VHDL and ModelSIM question

Hi,

I have been trying to learn the "generic" statement in VHDL and as a
result I  have made the  generic binary decoder (below) in VHDL. Note that
both the  encoded input A and decoded output Y are of type unsigned. Also
in the "for" loop I have used "to_integer"

Then I have a top level file that instantiates this decoder with SizeIn =
8 and SizeOut = 256.

The design is synthesized and implemented without any errors on XST.
However when I try to simulate the design using ModelSim I get warnings
and errors such as -

# WARNING[1]: Types do not match for port A
# WARNING[1]: A use of this default binding for this component
instantiation will result in an elaboration error.
# WARNING[1]: Types do not match for port Y
# WARNING[1]: A use of this default binding for this component
instantiation will result in an elaboration error.

** Failure: Default binding had errors for entity  "top_architecture" on
the component declaration of line 55. See the  compiler messages.

I think this is because of type "unsigned" When I remove "unsigned" and
make it type  "std_logic_vector," I cannot use the "to_integer" in
the "for" loop. XST does not synthesize and gives the following error


ERROR: HDLParsers:808 - to_integer cannot have such operands in this
context

Any ideas? I am absolutely frustrated with VHDL right now :D. Maybe the
best way would be to make it std_logic_vector and somehow get the same
functionality of "to_integer" by some other function.


Thanks in advance for your help.

-Yash

*************************************************************************
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity generic_decoder is
    generic (SizeIn, SizeOut: integer);
    port ( Clock, Rst : in std_logic;
           A : in unsigned(SizeIn - 1 downto 0);
           Y : out unsigned(SizeOut - 1 downto 0));
end generic_decoder;


architecture arch_generic_decoder of generic_decoder is

begin

	process (Clock, Rst)
	begin
		if (Rst = '1') then
			Y <= (others => '0');
		elsif (Clock'event and Clock = '1') then
			for N in 0 to SizeOut - 1 loop
				if (to_integer(A) = N) then
					Y(N) <= '1';
				else
					Y(N) <= '0';
				end if;
			end loop;
		end if;
	end process;

end arch_generic_decoder;