[UVM]driver与sequencer的交互

上次文章写道uvm_do macro,主要涉及sequence与sequencer的交互，这次讲driver与sequencer的交互。

在driver中会调用sequencer的task get_next_item,其主要包括task m_select_sequence

task uvm_sequencer::get_next_item(output REQ t);
  REQ req_item;

  // If a sequence_item has already been requested, then get_next_item()
  // should not be called again until item_done() has been called.

  if (get_next_item_called == 1)
    uvm_report_error(get_full_name(),
      "Get_next_item called twice without item_done or get in between", UVM_NONE);
  
  if (!sequence_item_requested)
    m_select_sequence();

  // Set flag indicating that the item has been requested to ensure that item_done or get
  // is called between requests
  sequence_item_requested = 1;
  get_next_item_called = 1;
  m_req_fifo.peek(t);//在finish_item中send_request把request item放入m_req_fifo中，在item_done中pop出
endtask

task uvm_sequencer_base::m_select_sequence();
   int selected_sequence;

    // Select a sequence
    do begin
      wait_for_sequences();
      selected_sequence = m_choose_next_request();
      if (selected_sequence == -1) begin
        m_wait_for_available_sequence();
      end
    end while (selected_sequence == -1);
    // issue grant
    if (selected_sequ