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-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.
Star Ocean and Street Fighter Alpha 2 were the only games that used this chip. It also served as a copy protection chip that made these titles extremely difficult to emulate.
Read more about this topic: List Of Super NES Enhancement Chips