mpg123's Assembly code is optimized with SIMD instructions to improve the performance of the iDCT part of the MPEG decoding.

[3] The license for the last release (0.59r / 19 June 1999) from the original authors included restrictive (non-commercial, no-derivative) terms, although the source code was available.

This led to the creation of mpg321, a similar program (based on MAD) licensed under the GPLv2.

[4] Official development of mpg123 and its library mpglib stalled, and serious security holes emerged.

[5][6] Patches were applied downstream for packages provided by various Linux and BSD operating systems.