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In the Japanese version of Sonic the Hedgehog 3, four worlds that had never before been seen in the United States were a standard part of the ROM. However, in the United States, Sega of America released Sonic the Hedgehog 3 without the levels, then later released a Game Geniesque cartridge that added the extra levels, and also functioned as it's own game; Sonic and Knuckles.

When you take a dumped Sonic and Knuckles ROM and a dumped Sonic the Hedgehog 3 image and type the following:

COPY /B S&K.BIN + SONIC3.BIN SONIC3&K.BIN You'll create a ROM image with the exact same properties as a Sonic the Hedgehog 3 ROM sitting in the Sonic & Knuckles piggy-back cartridge! The person to discover this was NO CARRIER who stated the following theories: Before I begin explaining why I think it works, we must make 2 assumptions:
  1. The program code and the modified graphics for Sonic 2 and 3 are located on the S&K cartrage. (I thing this is true because I don't see how the modifed graphics and all the changes in the program can be implemented on a Ready Only Memory medium.

  2. When you add a cartridge to the S&K cartridge unit, the Genesis treats it like one cartridge with a heck of alot of memory. In the sense of the ROM images, it is just one large file. (Note, by combining them, the total size of the two cartridges is smaller than the largest ROM size possible that the Genesis can control.) Remember, it is doing this without the aid of additional handware, just software.

At power-on of the Genesis, and after all the standard stand procedures are run, (Like master reset, ect.), the S&K cartrage looks to see if there is anything in the add-on port by trying to read the contents of the first bank or so. (It assumes that the smallest ROM size is under 1 Megabit or lower. So, by accessing a bank in this area assures that it is going to read something.)

If something exists, it will get back some kind of feedback. If it doesn't, it will get back all 1's or 0's because there are no banks and it considers them blank. (The output would more likly be floating high 's -- i.e. all 1's) Since there is cartridge in the add-on port, it runs the Sonic and Knuckles game. If it determines that add-on port is occupied, it will read the header of the cartridge. (Think of the head something like a file name: It's a means of identifing the cartridge if no label was present.)

If it is not Sonic the Hedgehog 2 or Sonic the Hedgehog 3, then it spits out the message Ya right. If it IS Sonic the Hedgehog 3 then it runs Sonic 3 program on the S&K cartridge. It uses the Sonic 3 cartridge for all its graphics and sounds, except for any new ones.

At this point, if it is not Sonic the Hedgehog 3, then it can only be Sonic the Hedgehog 2. So it runs the Sonic 2 program on the S&K cartrage. It uses the Sonic 2 cartridge for all its graphics and sounds, except for any new ones.

It doesn't work: What do I do?

From my tinkering around, Sonic 3 works fine with the S&K modifications, but you cannot save your game. Sonic 2 does not work at the moment. More likely, it is the emulator. Also, check to see if the header says Sonic the Hedgehog 2 or ...3. Those console game distributers like to change the Header with things like Sonic 2 for Console Zone or Sonic 2 supplied by ^BaTMaN^. Under my theory, this WILL cause problems. Find a program that lets you change headers of carts.

As for getting into the secret bonus level by using another cart other than Sonic 2 or 3, for the S&K cart:

No luck either.

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