What is the mechanism of the lock-on system of 'Sonic & Knuckles' that plays by directly connecting another cassette to the Mega Drive cassette?



Sega's popular action game '

Sonic the Hedgehog ' series title, ' Sonic & Knuckles ' released for Mega Drive in October 1994, by connecting a cartridge of another title in the same series to the cartridge, It adopted a ' lock-on system ' that allows you to play another version of the game. This lock-on system is explained in the developer forum Stack Exchange.

gaming - How exactly does Sonic & Knuckles' 'Lock-On Technology' work? - Retrocomputing Stack Exchange
https://retrocomputing.stackexchange.com/questions/1514/how-exactly-does-sonic-knuckles-lock-on-technology-work

The Mega Drive is Sega's home-use game hardware that you play by inserting a cartridge. The types of mega drive cartridges ranged from 2 megabits to 40 megabits depending on the capacity of the game. The cartridge of the first work ' Sonic the Hedgehog ' released in Mega Drive in 1991 is a 4 megabit rom cassette and has the following shape.

[Sega Hard Encyclopedia] Cartridge for Mega Drive
https://www.sega.jp/fb/segahard/md/cartridge.html



And the cartridge of 'Sonic & Knuckles' released in 1994 was an 18 megabit rom cassette. You can see what kind of shape it is by looking at the TV commercial image that was being broadcast at that time.

Japanese Sonic & Knuckles TV Commercial-YouTube


The cartridge has a slightly larger top than the 4 megabit ROM cassette.



This upper part is a connection terminal, and if you open the lid as follows, you can insert the cartridge.



When you connect the 'Sonic the Hedgehog 2' cartridge to the 'Sonic & Knuckles' cartridge, it looks like this. Sega called this system for connecting different cartridges the 'lock-on system.'



As announced in the TV commercial, 'Sonic & Knuckles' was released worldwide at the same time. In the United States, a mega drive was developed under the name 'Genesis', and 'Sonic & Knuckles' was also sold for Genesis. The cartridge of Genesis version 'Sonic & Knuckles' is the image below.



The reason why the cartridge of 'Sonic & Knuckles' became like this is that the third work '

Sonic the Hedgehog 3 ' had to cut half of the planned volume due to the circumstances of the development period. 'Sonic & Knuckles' is a sequel to 'Sonic the Hedgehog 3', and by combining it with the cartridge of 'Sonic the Hedgehog 3', you can play as a 'complete version of Sonic the Hedgehog 3'.

Specifically, you can play 'Sonic the Hedgehog 3' in zones 1 to 6 and 'Sonic & Knuckles' in zones 7 and beyond without interruption. In addition, data can be saved by connecting 'Sonic the Hedgehog 3'. Also, when I inserted the cartridge of the second work ' Sonic the Hedgehog 2 ', I was able to play 'Sonic the Hedgehog 2' with Knuckle instead of Sonic.



Furthermore, by inserting 'Sonic the Hedgehog' or other Mega Drive cartridges into 'Sonic & Knuckles', you could play a hidden stage. However, when inserting 'Sonic the Hedgehog' or other mega drive cartridges, it is a hidden element and is not described in the manual.

The picture below is the board of the Genesis version 'Sonic & Knuckles' cartridge. The upper part is the connection terminal for the lock-on system. The two large chips in the center are ROMs containing game data, with a capacity of 512 KB on the left and 2 MB on the right. Also, the chips '74AC08 Quad 2-input AND Gate', '74AC139 1-of-4 decoder/demultiplexer' and '74HC74 Dual D-type Flip-Flop' on the board detect the connected cartridge and map to the 24-bit

address space of the MC68000 , which is the CPU of the mega drive.



For example, when 'Sonic the Hedgehog 3' is connected to 'Sonic & Knuckles', 'Sonic & Knuckles' is originally programmed to work with 'Sonic the Hedgehog 3', so the connected 'Sonic the Hedgehog 3' code is executed as it is. However, the latter stage data and knuckle data are read from the ROM of 'Sonic & Knuckles'.



Also, when connecting the original 'Sonic the Hedgehog', it is programmed to read all the bonus game data recorded in the 'Sonic & Knuckles' cartridge. Also, if you insert another cartridge unrelated to 'Sonic the Hedgehog', it is possible to read only one stage data of a bonus game that can be played innumerably.

And if you connect 'Sonic the Hedgehog 2', you can play 'Knuckle version Sonic the Hedgehog 2' where you can play 'Sonic the Hedgehog 2' with knuckles. Actually, of the two ROMs installed in the 'Sonic & Knuckles' cartridge, the 512KB one contains the entire code of 'Knuckles version Sonic the Hedgehog 2'. Basically, execute the code of 'Knuckle version Sonic the Hedgehog 2' included in 'Sonic & Knuckles'. However, game data is read from the connected 'Sonic the Hedgehog 2' cartridge and reused. The only code running from the resulting connected cartridge is the sound driver.



Of course, the complete code of the game is recorded in the cartridge of 'Sonic the Hedgehog 2', but 'Sonic & Knuckles' contains variable data such as speed, jump height, wall climbing, graphic data and new map data adjusted for the knuckle version. This is also testified by Mr. Naoki Horii of M2 , who analyzed 'Sonic the Hedgehog 2' in porting work to the Wii virtual console.

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in Software,   Hardware,   Game, Posted by log1i_yk