算术逻辑单元.txt

4位ALU逻辑运算单元

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;
use ieee.std_logic_unsigned.all;
entity alu is
  port
(a: in unsigned(3 downto 0);
 b: in unsigned(3 downto 0);
 cin: in std_logic;
 s: in std_logic_vector(2 downto 0);
 bcdout: out std_logic_vector(3 downto 0);
 cout: out std_logic);
end alu;
architecture behavor of alu is
  signal c,y: std_logic_vector(3 downto 0);
  signal q: unsigned(3 downto 0);
begin
process(a,b,cin,s)
begin
case s is
  when "000"=>
             y(0)<=a(0) xor b(0) xor cin;
             c(0)<=(a(0) and b(0)) or (b(0) and cin) or (a(0) and cin);
             gen1:for i in 1 to 3 loop
             y(i)<=a(i) xor b(i) xor c(i-1);
             c(i)<=(c(i-1) and a(i)) or (c(i-1) and b(i)) or (a(i) and b(i));
             end loop;
             bcdout<=y(3)&y(2)&y(1)&y(0);
             cout<=c(3);
  when "001"=>
             y(0)<=a(0) xor b(0) xor cin;
             c(0)<=(cin and not a(0))  or (cin and b(0)) or (not a(0) and b(0));
             gen2:for i in 1 to 3 loop
             y(i)<=a(i) xor b(i) xor c(i-1);
             c(i)<=(c(i-1) and not a(i)) or (c(i-1) and b(i)) or (not a(i) and b(i));
             end loop;
             bcdout<=y(3)&y(2)&y(1)&y(0);
             cout<=c(3);
  when "010"=>
             if cin='0' then
                bcdout <=a(3)&a(2)&a(1)&a(0);
             else
                bcdout <=a+1;
             end if;
             cout<='0';
 when "011"=>
             if cin='0' then
                bcdout <=a(3)&a(2)&a(1)&a(0);
             else
                bcdout <=a-1;
             end if;
             cout<='0';
  when "100"=>
             y(3)<=a(3) and b(3);
             y(2)<=a(2) and b(2);
             y(1)<=a(1) and b(1);
             y(0)<=a(0) and b(0);
             bcdout<=y(3)&y(2)&y(1)&y(0);
  when "101"=>
             y(3)<=a(3) or b(3);
             y(2)<=a(2) or b(2);
             y(1)<=a(1) or b(1);
             y(0)<=a(0) or b(0);
             bcdout<=y(3)&y(2)&y(1)&y(0);
  when "110"=>
             y(3)<=a(3) xor b(3);
             y(2)<=a(2) xor b(2);
             y(1)<=a(1) xor b(1);
             y(0)<=a(0) xor b(0);
             q<=y(3)&y(2)&y(1)&y(0);
             bcdout<=y(3)&y(2)&y(1)&y(0);
  when "111"=>
             y(3)<=not a(3);
             y(2)<=not a(2);
             y(1)<=not a(1);
             y(0)<=not a(0);
             bcdout<=y(3)&y(2)&y(1)&y(0);
  when others=>
             bcdout<="0000";
  cout<='0';
end case;
end process;
end behavor;