nesdev.org

I guess you all know this trick where you put a Tennis cartridge in the Famicom toploader, leave the system on and then put a Super Mario cartridge in. 256 hidden or rather glitch levels appear. I wonder if this would work also with a european NES. As I got it the reason it does not work is the lock-out chip. Would the Mario-Tennis trick work if one disables the lock-out chip?

Previous thread about this topic.

CIC shouldn't matter, actually. Just have to not do a full power cycle, so that RAM retains its contents. Something like "power on with tennis" (or any other way of pre-loading RAM with the right values), hold reset, swap carts, release reset.