假设输入信号 a=” 0110 ” , b=” 1110 ” ,则以下程序执行后, c 的值为( )。 entity logic is port( a , b : in bit _vector(3 downto 0); c : out bit _vector(7 downto 0)); end logic; architecture a of logic is begin c(0) <= not a(0); c(2 downto 1) <= a(2 downto 1) and b(2 downto 1); c(3) <= '1' xor b(3) ; c(7 downto 4) <= "1111" when (a (2)= b(2)) else "0000"; end a;