Lions' Commentary on UNIX 6th Edition, with Source Code

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Lions' Commentary on UNIX 6th Edition, with Source Code by John Lions (1976) contains the complete source code of the 6th Edition Unix kernel plus a commentary. It is commonly referred to as the Lions book. Despite its age, it is still considered an excellent commentary on simple but high quality code.

For many years, the Lions Book was the only Unix kernel documentation available outside Bell Labs. Although the license of 6th Edition allowed classroom use of the source code, the license of 7th Edition specifically excluded such use, so the book spread by samizdat. It was commonly held to be the most copied book in computer science.

The book was reprinted in 1996 by Peer-To-Peer Communications.



Unix Operating System Source Code Level Six is the kernel source code, lightly edited by Lions to better separate the functionality — system initialization and process management, interrupts and system calls, basic I/O, file systems and pipes and character devices. All procedures and symbols are listed alphabetically with a cross reference.

The code as presented will run on a PDP-11/40 with RK-05 disk drive, LP-11 line printer interface, PCL-11 paper tape writer and KL-11 terminal interface, or a suitable PDP-11 emulator, such as SIMH.

A Commentary on the Unix Operating System starts with notes on Unix and other useful documentation (the Unix manual pages, DEC hardware manuals and so on), a section on the architecture of the PDP-11 and a chapter on how to read C programs. The source commentary follows, divided into the same sections as the code. The book ends with suggested exercises for the student.

As Lions explains, this commentary supplements the comments in the source. It is possible to understand the code without the extra commentary, and the reader is advised to do so and only read the notes as needed. The commentary also remarks on how the code might be improved.


The source code and commentary were originally produced in May 1976 as a set of lecture notes for Lions' computer science courses (6.602B and 6.657G) at the University of New South Wales Department of Computer Science.

UNIX News March 1977 announced the availability of the book to Unix licensees. Lions had trouble keeping up with its popularity, and by 1978 it was available only from Bell Labs.

When AT&T announced Unix Version 7 at USENIX in June 1979, the academic/research license no longer automatically permitted classroom use. Thus, licensees were no longer able to use the Lions notes for classes on operating systems.

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