The MSU1 is an SNES coprocessor enhancement, which adds mass storage, streaming video and CD-quality audio capabilities to the base system.
Unlike other coprocessors, I have created the MSU1 specification. It is in that sense, unofficial. It is meant to be utilized to create new software, or to enhance existing software.
The goal was to see what kind of software could be produced if the fabled SNES-CD attachment were ever brought to market. However, instead of attempting a feat as complex as engineering a complete system add-on, the MSU1 attempts to be as simplistic as possible. Rather than reply on video and audio codecs, MSU1 favors raw, uncompressed data. It relies on the realities of cheap storage in place of advanced codecs; and it relies on the combination of existing coprocessors to enhance computational ability. The intent of this simplification and flexibility in the specification is to allow a maximum number of potential hardware and software implementations of the specification.
Currently, support for the MSU1 is available both under emulation and on real SNES hardware.
higan/bsnes provides the canonical implementation under emulation.
sd2snes provides hardware support in the form of a flash cart utilizing SD cards for storage.
Mass storage allows the MSU1 to create games that smash the small ROM size limitations of existing games. No longer are you limited to a mere 4MB of ROM, you now have up to 4GB of data storage available. Games can be created with seemingly unlimited levels, innumerable textures, etc.
Streaming video utilizes mass storage to allow full-motion video sequences to be played on the SNES.
CD-quality audio gives us the ability to play fully orchestral music with no quality loss. It could alternatively be used for full voice acting support.
When enhancing existing games, enhanced soundtracks that far exceed the SNES' audio capabilities could be inserted into the games, full-motion animation sequences could be taken from CD-system ports (Mega CD, PCE-CD, PC-FX, PS1, etc) and added back to the SNES originals. You might imagine Chrono Trigger with all of the PS1 animation sequences running on the SNES with no loading delays. The best of both worlds.