The Computers: Software: Operating Systems: Realtime: MenuetOS: (KolibriOS MenuetOS )

KolibriOS is an operating system (OS) for x86 PCs. It was originally a MenuetOS distribution, with some improvements, by a Russian developer team. It has now become a MenuetOS fork project, by the KolibriOS Team. Both OSs are written in the Assembly programming language FASM. But C, Free Pascal, and Forth can be used for user programs. KolibriOS has a GNU General Public License, so it is free, open source software. KolibriOS MenuetOS Realtime Operating Systems Software Computers.

In a review piece on alternative operating systems (2009), Tech Radar called it "tremendously impressive" noting its performance and streamlined code-base. (wikipedia)