conv_integer bei einem 8-bit Multiplexer

Hallo leute,
ich versuche gerade mich in VHDL einzuarbeiten. Es sollte einen Code für 
einen 8-bit Multiplexer geschriben, das wäre dann kein Problem. Aber 
laut der AUfgabenstellung sollte für für die Auswahl des Quellsignales 
die Funktion conv_integer verwendet werden!

Wenn ich den Code in GHDL mit diesen Befehl analysiere, kommen folgende 
Fehler Meldungen.

ghdl -a --ieee=synopsys mux81_2.vhdl

Hat jemand eine Idee, wo das Problem liegen könnte?


PS: Ich habe hier im Forum gelesen, dass es besser die Funktion 
to_integer und die ieee.numeric_std benutzt werden sollen.



mux81_2.vhdl:20:44: can't match function call with type std_ulogic
mux81_2.vhdl:20:32: (location of function call)
mux81_2.vhdl:21:44: can't match function call with type std_ulogic
mux81_2.vhdl:21:32: (location of function call)
mux81_2.vhdl:22:44: can't match function call with type std_ulogic
mux81_2.vhdl:22:32: (location of function call)
mux81_2.vhdl:23:44: can't match function call with type std_ulogic
mux81_2.vhdl:23:32: (location of function call)
mux81_2.vhdl:24:44: can't match function call with type std_ulogic
mux81_2.vhdl:24:32: (location of function call)
mux81_2.vhdl:25:44: can't match function call with type std_ulogic
mux81_2.vhdl:25:32: (location of function call)
mux81_2.vhdl:26:44: can't match function call with type std_ulogic
mux81_2.vhdl:26:32: (location of function call)
mux81_2.vhdl:27:44: can't match function call with type std_ulogic
mux81_2.vhdl:27:32: (location of function call)
/usr/lib/ghdl/bin/ghdl: compilation error

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library ieee;

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use ieee.std_logic_1164.all;

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use ieee.std_logic_arith.all;

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entity mux81_2 is

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port (  a:   in  std_logic_vector (0 to 7);

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  sel: in  std_logic_vector (2 downto 0);

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  y:   out std_logic);

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end mux81_2;

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architecture BEHAVIOR of mux81_2 is

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begin

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  process(a, sel)

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  variable y_int: std_logic;

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  begin

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  case sel is  

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  when "000"  => y_int := conv_integer(a(0));

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  when "001"  => y_int := conv_integer(a(1));

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  when "010"  => y_int := conv_integer(a(2));

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  when "011"  => y_int := conv_integer(a(3));

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  when "100"  => y_int := conv_integer(a(4));

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  when "101"  => y_int := conv_integer(a(5));

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  when "110"  => y_int := conv_integer(a(6));

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  when "111"  => y_int := conv_integer(a(7));

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  when others => null;

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  end case;

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  y <= y_int;

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  end process;

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end BEHAVIOR;