Lock-On Technology
From Sega Retro
Lock-On Technology is a unique feature found on Sonic & Knuckles cartridges for the Sega Mega Drive/Genesis that allowed a player to connect an older Sonic game to the cartridge for extended or altered gameplay.
Contents
Operation
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 separately. Lock-On technology made Sonic & Knuckles 'the only backwards-compatible game cartridge', as the advertisements of the time claimed.
Lock-On with Sonic 3
See: Sonic 3 & Knuckles
Lock-On with Sonic 2
When Sonic 2 is connected, the cart combines the Sonic & Knuckles ROM, the Sonic 2 ROM, and a 256K ROM chip to boot into a new game entitled 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, all code needed for the game to function, 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, being more analogous to an IPS patch.
Multiple prototypes of this patch chip were released in the February 23rd 2008 Proto Release.
Lock-On with Sonic 1
A game later titled Blue Sphere is enabled upon pressing 
, 
, and 
simultaneously on the No Way screen.
Lock-On with other games
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. If Locked-On with a game that would cause the combined ROM to be larger than 4 Megabits, it will boot to Sonic & Knuckles.