theres only one person who know The only person who knows how to do this is a member of the segaxtreme forums named Arakon. He sells a custom made multi bios. So he would be the best to ask. Just go to the segaxtreme forums at http://www.phantasy-star-universe.com/ go to message boards then post on the sega cd part of the forum and ask him. He'll probably recomend you just buy it from him. Also ask him for the page on how to instal the multi bios.
Doesnt it just boil down to installing a switch on the highest address line? (and getting the two BIOSes to fit on the same chip)
Yes, all Arakon's does is what Antipasta just said. Its just that he hacked the splash screen (its WAY ugly IMO) and he also had someone remove the built in Mega Drive region protection so that you could run any MCD on any Mega Drive. I'm planning on doing the same without touching any graphics. Will use a 27c8096 (or whatever 8Mbit/16-bit are called) so that I can switch between 8 BIOS with a DIP switch block. ExCyber is the person to talk to about the region protection because he was the one that did it for Arakon. I asked him about it, got some information but I never got back around to cracking the protection. The best use of ROM "banking" is for making a LaserActive switchable BIOS
