ALU - Behavioural modelling

library ieee;
use ieee.std_logic_1164.all;

library work;
use work.Gates.all;

entity alu is
  generic(
    N : integer:=4
  );
  port (
    A : in std_logic_vector(N-1 downto 0);
    B : in std_logic_vector(N-1 downto 0);
    Y : out std_logic_vector(5 downto 0)
  ) ;
end alu;

architecture beh of alu is

  function max(A: in std_logic_vector; B : in std_logic_vector)
    return std_logic_vector is
      variable is_max : std_logic_vector(N-1 downto 0):= (others => '0');
    begin
      for i in N-1 downto 0 loop
        if ((A(i) xor B(i)) = '1') then
          if (A(i) = '1') then
            return A;
          else
            return B;
          end if;
        end if;
      end loop;
      return is_max;
  end max;

  function eq(A: in std_logic_vector; B : in std_logic_vector)
    return std_logic_vector is
      variable is_eq : std_logic_vector(N-1 downto 0):= (others => '0');
    begin
      for i in N-1 downto 0 loop
        if ((A(i) xor B(i)) = '1') then
          return is_eq;
        end if;
      end loop;
      return A;
  end eq;

  alias a3 : std_logic is A(N-1);
  alias b3 : std_logic is B(N-1);
begin

  alu1 : process( A,B )
  begin
    if ((b3 = '0') and (a3 = '0')) then
      Y <= "00"& max(A,B);
    elsif ((b3 = '0') and (a3 = '1')) then
      Y <= "00"& (A and B);
    elsif ((b3 = '1') and (a3 = '0')) then
      Y <= "00"& (not A);
    elsif ((b3 = '1') and (a3 = '1')) then
      Y <= "00"& eq(A,B);
    else
      Y <= (others => '0');
    end if;
  end process ; -- alu1
end;