Multics

Multics (Multiplexed Information and Computing Service) is a time-sharing operating system originally designed by MIT, Bell Labs and General Electric for the GE645 mainframe computer. Development was started in 1965. It is considered one of the most influential operating systems of all time, and it was the main inspiration for Unix.

Features

 * Multics was the first operating system to support a hierarchical file system. In addition, it supported long names and multiple names for segments, as well as symbolic links.
 * Dynamic linking is introduced. It allows processes to make external subroutines available to it just by loading the segment it's contained in.
 * On-line reconfiguration is supported, the CPU, memory and disks could be added or removed while the operating system kept running.
 * Several modern security systems were first implemented in Multics, and it had support for ring-based security.
 * It was one of the first operating systems to be written in a high-level programming language, known as PL/I.