将以下语句改成用generate语句实现。 library ieee; use ieee.std_logic_1164.all; entity example210 is port(a,clk:in std_logic; b:out std_logic); end example210; architecture a of example210 is component dff port(d,clk:in std_logic; q:out std_logic); end component; signal z:std_logic_vector(0 to 4); begin z(0)<=a;b<=z(4); dff1:dff port map(z(0),clk,z(1)); dff2:dff port map(z(1),clk,z(2)); dff3:dff port map(z(2),clk,z(3)); dff4:dff port map(z(3),clk,z(4)); end a;