assert statement

can anyone tell me what's wrong with this assert statements.

hereis my code:

----------------------------------------------------------------------------------
-- Company:
-- Engineer:
--
-- Create Date:    17:54:42 05/12/2008
-- Design Name:
-- Module Name:    fifoashani - 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;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

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

entity fifo is
****t(	wren : in std_logic;
		rden: in std_logic;
		reset : in std_logic;
		clk : in std_logic;
		data : in std_logic_vector(3 downto 0);
		stackfull: out std_logic;
		stackempty: out std_logic;
		dop: out std_logic_vector(3 downto 0));
end fifo;

architecture Behavioral of fifo is

type stack is array(0 to 7) of std_logic_vector(3 downto 0);
signal int_reg : std_logic_vector(3 downto 0);
signal int_stack : stack;
signal countup: integer :=0;
signal countdn: integer :=0;
signal stackempty_var: std_logic :='0';
signal stackfull_var : std_logic :='0';



begin

process(clk)


begin


if(clk'event and clk='1') then

	if (reset = '1') then
	stackfull_var <= '0';
	stackempty_var <= '0';
	countup <= 0;
	countdn <= 0;
	int_reg <= "0000";


	elsif(wren = '1') then
		assert not (countup>7)
		re****t " stackfull"
		severity NOTE ;

--		if(countup>7) then
--		stackfull_var <='1';

		int_stack(countup)<= data;
		countup <= countup + 1;


	elsif(rden = '1') then
	  assert (countdn >7)
	  re****t "stack empty"
	  severity NOTE ;
--		if(countdn > 7) then
--		stackempty_var <= '1';

		int_reg <= int_stack(countdn);
		countdn <= countdn + 1;
		end if;

end if;

end process;

dop <= int_reg;
stackfull <= stackfull_var;
stackempty <= stackempty_var;

end Behavioral;