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((Short description|Book))
((More citations needed|date=April 2023))

((Infobox book
| name             = A commentary on the Unix operating system
| image            = Brian Kernighan in 2012 at Bell Labs 2.jpg
| caption          = [[Brian Kernighan]] holding a copy of Lions's ''Commentary''
| author           = [[John Lions]]
| country          = ((ubl|Australia ((small|(original)))|United States ((small|(1996 reprint)))))
| language         = English; also available in Chinese and Japanese
| subject          = [[Unix]] operating system
| genre            = [[Computer Science]]
| publisher        = [[University of New South Wales]]
| pub_date         = 1976
| pages            =
| isbn             = 
| oclc             = 36099640
| dewey            = 005.43
| congress         = QA 76.76 .O63 L56
))

'''''A Commentary on the Sixth Edition UNIX Operating System'''''  by [[John Lions]] (later reissued as '''''Lions' Commentary on UNIX 6th Edition''''')<ref name="wcCommentary">((cite book |url=http://www.lemis.com/grog/Documentation/Lions/ |title=''A commentary on the Sixth Edition UNIX Operating System'' |date=1976–1977 |first=John |last=Lions |publisher=University of New South Wales: Department of Computer Science |oclc=494684468))</ref><ref name="wcSource">((cite book |title=''Unix operating system source code level six'' |url=https://www.worldcat.org/oclc/494684460 |date=1976–1977 |publisher=University of New South Wales: Department of Computer Science|oclc=494684460 ))</ref> is a highly influential<ref> Jonathan Corbet [https://lwn.net/Articles/881431/ Brian Kernighan on the origins of Unix] LWN.net. January 17, 2022.</ref> 1976 publication containing analytical commentary on the [[source code]] of the [[Version 6 Unix|6th Edition]] [[Unix]] [[computer]] [[operating system]] "resident nucleus"<ref name=UN77-03>((cite magazine |url=https://archive.org/details/unix_news_march-1977/page/n3/mode/2up |title=NEW BOOK |magazine=UNIX News |publisher=USENIX |date=Mar 1977 ))</ref> (i.e., [[kernel (operating system)|kernel]]) [[software]], plus copy formatted and indexed by Lions, of said source code obtained from the authors at [[AT&T]] [[Bell Labs]].  It is also commonly referred to as the '''Lions Book'''.((citation needed|date=May 2021))

Itself an exemplar of the early success of UNIX as portable code for a publishing platform, Lions's work was typeset using UNIX tools, on systems running code ported at the University, similar to that which it documented, see ''page vi''.<ref name="lehey">((cite web |url=http://www.lemis.com/grog/Documentation/Lions/ |title=''Commentary on the Sixth Edition UNIX Operating System'' |date=2015-09-14 |first=Greg |last=Lehey |publisher=Greg Lehey, Warren Toomey))</ref>

It was commonly held to be the most copied book in [[computer science]].((citation needed|date=May 2021)) Despite its age, Lions's book is still considered an excellent commentary on simple, high quality code.

Lions's work was most recently reprinted in 1996 by Peer-To-Peer Communications,<ref name="978-1-57398-013-5"/> and has been circulated, recreated or reconstructed variously in a number of media by other parties, e.g. see webpage of Greg Lehey.<ref name="lehey" />

[[File:Lions Commentary on UNIX 6th Edition with Source Code.jpg|thumb|''Lions Commentary on UNIX 6th Edition with Source Code'' 1996 reissue<ref name="978-1-57398-013-5">((cite book |last1=Lions |first1=John |title=Lions' Commentary on UNIX 6th Edition with Source Code |date=1996 |publisher=Peer-to-Peer Communications |isbn=978-1-57398-013-5 |language=en))</ref>]]

==Synopsis==
''UNIX Operating System Source Code Level Six'' is the kernel source code, lightly edited by Lions to better separate the functionality&nbsp;— 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 [[RK05]] disk drive, LP11 line printer interface, PCL11 paper tape writer and KL11 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, [[Digital Equipment Corporation|DEC]] hardware manuals and so on), a section on the architecture of the PDP-11 and a chapter on how to read [[C (programming language)|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.

==History==
[[File:Johnlions.jpg|thumb|[[John Lions]] with his students in 1980]]
[[File:Lions Commentary Unix.jpeg|thumb|A Japanese reprinting of John Lions ''A commentary on the Unix operating system'' and accompanying formatted source code displaying a license for use by certain licensees, and a directive to those licensees to restrict use by other parties, ''Unix operating system source code level six'']]

The source code and commentary were printed in book form in 1977, after first being assembled in May 1976, as a set of lecture notes<ref name="UN77-03" /> for Lions's [[computer science]] courses (6.602B and 6.657G, mentioned in the introduction of the book) at the [[University of New South Wales]].

UNSW had actually obtained [[History of Unix#1970s|UNIX]] source code in 1975, in response to Ken Robinson's 1974 query to Dennis Ritchie at Bell.<ref>((cite web |title=Remembering Ken Robinson |first=(Dean) |last=Faculty of Engineering, UNSW |publisher=University of New South Wales: Engineering |date=2020-09-18 |archive-date=2020-10-20 |url=https://www.engineering.unsw.edu.au/computer-science-engineering/news-events/news/remembering-ken-robinson |archive-url=https://web.archive.org/web/20201020153121/https://www.engineering.unsw.edu.au/computer-science-engineering/news-events/news/remembering-ken-robinson ))</ref> Bell Labs was a subsidiary of AT&T, under restrictions owing to its monopolistic nature as a national telecommunications infrastructure provider, and not permitted to conduct business in any other field,((citation needed|date=May 2021)) and so was not at liberty to profit from sale of software,((citation needed|date=May 2021)) however UNIX was being provided under license by another AT&T subsidiary, [[Western Electric]] at least by 1977.((clarify|date=May 2021))

The UNIX User's group, [[USENIX]]'s newsletter, ''UNIX News'', of March 1977,<ref name="UN77-03" /> announced the availability of the book to UNIX licensees. Note that the newsletter's own strongly worded circulation restriction notice, could only ever have applied within the framework of existing licenses to the licensees with agreements held with the mentioned organisations, not to non-licensees, as a matter of civil contract—the newsletter displays no evidence of governmental authority of the type which might allow general suppression of circulation, such as national security [[Top Secret]] classification.)

Difficulty in keeping pace with the book's popularity, meant that by 1978 it was available only from AT&T [[Bell Labs]].<ref name="Daemon-GNU-Penguin">[https://web.archive.org/web/20100505073306/http://www.groklaw.net/article.php?story=20050414215646742 The Daemon, The GNU and the Penguin - Chapters 2 & 3] ([[Peter H. Salus]])</ref>

For many years, the Lions book was the only UNIX kernel documentation available outside Bell Labs.((citation needed|date=May 2021)) Although the license of 6th Edition((citation needed|date=May 2021)) allowed classroom use of the source code, the license of [[Version 7 Unix|7th Edition]]((citation needed|date=May 2021)) specifically excluded such use, so subsequent to this, the book, based on the more liberally licensed version, spread widely through copy machine reproductions, made arguably under various excuses, including (but not limited to) generous educational licensing terms afforded the publishing institution by the source code owner((citation needed|date=May 2021)), as well as various [[Copyright law of Australia|copyright]] exemptions protecting discussion of mathematical work, though in the shadow of increasing political pressure to erode such rights, as technological means to ‘self-copy’ -- and even self-publish—works became cheaper, more efficient, and more prolific.((citation needed|date=May 2021)) UNIX ''itself'', was one of these, having been a successful innovation financed at Bell in order to facilitate publishing of technical manuals in-house.((citation needed|date=May 2021))

When AT&T announced [[Version 7 Unix|UNIX Version 7]] at [[USENIX]] in June 1979, the academic/research license no longer automatically permitted classroom use.

However, thousands of computer science students around the world spread photocopies. As they were not being taught it in class, they would sometimes meet after hours to discuss the book. Many pioneers of UNIX and [[open-source software|open source]] had a treasured multiple-generation photocopy.<ref name=jargon>((cite web
| url=http://www.catb.org/~esr/jargon/html/L/Lions-Book.html
| title=Lions Book
| work=[[Jargon File]]
| date=December 29, 2003
| access-date=2020-01-11))</ref>

Other follow-on effects of the license change included [[Andrew S. Tanenbaum]] creating [[Minix]]. As Tanenbaum wrote in ''Operating Systems'' (1987):

((quote|When AT&T released Version 7, it began to realize that UNIX was a valuable commercial product, so it issued Version 7 with a license that prohibited the source code from being studied in courses, in order to avoid endangering its status as a trade secret. Many universities complied by simply dropping the study of UNIX, and teaching only theory.))

Various UNIX people, particularly [[Peter H. Salus]], [[Dennis Ritchie]] and Berny Goodheart, lobbied Unix's various owners (AT&T, [[Novell]], the [[Santa Cruz Operation]]) for many years to allow the book to be published officially. In 1996, the Santa Cruz Operation finally authorised the release of the twenty-year-old 6th Edition source code (along with the source code of other versions of "[[Ancient UNIX]]"), and the full code plus the 1977 version of the commentary was published by Peer-To-Peer Communications (((ISBN|978-1-57398-013-5))).<ref name="978-1-57398-013-5"/> The reissue includes commentary from Michael Tilson (SCO), Peter Salus, Dennis Ritchie, [[Ken Thompson (computer programmer)|Ken Thompson]], Peter Collinson, Greg Rose, [[Mike O'Dell]], Berny Goodheart and [[Peter Reintjes]].

=="You are not expected to understand this"==
The infamous program comment "You are not expected to understand this" occurs on line 2238 of the source code (''Lions' Commentary'', p.&nbsp;22) at the end of a comment explaining the process exchange mechanism. It refers to line 325 of the file slp.c.<ref>((cite web|url=https://github.com/dspinellis/unix-history-repo/blob/Research-V6/usr/sys/ken/slp.c#L325|title= unix-history-repo/usr/sys/ken/slp.c
|author=Ken Thompson|website= [[GitHub]]
|access-date=2018-10-12))</ref> The source code reads:<ref name="odd">((cite web |url=https://www.bell-labs.com/usr/dmr/www/odd.html |title=Odd Comments and Strange Doings in Unix|author=Dennis Ritchie |date=2002-06-22 |access-date=2021-05-14))</ref><ref>https://swtch.com/unix/</ref>

<syntaxhighlight lang="c">
        /*
         * If the new process paused because it was
         * swapped out, set the stack level to the last call
         * to savu(u_ssav).  This means that the return
         * which is executed immediately after the call to aretu
         * actually returns from the last routine which did
         * the savu.
         *
         * You are not expected to understand this.
         */
        if(rp->p_flag&SSWAP) {
                rp->p_flag =& ~SSWAP;
                aretu(u.u_ssav);
        }
</syntaxhighlight>

A major reason why this piece of code was hard to understand was that it depended on a quirk of the way the C-compiler for the [[PDP-11]] saved registers in procedure calls. This code failed when ported to other machines and had to be redesigned in [[Version 7 Unix|Version 7 UNIX]].<ref>((Cite journal |doi = 10.1002/j.1538-7305.1978.tb02141.x|title = UNIX Time-Sharing System: Portability of C Programs and the UNIX System |url=https://www.bell-labs.com/usr/dmr/www/portpap.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.bell-labs.com/usr/dmr/www/portpap.pdf |archive-date=2022-10-09 |url-status=live |journal = Bell System Technical Journal|volume = 57|issue = 6|pages = 2021–2048|year = 1978|last1 = Johnson|first1 = S. C.|last2 = Ritchie|first2 = D. M.|s2cid = 17510065 ))</ref> [[Dennis Ritchie]] later explained the meaning of this remark:<ref name="odd"/>

((quote|"You are not expected to understand this" was intended as a remark in the spirit of "This won't be on the exam", rather than as an impudent challenge.))

==Further reading==
* [[Andrew S. Tanenbaum]], ''Operating Systems: Design and Implementation'', ([[Prentice Hall]], ((ISBN|0-13-637331-3)), June 1987)
* [https://www.salon.com/1999/11/30/lions_2/ Code Critic] (Rachel Chalmers, ''[[Salon (website)|Salon]], 30 November 1999)
* [https://www.groklaw.net/article.php?story=20050502114023686 The Daemon, The GNU and the Penguin - Ch. 6: 1979] ([[Peter H. Salus]])
* [[Brian W. Kernighan]] and [[Dennis Ritchie]], ''[[The C Programming Language]]'', ((ISBN|0-13-110362-8))

==See also==
* [[xv6]], a modern reimplementation of [[Version 6 Unix|Sixth Edition UNIX]] in [[ANSI C]] for [[multiprocessing|multiprocessor]] [[x86]] and [[RISC-V]] systems.

==References==
((Reflist))

== External links ==
*https://pdos.csail.mit.edu/6.828/2006/v6.html
*https://dl.acm.org/doi/10.5555/238143

((DEFAULTSORT:Commentary on the UNIX Operating System, A))
[[Category:1976 non-fiction books]]
[[Category:Unix books]]
[[Category:Computer programming books]]

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