Default Sequence

There are two usual ways to run the sequence:

• Using default_sequence
• Using start method

How default_sequence works?

Inside uvm_sequencer_base.svh
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function void uvm_sequencer_base::start_phase_sequence(uvm_phase phase);
...
(uvm_config_db #(uvm_object_wrapper)::get(
               this, {phase.get_name(),"_phase"}, "default_sequence", wrapper) && wrapper != null)
...
--------------------------------------------------------------------------------------

This API will do the following:

• get default_sequence 
• cast this uvm_object_wrapper into uvm_sequence_base
• start the sequence

Who calls this above method?

Inside uvm_task_phase.svh:
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  virtual function void execute(uvm_component comp,
                                                 uvm_phase phase);
...
      if ($cast(seqr,comp))
          seqr.start_phase_sequence(phase);
...
--------------------------------------------------------------------------------------
execute method is called for all phases of component.

And if the component is Sequencer it will call start_phase_sequence() method, to check if the default_sequence has been set for this method.

If we run sequence using both the approach:
Registering default sequence in main phase of sequencer and starting another sequence in main phase of testcase, default sequence will be executed first as main phase is called in bottom-up hierarchy.