Lions Book N. "Source Code And Commentary On Unix Level 6", By John Lions.

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Lions Book n.

"Source Code and Commentary on Unix
level 6", by John Lions. The two parts of this book contained (1)
the entire source listing of the Unix Version 6 kernel, and (2) a
commentary on the source discussing the algorithms. These were
circulated internally at the University of New South Wales
beginning 1976-77, and were, for years after, the only
detailed kernel documentation available to anyone outside Bell
Labs. Because Western Electric wished to maintain trade secret
status on the kernel, the Lions Book was only supposed to be
distributed to affiliates of source licensees. In spite of this,
it soon spread by samizdat to a good many of the early Unix

[1996 update: The Lions book lives again! It was put back in print
as ISBN 1-57398-013-7 from Peer-To-Peer Communications, with
forewords by Dennis Ritchie and Ken Thompson. In a neat bit of
reflexivity, the page before the contents quotes this entry.]