KeyKOS

KeyKOS is a persistent, pure capability-based operating system for the IBM S/370 mainframe computers. It allows emulating the VM, MVS, and POSIX environments. It is a predecessor of the Extremely Reliable Operating System (EROS), and its successors, the CapROS and Coyotos operating systems. KeyKOS is a nanokernel-based operating system.
The development of KeyKOS started under the name GNOSIS in Tymshare, Inc. during the 1970s and continued under the name KeyKOS after being bought by Key Logic.