--
--------------------------------------------------------------------------------/
-- DESCRIPTION   :  Signed mulitplier:
--                  A (A) input width : 4
--                  B (B) input width : 4
--                  Q (data_out) output width : 7
-- Download from :  http://www.pld.com.cn
--------------------------------------------------------------------------------/

library IEEE;
use IEEE.std_logic_1164.all;

entity one_bit_adder is
 port (
  A: in STD_LOGIC;
  B: in STD_LOGIC;
  C_in: in STD_LOGIC;
  S: out STD_LOGIC;
  C_out: out STD_LOGIC
 );
end one_bit_adder;

architecture one_bit_adder of one_bit_adder is
begin

 S <= A xor B xor C_in;
 C_out <= (A and B) or (C_in and (A xor B));

end one_bit_adder;

library IEEE;
use IEEE.std_logic_1164.all;

entity multi is
 port (
  A: in STD_LOGIC_VECTOR (3 downto 0);
  B: in STD_LOGIC_VECTOR (3 downto 0);
  data_out: out STD_LOGIC_VECTOR (6 downto 0)
 );
end multi;

architecture multi_arch of multi is
signal A_MULT_B0: STD_LOGIC_VECTOR (2 downto 0);
signal A_MULT_B1: STD_LOGIC_VECTOR (2 downto 0);
signal A_MULT_B2: STD_LOGIC_VECTOR (2 downto 0);

signal S_TEMP1: STD_LOGIC_VECTOR (1 downto 0);
signal S_TEMP2: STD_LOGIC_VECTOR (1 downto 0);

signal C_TEMP : STD_LOGIC_VECTOR (6 downto 0);

signal C0_out_B0, C1_out_B0, C2_out_B0 : STD_LOGIC;
signal C0_out_B1, C1_out_B1, C2_out_B1 : STD_LOGIC;

signal ZERO: STD_LOGIC;

component one_bit_adder
 port (
  A: in STD_LOGIC;
  B: in STD_LOGIC;
  C_in: in STD_LOGIC;
  S: out STD_LOGIC;
  C_out: out STD_LOGIC
  );
end component;
begin
 U_0_0 : one_bit_adder port map (A => A_MULT_B0(1), B => A_MULT_B1(0), C_in => ZERO, S => C_TEMP(1), C_out => C0_out_B0);
 U_0_1 : one_bit_adder port map (A => A_MULT_B0(2), B => A_MULT_B1(1), C_in => C0_out_B0, S => S_TEMP1(0), C_out => C1_out_B0);
 U_0_2 : one_bit_adder port map (A => ZERO, B => A_MULT_B1(2), C_in => C1_out_B0, S => S_TEMP1(1), C_out => C2_out_B0);

 U_1_0 : one_bit_adder port map (A => A_MULT_B2(0), B => S_TEMP1(0), C_in => ZERO, S => C_TEMP(2), C_out => C0_out_B1);
 U_1_1 : one_bit_adder port map (A => A_MULT_B2(1), B => S_TEMP1(1), C_in => C0_out_B1, S => S_TEMP2(0), C_out => C1_out_B1);
 U_1_2 : one_bit_adder port map (A => A_MULT_B2(2), B => C2_out_B0, C_in => C1_out_B1, S => S_TEMP2(1), C_out => C2_out_B1);

 A_MULT_B0(0) <= A (0) and B (0);
 A_MULT_B0(1) <= A (1) and B (0);
 A_MULT_B0(2) <= A (2) and B (0);

 A_MULT_B1(0) <= A (0) and B (1);
 A_MULT_B1(1) <= A (1) and B (1);
 A_MULT_B1(2) <= A (2) and B (1);

 A_MULT_B2(0) <= A (0) and B (2);
 A_MULT_B2(1) <= A (1) and B (2);
 A_MULT_B2(2) <= A (2) and B (2);

 ZERO <= '0';
 C_TEMP(0) <= A_MULT_B0(0);
 C_TEMP(4 downto 3) <=  S_TEMP2(1 downto 0);
 C_TEMP(5) <= C2_out_B1;

 C_TEMP(6) <= A(3) xor B(3);

 data_out <= C_TEMP;

end multi_arch;

这种方法就是分解成许多

 S <= A xor B xor C_in;
 C_out <= (A and B) or (C_in and (A xor B));

然后接起来，

当然也可以用系统带的*指令。