function function_name (parameter_list) return type is
	declarations
begin
	sequential statements
end function_name;

1. A function can only have input parameters, so the mode (direction) is not specified. 
2. A function may declare local variables.
3. A function may contain any sequential statement except signal assignment and wait.



library ieee;
use ieee.std_logic_1164.all;

package MY_PACK is
  function PARITY (X : std_logic_vector)
                   return std_logic;
end MY_PACK;

package body MY_PACK is
  function PARITY (X : std_logic_vector)
                   return std_logic is  

  variable TMP : std_logic;
  begin

       --TMP:= '0';
       TMP := X(0);
       --for J in X'range loop
       for J in 1 to X'high loop
             TMP := TMP xor X(J);
        end loop;          -- works for any size X
        return TMP;
 
 
  end PARITY;

end MY_PACK;


library ieee;
use ieee.std_logic_1164.all;
use work.MY_PACK.all;

entity PAR is
   port( db:  in std_logic_vector(7 downto 0);
            dw: in std_logic_vector(15 downto 0);
            pb, pw: out std_logic);
end PAR;

architecture ARCH1 of PAR is
begin
  pb <= PARITY(db);
  pw <= PARITY(dw);
end ARCH1;