Lock-On Technology

Lock-On Technology is a special feature found on Sonic & Knuckles for the Sega Genesis that allowed a player to connect an older Sonic game, such as Sonic the Hedgehog 3, to the cartridge for extended gameplay. Unlike other methods of connecting cartridges (for example, in the Game Genie cheat device), Lock-On works by using special circuitry that combines multiple ROM chips into a single address space, depending on which game is inserted into the cartridge's slot. The result is that the console thinks that one big cartridge is plugged in. Devices such as the Game Genie worked in an entirely different way, running first the device's internal program, then running the inserted cartridge seperately. Lock-On technology made Sonic & Knuckles 'the only backwards-compatabile game cartridge', as the advertisements of the time claimed.

Lock-On technology enables a special 256K ROM chip when Sonic the Hedgehog 2 is connected. This extra ROM chip is required for proper gameplay in Knuckles in Sonic 2. This chip contains 'patch code' such as object placement data (for example, adding a 1-Up to the top of a building in the Chemical Plant Zone), Knuckles sprites and mappings, and some Casino Night Zone collision data. This chip alone is available in 'dumped' form; however, it is unplayable in emulators, as it is not a complete game in and of itself. An analogy would be that of trying to run an IPS patch in an emulator - because essential game code is not present, it cannot be run.

When a non-Sonic game (or Sonic the Hedgehog) is plugged in, the result is a game of Blue Sphere special stages. These levels are randomly produced using information from the inserted cartridge's ROM header.