The first part of this text consists of the C source code of an old UNIX kernel (UNIX 6). The second part provides explanations about this code. The explanations (as well as the source code) start at physical address 0 and cover essentials such as (the PDP-11 equivalent to) paging, context/coroutine switches, device drivers, system calls. The treatment is in-depth; I read the second part sequentially and some pages took me over an hour to work through (with constant shifting back and forth between the first and second part).

Besides multicore, I would say that UNIX 6 already contains all essential parts of modern operating systems. Due to its simplicity, I thus found it well suited to learn about the fundamental workings of a real kernel, still today after over 40 year. It is also amazing how much of the concepts from this original UNIX version have survived into todays UNIX variants. A text from a time when operating systems (UNIX 6) and programming languages (C) had not yet parted.

To mention something negative, the author occasionally asks questions where it is not clear whether this is a point where author himself does not know the answer, or if he leaves the answer as an exercise to reader. Also, from a contemporary perspective, I found the use of the PDP-11 architecture annoying. ( )

The automatic entry says there are 254 pages, and I'm reluctant to count them. There are no page numbers. It's close to what most of us held in our hands, in photocopies of photocopies of photocopies. I had access to a 14-generation copy. I didn't own it, but at least I knew someone who had one.

When Peter Salus (and others) finally managed to get all the obstacles out of the way, and have this republished, in 1996, I ordered it from Amazon as an advance copy, holding my breath until it was finally published, and sent to me. I'm glad that it was published again, while John still lived. He was ill, even while the publication of this was going forward.

This work has (as Peter Salus pointed out in his book, "A Quarter Century of UNIX") one of the best comments ever written in code.

From Sheet 22:
/*
2238 * You are not expected to understand this.
2239 */
2240 if(rp->p_flag&SSWAP) {
2241 rp->p_flag =& ~SSWAP;
2242 aretu(u.u_ssav);
2243 }

RIP John. Thank you. ( )