mpg123 appeared after the first MP3 player for UNIX, MAPlay, and quickly came into widespread use, as it relied heavily on assembler optimizations to perform the iDCT part of the MPEG decoding quicker than any other MP3 player. Today, mpg123 contains assembler optimizations for almost every popular CPU on the market. This makes it probably the most resource-efficient MP3 player and library there is. The core of mpg123 is used by XMMS for MP3 playback.