Nachos Operating System was developed by Thomas Anderson at University of Washington. Its first release came in 1992.
This is my implementation of Nachos, with the following modifications:

1.Implemented various system calls such as sleep(), exec(), join(), fork(), read(), write() and exit().
2.Implemented semaphores, thread scheduling and process management routines, 
  extending functionality of Nachos Operating System to handle Multithreading and User Programming.
3.Extended Nachos OS to implement Round Robin Scheduling, Multi Feedback Scheduling.
4.Built a Console Driver for Nachos.
5.Extended the functionality of the Nachos File system, implementing File Extension, Directory Structure,  Single and 
  Double Indirect Blocks.
6.Extended Nachos to support demand paged, memory mapped files by adding two new system calls- Mmap() and
  Munmap().

More information can be found at this link:
https://en.wikipedia.org/wiki/Not_Another_Completely_Heuristic_Operating_System