FUZIX is an actively developed fork of the UZI operating system which was one of the smallest UNIX-alike operating systems of the mid-1980’s, dedicated to run on the Z80 CPU.
FUZIX was ported to many of the popular Z80-based hardware platforms, eventually it went beyond the Z80 world and now supports more of the 8-, 16- and even 32-bit (e.g. arm-v7m, m68k) architectures.