add4b.vhd

可以很好实现一个二进制转换成BCD码的程序

-----4 bit adder
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
use ieee.std_logic_arith.all;

entity add4b is
 port(cin :in std_logic; --carry in flag;
      a,b :in std_logic_vector(3 downto 0);
      sum :out std_logic_vector(3 downto 0);
      cout:out std_logic--carry out flag;
     );
end entity add4b;

architecture behave of add4b is
signal aa,bb :std_logic_vector(4 downto 0); 
signal ss :std_logic_vector(4 downto 0);
begin
aa<='0'&a;
bb<='0'&b;
ss<=aa+bb+cin;
sum<=ss(3 downto 0);
cout<=ss(4);
end;