With VHDL-2008, the case statement has a way to deal with explicit don’t care assignments. When using case? , the tool now evaluates explicit don’t care terms, as in the following example:
process(clk) begin
if clk’event and clk=’1’ then
case? my_reg is
when "01--" => out1 <= in1;
when "000-" => out1 <= in2;
when "1111" => out1 <= in3;
when others => out1 <= in4;
end case?;
end if;
end process;
Note: For this statement to work, the signal in question must be assigned an explicit don’t care .