The problem of multi_cycle custom instruction

This is only template for guidance(haven't checked syntax)

library ieee;
use ieee.std_logic_1164.all;
 
entity move 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);
      clk:      in    std_logic;
      clk_en :in    std_logic;
      start:   in    std_logic;
      reset:   in   std_logic;
      done:   out std_logic
);
end move;
 
architecture behave of move is
 
signal t: std_logic_vector(3 downto 0);
signal a: std_logic_vector(31 downto 0);
signal b: std_logic_vector(31 downto 0);
 
begin
 
process(clk,reset)
begin
if(reset = '1')then
   a <= (others => '0');
   b <= (others => '0');
   t <= (others => '0');
   done <= '0';
   result <= (others => '0');
elsif(clk'event and clk='1')then
   if(clk_en = '1')then
 
     if(start = '1')then
          a <= dataa;
          b <= datab;
          t <= "0000";
          done <= '0';
          result <= (others => '0');
     end if;
 
      if(t < "1111")then
            a <= a+1;
            b <= b+1;
            t <= t+1;
            done <= '0';
      else
           done <= '1';
           result <= a+b;
      end if;
   end if;
end if;
end process;
 
end behave;