I've been meaning to post an example of this for a while. It shows how to use your own custom assembly display kernel in bB.   Care has been taken to use the bB overscan and vblank timings, so you should be able to flip back and forth between your custom kernel and a stock kernel without any screen roll.   The program displays the custom rainbow kernel until the player holds down the joystick button, at which point the standard bB kernel is displayed.   Enjoy!     rainbowbasic.bas rain