The problem of custom instruction

I want to define instruction of multiplication,but there is a problem that I cann't solve.

the code is:

library ieee;

use ieee.std_logic_1164.all;

use ieee.std_logic_unsigned.all;

use ieee.std_logic_arith.all;

entity mult is

port(dataa:in std_logic_vector(31 downto 0);

datab:in std_logic_vector(31 downto 0);

result:out std_logic_vector(31 downto 0);

reset:in std_logic:='0';

start:in std_logic:='0';

clk_en:in std_logic:='0';

clk:in std_logic);

end mult;

architecture behave of mult is

signal rt:std_logic_vector(31 downto 0);

begin

rt<=dataa(15 downto 0) * datab(15 downto 0);

result<=rt;

end behave;

please help me!

thanks in advance!

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