// Note the start of a marking phase. Record the
  // start of the unmarked area of the region here.

  void note_start_of_marking(bool during_initial_mark) {

        init_top_at_conc_mark_count();
        _next_marked_bytes = 0;
        if (during_initial_mark && is_young() && !is_survivor())
          _next_top_at_mark_start = bottom();
        else
          _next_top_at_mark_start = top();
      }