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S-DD1

The S-DD1 chip is a powerful ASIC decompressor made by Nintendo for use in some Super Nintendo Entertainment System Game Paks. Designed to handle data compressed by ABS Lossless Entropy Algorithm, a form of arithmetic coding developed by Ricoh, its use was necessary in games where massive amounts of sprite data had to be compressed with a 32 or 48 megabit data limit in mind. This data is decompressed on-the-fly by the S-DD1 and given directly to the picture processing unit.

The S-DD1 mediates between the Super NES's core CPU (the Ricoh 5A22) and the game's ROM via two buses. However, the controlling 5A22 processor may still request normal, uncompressed data from the game's ROM even if the S-DD1 is already busy with a decompression operation. This form of parallelism allows sprite data to be decompressed while other types of data are quickly passed to the main CPU.

It also served as a de facto copy protection that made Star Ocean and SFA2 extremely difficult to pirate and/or emulate for years.

List of games that use the S-DD1 chip

• (1996) Star Ocean
• (1996) Street Fighter Alpha 2

References

• Snes9x Development Forum
• Andreas Naive's documentation of how the chip and ABS algorithm work