Xv6

xv6 is a modern reimplementation of Sixth Edition Unix in ANSI C for multiprocessor x86 and RISC-V systems. It was created for pedagogical purposes in MIT's Operating System Engineering course.
Purpose.
MIT's Operating System Engineering course formerly used the original V6 source code. xv6 was created as a modern replacement, because PDP-11 machines are not widely available and the original operating system was written in archaic pre-ANSI C. Unlike Linux or BSD, xv6 is simple enough to cover in a semester, yet still contains the important concepts and organization of Unix.
Self-documentation.
One feature of the Makefile for xv6 is the option to produce a PDF of the entire source code listing in a readable format. The entire printout is only 99 pages, including cross references. This is reminiscent of the original V6 source code, which was published in a similar form in "Lions' Commentary on UNIX 6th Edition, with Source Code."
Educational use.
xv6 has been used in operating systems courses at many universities including
University of Leeds, Stony Brook University, Northwestern University,
The George Washington University, Northeastern University, Yale University, Columbia University, Ben-Gurion University, Johns Hopkins University, Portland State University, Tsinghua University,
Southern Adventist University, the University of Wisconsin–Madison, Binghamton University, the University of Utah,
 University of California, Irvine, University of California, Riverside, Georgia Tech, IIIT Hyderabad, IIIT Delhi, IIIT Bangalore, IIIT Allahabad, MNNIT Allahabad IIT Bombay, IIT Delhi, IIT Madras, IIT Bhubaneswar and PEC Chandigarh in India,
the Linnaeus University in Sweden,
the University of Otago in New Zealand,
the National University of Córdoba, the National University of Río Cuarto, in Argentina,
the Federico Santa María Technical University in Chile, the Università degli Studi di Palermo University of Illinois at Chicago, Rutgers University, the Università degli Studi di Modena e Reggio Emilia in Italy, the Iran University of Science and Technology (IUST), and the University of Tehran in Iran, Slovak University of Technology in Bratislava in Slovakia, Federal University of Minas Gerais in Brazil, the 
Technion – Israel Institute of Technology, the Ben Gurion University In The Negev in Israel, and the University of Pittsburgh.
External links.
x86 (unmaintained):
RISC-V: