined tests. Doing so provides the ability to select which test to execute using the UVM_TESTNAME command line or argument to the uvm_root::run_test task.
For example
prompt> SIM_COMMAND +UVM_TESTNAME=test_bus_retry
The global
run_test() task should be specified
inside an initial block such as
Multiple tests, identified by their type name, are compiled in and then selected for execution from the command line without need for recompilation. Random seed selection is also available on the command line.
If +UVM_TESTNAME=test_name is specified, then an object of type ‘test_name’ is created by factory and phasing begins. Here, it is presumed that the test will instantiate the test environment, or the test environment will have already been instantiated before the call to run_test().
If the specified test_name cannot be created by the uvm_factory, then a fatal error occurs. If run_test() is called without UVM_TESTNAME being specified, then all components constructed before the call to run_test will be cycled through their simulation phases.
9. Phasing Overview
UVM implements an automated mechanism for phasing the execution of the various components in a testbench
9.6 UVM Common Phases
The common phases are the set of function and task phases that all uvm_components execute together. All uvm_components are always synchronized with respect to the common phases.