pls pls save me from this index value warning

----------------------------------------------------------------------------------
-- Company: 
-- Engineer: 
-- 
-- Create Date:    18:04:49 02/15/2013 
-- Design Name: 
-- Module Name:    ram - Behavioral 
-- Project Name: 
-- Target Devices: 
-- Tool versions: 
-- Description: 
--
-- Dependencies: 
--
-- Revision: 
-- Revision 0.01 - File Created
-- Additional Comments: 
--
----------------------------------------------------------------------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

-- Uncomment the following library declaration if using
-- arithmetic functions with Signed or Unsigned values
--use IEEE.NUMERIC_STD.ALL;

-- Uncomment the following library declaration if instantiating
-- any Xilinx primitives in this code.
--library UNISIM;
--use UNISIM.VComponents.all;

package memory is 
type mem_data is array (23 downto 0) of bit_vector(7 downto 0);
end memory;
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_arith.ALL;
use IEEE.STD_LOGIC_unsigned.ALL;
use work.memory.all;


entity ram is

port(
     add_bus:in std_logic_vector(31 downto 0);
	  data_bus1:in bit_vector(31 downto 0);
	  data_bus2,data_bus0:out bit_vector(31 downto 0);
	  mem_ctrl:in bit_vector(2 downto 0);
	  data_out:out std_logic_vector(31 downto 0);
	  clk:in std_logic
	  );
	  
end ram;

architecture Behavioral of ram is

begin
process(add_bus,data_bus1,mem_ctrl,clk)

variable i1,i2,i3:integer;
variable i:integer ;
variable ram_data:mem_data :=  (("00000000"),
									 ("00000000"),
									 ("00000000"),
									 ("00000100"),
									 ("00000000"),
									 ("00000000"),
									 ("00000000"),
									 ("01000000"),
									 ("00000000"),
									 ("00000000"),
									 ("00000000"),
									 ("00000010"),
									 ("00000000"),
									 ("00000000"),
									 ("00000000"),
									 ("00000001"),
									 ("00000000"),
									 ("00000000"),
									 ("00000000"),
									 ("10000000"),
									 ("01111110"),
									 ("01100100"),
									 ("01100000"),
									 ("00000000"));
begin
case add_bus is
 when "00000000000000000000000000000000" => i:= 0;
 when "00000000000000000000000000000001" => i:= 1;
 when "00000000000000000000000000000010" => i:= 2;
 when "00000000000000000000000000000011" => i:= 3;
 when "00000000000000000000000000000100" => i:= 4;
 when "00000000000000000000000000000101" => i:= 5;
 when "00000000000000000000000000000110" => i:= 6;
 when "00000000000000000000000000000111" => i:= 7;
 when "00000000000000000000000000001000" => i:= 8;
 when "00000000000000000000000000001001" => i:= 9;
 when "00000000000000000000000000001010" => i:= 10;
 when "00000000000000000000000000001011" => i:= 11;
 when "00000000000000000000000000001100" => i:= 12;
 when "00000000000000000000000000001101" => i:= 13;
 when "00000000000000000000000000001110" => i:= 14;
 when "00000000000000000000000000001111" => i:= 15;
 when "00000000000000000000000000010000" => i:= 16;
 when "00000000000000000000000000010001" => i:= 17;
 when "00000000000000000000000000010010" => i:= 18;
 when "00000000000000000000000000010011" => i:= 19;
 when "00000000000000000000000000010100" => i:= 20;
 when "00000000000000000000000000010101" => i:= 21;
 when "00000000000000000000000000010110" => i:= 22;
 when "00000000000000000000000000010111" => i:= 23;
 when others =>
 null;
 end case;
 i1:=i+1;
 i2:=i+2;
 i3:=i+3;
 
 if(clk'event and clk = '1') then
 if(i<= 20) then
 if (mem_ctrl(2) = '1') then
 --some = := (data_bus1(7 downto 0));
 ram_data(i) := (data_bus1(7 downto 0));
ram_data(i1) := (data_bus1(15 downto 8));
--ram_data(i1) := (data_bus1(15 downto 8));
ram_data(i2) := (data_bus1(23 downto 16));
 ram_data(i3) := (data_bus1(31 downto 24));
 data_out <=to_stdlogicvector(ram_data(i3)&ram_data(i2)&ram_data(i1)&ram_data(i));
-- wait for 0ns;
 end if;
 if (mem_ctrl(1) = '1') then
 --wait for 10ns;
 data_bus0 <= (ram_data(i3)&ram_data(i2)&ram_data(i1)&ram_data(i));
--wait for 0ns;
end if;
else 
null;
end if;
if(i<23) then
if (mem_ctrl(0) = '1') then
 --wait for 20ns;
 data_bus2 <= ("000000000000000000000000"&ram_data(i));
 --wait for 0ns;
 end if;
 end if;
if (i >20) then
null;
end if;
end if;
end process;
end Behavioral;