MSU1, also named "Media Streaming Unit revision 1", is a homemade enhancement chip made by byuu for the SNES.
It allows the SNES to have 4 GB of storage space and CD quality Stereo Audio.
You can add MSU1 registers just by having <ROMname>.msu with the ROM. (since bsnes v081)
$2000 - Status (bit 7 - Data Port Busy?; bit 6 - Audio Port Busy?; bit 5 - Audio is repeating; bit 4 - Audio is playing; bit3-0 - Revision)
$2001 - Data Port (Gives byte from the position in 4GB Data)
$2002-$2007 - Identification "S-MSU1"
$2000-$2003 - 4GB 32-bit Offset value ($2003 must be written to seek)
$2004-$2005 - Choose Audio Track ($2005 must be written to seek)
$2006 - Audio Volume
$2007 - Play, Repeat Audio. (bit 0 = If 1, Track plays; bit 1 = If 1, Track repeats)