architecture RTL_CASE of DECODER is
begin
  process(KEYPAD)
  begin
    case KEYPAD is
      when "1000000000" => KEY <= …;
      when "0100000000" => KEY <= … ;
      when "0010000000" => KEY <= … ;
      when "0001000000" => KEY <= … ;
      when "0000110000" => KEY <= … ;
      …
      when "0000000001" => KEY <= … ;
    end case;
  end process;
end RTL_CASE;