Generic multiplexer

Hello everybody,
I want to make generic multiplexer. I am using FOR/GENERATE to make
multiple code lines and conditional generate to make last line
(without ELSE). Please, suggest what to do and why my code doesn't
work. Thank you in advance.
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library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity gen_mux1 is
generic(n: integer:=3);
port (input : in std_logic_vector(2**n-1 downto 0);
sell : in std_logic_vector(n-1 downto 0);
output: out std_logic);
end gen_mux1;

architecture Behavioral of gen_mux1 is
begin

a: FOR i IN 0 TO 2**n-1 GENERATE
b1: IF i /= 2**n-1 GENERATE
output <= input(i) WHEN i = conv_integer(sel) ELSE
END GENERATE;
b2: IF i = 2**n-1 GENERATE
output <= input(i) WHEN OTHERS
END GENERATE;
END GENERATE;
end Behavioral;
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P.S. I want code to be concurrent.

 

[Andy]

Use ieee.numeric_std instead of std_logic_[arith, unsigned].

Multiple concurrent assignments to the same signal (created by the
generate loop) create multiple drivers for that signal. Your attempt
could be modified to create a tristate mux (bus), but that is probably
not what you want either:

output <= input(i) when i = to_integer(unsigned(sel)) else 'Z';

Not sure what you're trying to do with 2nd If-generate...

You don't need the generate statements (loop or if) anyway:

-- since sel is constrained to be within input'range:
use ieee.numeric_std.all;
architecture concurrent of gen_mux1 is
begin
output <= input(to_integer(unsigned(sel)));
end architecture;

-- or this will work if value of sel could be
-- outside of input'range:
use ieee.numeric_std.all;
architecture sequential of gen_mux1 is
begin
process (input, sel) is
begin
output <= '0'; -- default assignment
for i in input'range loop
if i = to_integer(unsigned(sel)) then
output <= input(i);
end if;
end loop;
end process;
end architecture;

Andy