$B080 is start of demo cycle
$B29D is end of demo cycle [3 bytes available]
$AD7A is after user presses a key
no support for pressing joystick buttons during demo

$0863: 30 -> 00 to disable sound
$AD7A: C993F0 -> 4C0001 to exit after user presses a key
$B29D: 4C80B0 -> 4C0001 to exit at end of demo cycle