Seminal time-sharing OS Multics - the Multiplexed Information and Computing Service - has been resurrected in a new simulator.
As The Register reported in 2011, Multics' sprang from MIT's decision to eschew an IBM mainframe, buy one from GE instead and write an OS for the machine. The operating system's source code was released in 2007, when we noted Multics' place in history as one of the first OSes “...to introduce concepts such as a hierarchical file system and dynamic linking. It was also the first to use the modern standard of per-process stacks in the kernel, with a separate stack for each security ring.”
As our own Liam Proven wrote back in 2011, “Unix was conceived as a sort of anti-Multics – 'Uni' versus 'multi', geddit? Unix was meant to be small and simple, as opposed to the large, complicated Multics. Consider the labyrinthine complexity of modern Unix and ponder what Multics must have been like.”
Ponder no more! The Multicians site offers links to the new simulator here, promising “a complete multi-user operating system, running on a simulated Honeywell DPS8M processor.”
A few tweaks have been made to Multics, notably support for dates in the 21st century. The folks behind the simulation say it's sufficiently robust that “Several shared systems have provided stable service for months, shutting down only to install new releases.” ®