用VHDL语言设计两个加法器，1）4位串行进位加法器；2）4位超前进位加法器。

好的，我可以给您提供一些 VHDL 代码示例来实现这两种加法器。

1）4位串行进位加法器

entity serial_adder is
    port (
        a, b: in std_logic_vector(3 downto 0);
        cin: in std_logic;
        sum: out std_logic_vector(3 downto 0);
        cout: out std_logic
    );
end entity;

architecture behavioral of serial_adder is
    signal c: std_logic_vector(3 downto 0);
begin
    sum(0) <= a(0) xor b(0) xor cin;
    c(0) <= (a(0) and b(0)) or (a(0) and cin) or (b(0) and cin);
    sum(1) <= a(1) xor b(1) xor c(0);
    c(1) <= (a(1) and b(1)) or (a(1) and c(0)) or (b(1) and c(0));
    sum(2) <= a(2) xor b(2) xor c(1);
    c(2) <= (a(2) and b(2)) or (a(2) and c(1)) or (b(2) and c(1));
    sum(3) <= a(3) xor b(3) xor c(2);
    cout <= c(3);
end architecture;

2）4位超前进位加法器

entity lookahead_adder is
    port (
        a, b: in std_logic_vector(3 downto 0);
        sum: out std_logic_vector(3 downto 0);
        cout: out std_logic
    );
end entity;

architecture behavioral of lookahead_adder is
    signal g: std_logic_vector(2 downto 0);
    signal p: std_logic_vector(2 downto 0);
begin
    g(0) <= a(0) and b(0);
    p(0) <= a(0) or b(0);
    g(1) <= a(1) and b(1);
    p(1) <= a(1) or b(1);
    g(2) <= a(2) and b(2);
    p(2) <= a(2) or b(2);
    
    sum(0) <= a(0) xor b(0);
    sum(1) <= a(1) xor b(1) xor g(0);
    sum(2) <= a(2) xor b(2) xor (g(0) and g(1)) xor (g(0) and p(1) and p(2));
    sum(3) <= a(3) xor b(3) xor (g(0) and g(1) and g(2)) xor (g(0) and g(1) and p(2)) xor (g(0) and p(1) and p(2)) xor (p(0) and g(1) and p(2)) xor (p(0) and p(1) and g(2));
    cout <= g(2) or (g(1) and (g(0) or p(0))) or (p(1) and p(0));
end architecture;

注意，这只是示例代码，并不保证可以直接使用。此外，为了方便起见，这些代码并没有进行模拟或验证。如果您要在实际项目中使用这些代码，请先进行必要的测试和验证。