VHDL: How can I shorten a 32bit expression?

Question

i have the following expression and want so shorten it down to a shorter one.

Can anyone help me?

architecture behavioral of mux4 is
begin
    with sel select
        y <= "a" when "00",
             "b" when "01",
             "c" when "10",
             "d" when "11",
             "XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX" when others;
end architecture behavioral

I want to avoid writing 32 time the character X. I know that it can be replaced with another expression but i can't remember.

Answer 1

To shorten the default expression, usually (others => 'X') will create a value with X in all unspecified members (here, all members since you haven't specified any bits explicitly.

architecture behavioral of mux4 is
begin
    with sel select
        y <= "a" when "00",
             "b" when "01",
             "c" when "10",
             "d" when "11",
             (others => 'X') when others;
end architecture behavioral;

Note : there are cases where VHDL can't determine the size of the expression from the context, so this won't always work. There is an alternative : since it's obvious you want the value to be the same size as y, you can create such a value filled with 'X' very easily using the 'range attribute, as in:

    with sel select
        y <= "a" when "00",
             (y'range => 'X') when others;