architecture Behavioral of feladat_2 is
    signal c: STD_LOGIC_VECTOR(4 downto 0) := (others => '0');
    signal tmp: STD_LOGIC_VECTOR(3 downto 0) := (others => '0');
begin
process(alusel,a,b)
    variable res: STD_LOGIC_VECTOR(3 downto 0) := (others => '0');
    variable tmp_s: STD_LOGIC_VECTOR(4 downto 0) := (others => '0');
    variable mul_tmp: STD_LOGIC_VECTOR(7 downto 0) := (others => '0');
    variable mul_res: STD_LOGIC_VECTOR(15 downto 0) := (others => '0');
begin
    cf <= '0';
    ovf <= '0';
    z <= (others => '0');
    res := "0000";

    case alusel is
        when "000" =>
            res := a;
        when "001" => -- a + b
            tmp_s := ('0' & a) + ('0' & b);
            res := tmp_s(3 downto 0);
            cf <= tmp_s(4);
            ovf <= (tmp_s(3) xor a(3) xor b(3)) xor tmp_s(4);

        when "010" => -- a - b
            tmp_s := ('0' & a) - ('0' & b);
            res := tmp_s(3 downto 0);
            cf <= tmp_s(4);
            ovf <= (tmp_s(3) xor a(3) xor b(3)) xor tmp_s(4);

        when "011" => -- b - a
            tmp_s := ('0' & b) - ('0' & a);
            res := tmp_s(3 downto 0);
            cf <= tmp_s(4);
            ovf <= (tmp_s(3) xor a(3) xor b(3)) xor tmp_s(4);

        when "100" => -- a * b
            res := (others => '0');
            mul_tmp := a * b;

            z(7 downto 0) <= mul_tmp(7 downto 0);
            -- using mul_res as copy of z but as variable so usable as right hand side operand
            -- this way avoiding making z inout parameter
            mul_res(7 downto 0) := mul_tmp(7 downto 0);

            -- sign extension (since two 4 bit number multiplication will fit only on 8 bit therefore
            -- I assume the result need sign extensions since the result represented in 16 bit
            if mul_tmp(7) = '1' then
                z(15 downto 8) <= "11111111";
                mul_res(15 downto 8) := "11111111";
            else -- altough technically the value can be be undefined or X, we ignore that, assuming '0'
                z(15 downto 8) <= "00000000";
                mul_res(15 downto 8) := "00000000";
            end if;
        when "101" =>
            res := a and b;
        when "110" =>
            res := a or b;
        when "111" =>
            res := a xor b;
        when others =>
            res := "0000";
    end case;

    y <= res; -- saving the result to the output singal

    -- setting negative flag
    if res(3) = '1' or mul_res(15) = '1' then
        nf <= '1';
    else
        nf <= '0';
    end if;

    -- setting zero flag
    if res = "0000" and mul_res = "0000000000000000" then
        zf <= '1';
    else
        zf <= '0';
    end if;

end process;
end Behavioral;