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Sonic CD Plus Plus

From Sonic Retro

Revision as of 10:46, 22 June 2010 by Shobiz (talk | contribs) (Oops, missed the extra newline at the top)
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Sonic CD Plus Plus
Last release: 20 June 2009
System: Sega Mega CD
Original game: Sonic the Hedgehog CD
Credits: qiuu, snkenjoi

SonicCD++ (or Sonic CD Plus Plus) is a hack of Sonic the Hedgehog CD by qiuu and snkenjoi, adding the Sonic the Hedgehog 2 (16-bit) style Spin Dash and the newer Homing Attack into the game. This enables the gameplay to be significantly speedier, while Sonic CD is known to be one of the more slow-paced games in the classic Sonic series.

New features

Development history

Everything started with snkenjoi asking qiuu whether he had attempted making a Sonic the Hedgehog CD hack yet (which he had of course not), upon which qiuu brought up Mad Echidna's
Sonic Retro
offer from a year back
to pay $100 to the first person to put the Spin Dash from Sonic the Hedgehog 2 (16-bit) 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 image a lot easier. Being done with that, qiuu set up an SVN repository marking the starting point of the two's collaboration. With the Spin Dash already halfway implemented, qiuu implemented the Homing Attack, which turned out to be significantly less difficult to add. 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 fit them in! The authors thank Tweaker for helping with this by mentioning the unused Sonic sprites, and MKAmeX for contributing Spin Dash sprites for the shrunken form Sonic assumes during some of Metallic Madness Zone.

Despite only taking around a week 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 MMD files within the Sonic CD ISO image, though one or two were used as a reference at times. In essence, the technique that was used is hex hacking without a Hex editor: Code that is to be inserted is written in assembly language, and then assembled into bytecode. The location the code or data is inserted into is automatically detected by the program, which identifies routines by their 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 additionally has to be patched after having been assembled. For the game to run this code, either existing jumps in game have to be hijacked, or code must 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

Download

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Download Sonic CD++
Multiple downloads available

US version (18.51 MB) (info)
EU version (17.2 MB) (info)
JP version (17.2 MB) (info)

External links