Operating System Design/Case Studies/GNU Hurd
GNU Hurd is a kernel for GNU, a free Unix-like operating system developed by the GNU Project. Hurd was initiated in 1990 to add the only missing feature in the GNU operating system ‒ a kernel. Although under development for over 20 years, there is still no stable release.
It consists of a group of 24 server processes that run on the Mach microkernel. These servers provide most OS features, from authentication to filesystems.