Sonic CD Plus Plus

Sonic CD Plus Plus
Last release: June 20, 2009
System: Sega Mega CD
Original game: Sonic CD
Credits: qiuu, snkenjoi

SonicCD++ (or Sonic CD Plus Plus) is a hack of Sonic CD by qiuu and snkenjoi inserting the Sonic 2-style spin dash and the homing attack into Sonic CD. This causes the gameplay to be significantly speedier, while Sonic CD is known to be one of the more slow-paced Sonic games.

Features

• Sonic 2-style spin dash (pressing down to duck, then // to charge)
• 'Megamix style' homing attack (pressing // while in a ball)
• Air Roll (when in the air without being in a ball, press /// to curl into a ball)
• Air Speedcap removed

Development history

Everything started with snkenjoi asking qiuu whether he had attempted making a Sonic CD hack yet (which he had of course not), upon which qiuu brought up Mad Echidna's

offer from a year back to pay $100 to the first person to put the Sonic 2 spin dash into Sonic CD. While qiuu doubted that it was up to date, it got snkenjoi motivated to start attempting to insert the move into the game. So, had it not been for either of these two (snkenjoi and MadEchidna), the project wouldn't have even started in the first place.

So for some time, qiuu mostly kept passive, merely giving advice to snkenjoi every now and then, but seeing the progress, he felt more and more like joining in. So parallel to snkenjoi's work, he started writing a tool that would make finding and inserting new content into the Sonic CD ISO a lot easier. Being done with that, qiuu set up an SVN repository marking the starting point of our work together. With the spin dash already halfway in progress, qiuu implemented the homing attack which turned out to be significantly less difficult than the spin dash. From here, it was simply a case of further developing the code in order to insert the additional features into the game - and notably, repeatedly trying to find free space to put them in (thanks to Tweaker for mentioning the unused Sonic sprites). We'd also like to thank MKAmeX for his contribution of spindash sprites for the small sonic seen in MMZ2.

Despite only taking around a week's time to accomplish, most of the development time was spent fixing bugs that had appeared. The final touch, again upon Tweaker's suggestion, was adding the air roll, which took barely ~20 minutes in the end.

In the end, it was a fun challenge.

Insight into the technical aspects of the hacking process

Contrary to what one may believe, no working disassembly was made of any of the MMDs within the Sonic CD ISO, though one or two were used as a reference at times. In essence, it is hex-hacking without a hex editor. Code that is to be inserted is written in ASM, and then assembled into bytecode. The location the code or data is inserted into is automatically detected by the program identifying routines by its bytecode. From here, the new code can be inserted into locations of known unused space. If a section of the code refers to routines or data from within the MMD, it has to be patched in addition after having been assembled. For the game to run this code, either existing jumps in game have to be hijacked or code be overwritten to point to the locations of the injected code, with the overwritten data being relocated at the jump destination to ensure correct program flow.

Source code is available upon request.

Credits

• qiuu & snkenjoi: main coding and implementation
• MKameX: sprites for the spindash of small Sonic
• MadEchidna: motivation through his $100 code bounty
• Tweaker: various suggestions and advice

External Links

Sonic CD++ Release Thread at Sonic Retro