Lock-on Technology

23 years ago, on October 18, Sonic & Knuckles was released on the Sega Genesis and Mega Drive worldwide. As sort of a celebration, I thought it'd be neat to do a short series about the game which not only served as a conclusion to the adventure that started in Sonic 3 a few months prior, but also introduced the concept of add-on content to video games as early as 1994.

Let's start with a little history: how exactly did Sonic & Knuckles come to be?

Fresh off the heels of Sonic 2's commercial success in 1992, Sega and Sonic Team set out to surpass themselves and produce the biggest, most polished Sonic game ever made. After the dubious outcome of the Sega Technical Institute's call for local graphic designers, which resulted in Sonic 2 having a rougher, inconsistent visual style, Sonic Team chose to once again use a team of solely Japanese artists to bring Sonic 3's worlds to life.

However, in the process of creating the largest Sonic levels ever, and employing a more cinematic, story-driven design, the development team ran into issues. Cartridges were expensive to produce, with ROM space at a premium. Sonic 2 was released on an 8 megabit cartridge, whereas Sonic 3 was projected to require a 24 megabit cartridge, three times as large. It also became apparent that development would take longer than had previously been anticipated.

Meanwhile, Sega of America had organized various promotions, including a tie-in with McDonald's Happy Meal, which could not be rescheduled. Sonic Team faced the difficult decision to split the game in two parts, the first of which would be released in February 1994. This became Sonic the Hedgehog 3.

As development continued, a question remained: how would the full version of the game be delivered to customers? A 24 megabit "Limited Edition" was considered but dropped, likely to keep production costs low. Releasing "Part Two" as a standalone title would be far more viable, but crush any possibility of playing Sonic 3 as a single, continuous game.

Luckily, Sega's hardware division would find a way for Sonic Team to both have their cake and eat it. And thanks to the same marketing team that popularized terms as such "16-bit graphics" and "blast processing", it had a name:

Lock-on Technology was a groundbreaking feature built directly into the Sonic & Knuckles cartridge. Like cheat devices that were available at the time, such as Action Replay and Game Genie, the top of Sonic & Knuckles's cartridge had an additional slot that allowed players to insert another game cartridge, effectively "locking on" the two titles.

Unlike such devices though, which only made basic modifications to a game, Sonic & Knuckles could lock on with other Sonic cartridges to create a game greater than the sum of its parts. When locked on with Sonic 3, the full version of the game was unlocked, seamlessly joining the two halves together, and allowing Knuckles to play in Sonic 3's stages. And when locked on with Sonic 2, it would somehow retroactively add Knuckles as a playable character in Sonic 2.

But how does it work?

Over the next few posts, I'll explain how Lock-on Technology works from a software perspective, how Sonic & Knuckles uses it to combine two games into one, and how Sonic Team was able to patch their titles on original hardware.