非阻塞赋值语句中延时在左边和右边的差别

yimiqiba

请教各位非阻塞赋值语句中延时在左边和右边的差别？例如下面两个例子：

例一：
module exchange;
reg[3:0] a, b;
initial begin
a=1; b=4;
#2 a=3; b=2;
#20 $finish;
end
initial
$monitor($time, "\t%h\t%h", a, b);
initial begin
a <= #5 b;
b <= #5 a;
end
endmodule

例二：

module exchange;
reg[3:0] a, b;
initial begin
a=1; b=4;
#2 a=3; b=2;
#20 $finish;
end
initial
$monitor($time, "\t%h\t%h", a, b);
initial begin
#5 a <= b;
#5 b <= a;
end
endmodule