求解一下 sdram 控制器代码段中的 command 中一段 关于仲裁 的 代码，怎么实现的？代码段如下： // set command_delay shift register and command_done flag
// The command delay shift register is a timer that is used to ensure that
// the SDRAM devices have had sufficient time to finish the last command.                 if ((do_refresh == 1) | (do_reada == 1) | (do_writea == 1) | (do_precharge == 1)
                     | (do_load_mode == 1))
                begin
                        command_delay <= 8'b11111111;
                        command_done  <= 1;
                        rw_flag <= do_reada;                                                 
                end
               
                else
                begin
                        command_done        <= command_delay[0];                // the command_delay shift operation
                        command_delay  <= (command_delay>>1);
                end
               
 
 // start additional timer that is used for the refresh, writea, reada commands              
                if (command_delay[0] == 0 & command_done == 1)
                begin
                 rp_shift <= 4'b1111;
                 rp_done <= 1;
                end
                else
                begin      
     if(SC_PM == 0)
     begin
      rp_shift <= (rp_shift>>1);
                     rp_done  <= rp_shift[0];
     end
     else
     begin
      if( (ex_read == 0) && (ex_write == 0) )
      begin
       rp_shift <= (rp_shift>>1);
                      rp_done  <= rp_shift[0];
      end
      else
      begin
       if( PM_STOP==1 )
       begin
        rp_shift <= (rp_shift>>1);
                        rp_done     <= rp_shift[0];
        ex_read  <= 1'b0;
        ex_write <= 1'b0;
       end     
      end
     end
                end
   end
        end
end
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