initial语句中的并行执行和串行执行

reg[7:0] databus;
 reg[7:0] db;


    initial fork
    databus = 8'b00;
    #10 databus = 8'h45;
    #20 repeat(10) #10 databus = databus +1;
    #25repeat(5) #20 databus = databus << 1;
    #140 databus =8'h0f;
   join
   
   initial 
   begin
    db = 8'b00;
      #10 db = 8'h45;
      #20 repeat(10) #10 db = db +1;
      #25repeat(5) #20 db = db << 1;
      #140 db =8'h0f;
   end

一样的代码，只不过一个是并行一个是串行执行，在vivado中仿真的波形以下：