and open source had a treasured multiple-generation photocopy. Other follow-on effects of the license change included Andrew S. Tanenbaum creating Minix. As Tanenbaum wrote in Operating Systems (1987): 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 (). The reissue includes commentary from Michael Tilson (SCO), Peter Salus, Dennis Ritchie, 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. 22) at the end of a comment explaining the process exchange mechanism. It refers to line 325 of the file slp.c. The source code reads: /* * 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); } 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. Dennis Ritchie later explained the meaning of this remark: See also xv6, a modern reimplementation of Sixth Edition UNIX in ANSI C for multiprocessor x86 and RISC-V systems. References Further reading Andrew S. Tanenbaum, Operating Systems: Design and Implementation, (Prentice Hall, , June 1987) Code Critic (Rachel Chalmers, Salon, 30 November 1999) The Daemon, The GNU and the Penguin - Ch. 6: 1979 (Peter H. Salus) Brian W. Kernighan and Dennis Ritchie, The C Programming Language'', 1976 non-fiction books Unix books Computer programming books