When the Sonic 2 cartridge is locked on to Sonic & Knuckles, its code becomes mostly unusable, and this is before we've even considered how we're going to add the Knuckles object to the game. This is due to the design of the lock-on system, which maps the locked-on ROM to a different base address, making most of the code on the ROM unusable. Mode that allows it to read stuff from the Sonic 3 cartridge. Last time, I tried to make it abundantly clear that Sonic 3 & Knuckles is really just Sonic & Knuckles running in a special Not that it contains anything remotely useful towards running Sonic 3 & Knuckles, anyway. This is a bit facetious, because as we've previously seen, when Sonic 3 is locked on, the two main ROMs take up the entirety of the Mega Drive's ROM address space, leaving no room for the auxiliary ROM. I do not know any specifics about how the auxiliary ROM gets mapped onto the Mega Drive's address space, beyond the fact that it's there by the time S&K detects the Sonic 2 ROM header in the lock-on slot:Īs an aside, I assume this is where the "34 meg" figure on the back of the Sonic & Knuckles box comes from: 16 megs from the Sonic 3 ROM + 16 megs from the main Sonic & Knuckles ROM + 2 megs from the auxiliary ROM. Surprise! The Sonic & Knuckles cartridge actually contains two separate ROM chips: the main 16 megabit ROM and an auxiliary 2 megabit ROM. Wait, what? Where did that third ROM come from? You can play random Blue Sphere levels, if that’s your thing.When the Sonic 2 cartridge is locked on to Sonic & Knuckles, special hardware in the Sonic & Knuckles cartridge maps the two games' ROMs onto the Mega Drive's address space as illustrated by the following diagram: There are only a few games made before Sonic & Knuckles that do not work, such as Phantasy Star IV: The End of the Millennium & Super Street Fighter II: The New Challengers. Most games made after Sonic & Knuckles was released are larger than 2MB and thus will not work with its lock-on feature. Locking-On with a game that uses a battery pack erases saved games. If Locked-On with a game larger than 2 Megabytes, it will boot to Sonic & Knuckles on real hardware, as the Sonic & Knuckles cart’s mapper will map the last 2MB of the locked-on ROM instead of the first 2MB. When a non-Sonic game is plugged in, the result is a single stage of Blue Sphere, produced randomly from information in the inserted cartridge’s ROM header, along with the stage’s ‘code’ for use in the full Blue Sphere. This is a passage I got from Sega Retro regarding connecting Sonic & Knuckles with a non-Sonic game. You can simulate the experience of entering codes one character at a time into a Game Genie menu and then playing Sonic the Hedgehog with infinite rings. There’s no real benefit to normal end-users but I can think of two uses for really dedicated gamers. It’s just there to digitally preserve ‘Lock-On-Technology’ for the Genesis.
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