TUHS

tuhs@tuhs.org

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by Warren Toomey

[ This whole article is just a flight of fancy; feel free to ignore it or at least treat it as the whimsy that it is. ] The PDP-7 Unix system is the first step on the evolution of Unix as we know it. We have a snapshot of the system around the end of 1970/beginning of 1971 at http://www.tuhs.org/Archive/PDP-11/Distributions/research/McIlroy_v0/ and a reconstructed and working partial system at https://github.com/DoctorWkt/pdp7-unix PDP-7 Unix was a playground for Ken, Dennis and others to try out ideas and implementations, and it was quickly superseded by 1st Edition PDP-11 Unix. Details of how it evolved are at https://www.bell-labs.com/usr/dmr/www/hist.html and https://www.bell-labs.com/usr/dmr/www/chist.html All fine and good. However, I keep wondering, how far could they have taken Unix on the PDP-7 platform? The Kernel ---------- The reconstructed kernel only occupies 3070 words of 4096 words available, so there is room left for more code. There is already an alternative reconstruction where the "dd" concept has been replaced with the ".." concept (see https://github.com/DoctorWkt/pdp7-unix/tree/master/src/alt) PDP-7 Unix doesn't have the concept of absolute or relative filenames (e.g. /usr/bin/ls or a/b/c or ../../file), Could the nami kernel function be modified to do this? It would probably mean changing from two characters packed into a word to a single character per word (to make searching for '/' easier), and this would turn it into something more recognisable as Unix. What about pipes? They should not be too hard to implement. Even sixteen pipes with a 16-word buffer each would only be 256+ extra data words in the kernel. And a hundred words of code? There are only a few special devices in the kernel: ttyin, ttyout, keyboard, display, pptin, pptout. What about a disk block device? Was there a PDP-7 tape device, and if so, why not a tape driver and block device? Filesystem ---------- The implementation of the filesystem is, in some places, quite inefficient. The free block list is implemented as follows. In each block, there are 10 free block numbers then a pointer to the next part of the free list. However, each block can hold 64 block numbers, so why are only 10 free block numbers stored in each block? By using the whole of a block to store free block numbers, there would actually be more free blocks to use! Each i-node (size 12 words, 7 of which are direct or indirect pointers) has one word which holds a unique value. This doesn't seem to be used at all. If it was reused as a block pointer, this would allow files to be up to 8*64=512 (small) or 8*64*64=32768 (large) words in length, instead of 7*64=448 words (small) or 7*64*64=28672 (large) words. The system is set up to only use one side of the two-sided disk device. It looks like the other side was used to backup (snapshot) a working system in case of catastrophic filesystem corruption: they could simply copy the blocks from the "snapshot" side back to the working side. We could double the size of the filesystem quite easily. Macro Assembler --------------- The kernel is written using fairly tight assembly code, and there probably isn't a way to translate it into a high-level language. The PDP-7 has an arcane instruction set, and the existing assembler syntax is nothing special. What about a macro assembler that makes it easier to write code, especially readable code? Here are some ideas based on the existing kernel: u.rq := 8 ==> lac 8; dac u.rq function swap ==> swap: 0 { return; jmp swap i } subroutine .fork ==> fork: line 1 // i.e. not a function { line 1 } loop ==> 1: { } jmp 1b if (sad dm1) ==> sad dm1 { jmp 1f code1 code1 } else { 1: code2 code2 } betwen(o101,o132) ==> jms betwen; o101; o132 There are probably a bunch more that could be added. The aim is to make the control structures easier to read and write. The programmer still has to grok the PDP-7 instruction set. B (or other) Language --------------------- PDP-7 Unix has a B compiler while compiles source down to a virtual instruction set which is interpreted by the B interpreter. We have the B interpreter code, and Robert Swierczek managed to rewrite the B compiler, see https://github.com/DoctorWkt/pdp7-unix/blob/master/src/other/b.b At first glance, the PDP-7 architecture is not that amenable to high level languages, but it turns out that it is indeed possible to write a compiler for a C subset that targets the PDP-7, see https://github.com/DoctorWkt/a-compiler. So, could the B compiler be modified to actually output PDP-7 assembly code? If so, we could rewrite the utilities (cp, mv, ls etc.) in a high-level language and make the system easier to maintain. I would recommend treating int and char as the same and only storing one char per word. And then, even though the PDP-7 architecture doesn't support it, how hard would it be to add int/char types and bring the language one step closer to C? Conclusion ---------- All of this is pie in the sky. It can certainly be done, but a) who has the time and b) it would be a "tour de force", nothing really useful. But imagine if, at the beginning of 1970, Unix had a proper B or C compiler, utilities written in this high-level language, a kernel written in a semi-high level language, and a system with pipes and proper pathnames. Cheers, Warren

8 years, 4 months

Leap Second

by Dave Horsfall

Let me be the first to say that the International Earth Rotation Service has announced that there will be a Leap Second inserted at 23:59:59 UTC on the 31st December, due to the earth slowly slowing down. It's fun to listen to see how the time beeps handle it; will your GPS clock display 23:59:60, or will it go nuts like my last one did (I had to power cycle the thing)? My recording of the last one: horsfall.org/leapsecond.webm . -- Dave Horsfall DTM (VK2KFU) "Those who don't understand security will suffer."

8 years, 4 months

tuhs@minnie.tuhs.org Re: User maintained programs in the Second Edition

by Doug McIlroy

> So was "/usr/bin" initially only for user-contributed binaries, or was it from its inception a place for binaries that were not essential for system boot and could not fit in the root partition? The latter is my understanding, but early on the two interpretations would have been nearly coextensive. Remember, though, that even Ken wrote some "user-contributed" code. Doug

8 years, 4 months

Re: [TUHS] phototypesetter C (was nm on Third Edition .o files)

by jnc＠mercury.lcs.mit.edu

> I don't think I ever heard the appellation "phototypesetter C" > before. Interesting data point; thanks for passing that along. > Certainly C and the phottypesetter developed independently, though in > the same room. But the explanation that they got linked by appearing in > the same tape release makes perfect sense. I have this vague memory of being told, or reading somewhere, that many of the changes from 'vanilla V6' C to 'phototypsetter' C were added because they were needed for that project, hence the name. Alas, I have no idea where I might have gotten that from (I had a quick look at a few likely documentary sources, but no joy). It's quite possible that this was a supposition on someone outside Bell's part (or perhaps inside Bell, but outside the Unix group), because the two came out in the same tape. Reading the notes about the upgrades (in particular, "newstuff.nr") makes it seem like a more likely driver of _some_ of the changes was the Interdata port (which was also happening at around the same time, if I have the timeline correct). And of course some might have been driven by general utility (e.g. the ability to initialize structures). It would be interesting to see if anyone remembers why these changes were made. Noel

8 years, 4 months

Validating the s2-bits tape epoch

by Diomidis Spinellis

The document http://www.tuhs.org/Archive/PDP-11/Distributions/research/1972_stuff/Readme discusses the uncertainty regarding the epoch used for the file timestamps. "The biggest problem here is to pin down the epoch for the files. In the early version of UNIX, timestamps were in 1/60th second units. A 32-bit counter using these units overflows in 2.5 years, so the epoch had to be changed periodically, and I believe 1970, 1971, 1972 and 1973 were all epochs at one stage or another." "Given that the C compiler passes, and the library, are dated in June of the epoch year, and that Dennis has said ``1972-73 were the truly formative years in the development of the C language'', it's therefore unlikely that the epoch for the s2 tape is 1971: it is more likely to be 1972. The tape also contains several 1st Edition a.out binaries, which also makes it unlikely to be 1973." "Therefore, Warren's decoding of the s2-bits file, in s2-bits.tar.gz, uses 1972 as the epoch. However, Dennis decoding in s2.tar.gz uses 1973." "Finally, the date(1) a.out on the tape uses 1971 as its archive. How annoying! After a bit of discussion, Dennis and Warren have agreed that 1972 is the most probable epoch for s2-bits." I thought I could validate the epoch by looking at the distribution of weekdays for the three alternative years (1971 to 1973). Here are the results. wget http://www.tuhs.org/Archive/PDP-11/Distributions/research/1972_stuff/Readme for guess in 1971 1972 1973 ; do echo $guess EPOCH=$(date +'%s' -d "$guess/01/01 00:00 UTC") awk '/\/core/,/\/etc\/init/ { if ($9) print strftime("%a", '$EPOCH' + $9 / 60)}' Readme | sort | uniq -c | sort -n done 1971 1 Sat 6 Mon 8 Thu 8 Tue 17 Fri 21 Wed 34 Sun 1972 1 Sun 6 Tue 8 Fri 8 Wed 17 Sat 21 Thu 34 Mon 1973 1 Tue 6 Thu 8 Fri 8 Sun 17 Mon 21 Sat 34 Wed As you can see, unless weekends at the Bell Labs were highly atypical, 1972 has the most probable distribution of work among the days of the week.

8 years, 4 months

Utility of C changes (Was: phototypesetter C)

by jnc＠mercury.lcs.mit.edu

> of course some [of the changes to C in this time period] might have been > driven by general utility (e.g. the ability to initialize structures). I was thinking about this, with my memory of the changes refreshed by re-reading those 'changes to C' notes, and it's interested how many of them were ones I personally (and most of the people working with me) didn't use. Here is a list of the changes described in those 3 documents: 'newstuff': - Longs - Unsigneds - Blocks (locals declared inside a non-top-level {} pair) - Initialization of structures - Initialization of automatic variables - Bit fields - Macro functions - Macro conditionals (#ifdef) - Arguments in registers - Typedefs - 'Static' scope 'Changes': - Multi-line macros - Undefine - Conditional expressions (#if) - Unions - Casts - Sizeof() on abstractions - '=' in initializations - Change binary operators from trailing to leading - 'extern' does not allocate storage (This note also includes unsigneds, blocks, and structure initializations, from the earlier? note.) 'cchanges': - Structure assignment and argument/return-value - Enum Of these, I never really used quite a few: blocks, automatic initializations, typedefs, unions, structure assignment/etc, or enum. I'm not sure if I ever used bit fields, either. Some of these are understandable; e.g. automatic initializations are just syntactic sugar (as are register arguments, but I did use those). Typedef is also effectively syntactic sugar; you can always use a macro and get almost (entirely?) the same result. In fact, I devised an entire system of types to make the code I was working on (almost entirely networking code, so lots of packet headers from other machines, etc) more rigorous - and later it turned out it had made it much more portable; it all used macros, not typedef. I don't think I ever used typedef... (The details of that might be of some interest: instead of int, long, etc we had things of the form {type}{len}, where {type}pe} was 'int', 'uns', 'bit', etc and length was 'b', 's', 'l', or two other interesting ones 'w' and 'f' - 'w' meant the machine's native word length, and 'f' meant whatever was fastest on the machine. So 'unsl' mean '32-bit unsigned'. Depending on the machine, the compiler couldn't always produce them all - e.g. the PDP-11 didn't have unsl - so sometimes you had to live with non-optimal replacements. There were also un-typed types, i.e. 'byte', 'swrd', 'lwrd' - 8, 16 and 32 bits - and 'word' - the machine's native length.) Unions didn't get used much either, in our stuff, although one would think it would be useful in network code - you get a packet with a pile of bits inside it, which can be one of N different formats, seems like a perfect application for a union. The problemis that it tied two different subsystems intimately together. If you have protocol A and protocol B, if you use a union to define the header format, the union has to have both A and B in it. Now if you want to add protocol C, that requires modifying that union definition. It was much easier to just take a pointer to the outer packet's data area, and assign (with cast) it to a new pointer variable which was of the correct type for the header you were trying to process. Some of the new things were incredibly useful, though - or, in fact, one couldn't get by without them. Casts were incredibly useful once the compiler got pickier. Initialization of structures was huge - other than 'bdevsw'-like hacks, there was just no other way to do that. Noel

8 years, 4 months

Re: [TUHS] User maintained programs in the Second Edition

by Doug McIlroy

> Does anyone remember whether at the time these were installed in the > system-wide /bin directory, Those that I remember were in /usr/bin. Doug

8 years, 4 months

Re: [TUHS] Dennis' Draft of the Unix Timesharing System: not so draft?

by jnc＠mercury.lcs.mit.edu

> From: Warren Toomey > Ritchie, D.M. The UNIX Time Sharing System. MM 71-1273-4. > which makes me think that the draft version Doug McIlroy found Not really a response to your question, but I'd looked at that 'UnixEditionZero' and was very taken with this line, early on: "the most important features of UNIX are its simplicity [and] elegance" and had been meaning for some time to send in a rant. The variants of Unix done later by others sure fixed that, didn't they? :-( On a related note, great as my respect is for Ken and Doug for their work on early Unix (surely the system with the greatest bang/buck ratio ever), I have to disagree with them about Multics. In particular, if one is going to have a system as complex as modern Unices have become, one might as well get the power of Multics for it. Alas, we have the worst of both worlds - the size, _without_ the power. (Of course, Multics made some mistakes - primarly in thinking that the future of computing lay in large, powerful central machines, but other aspects of the system - such as the single-level store - clearly were the right direction. And wouldn't it be nice to have AIM boxes to run our browers and mail-readers in - so much for malware!) Noel

8 years, 4 months

User maintained programs in the Second Edition

by Diomidis Spinellis

The Second Edition manual has a section titled "User Maintained Programs" listing the following utilities and games: basic, bc, bj, cal, chash, cre, das, dli, dpt, moo, ptx, tmg, and ttt. In the Introduction, the reader is asked to consult their authors for more information. Does anyone remember whether at the time these were installed in the system-wide /bin directory, or whether they were only available in their owners' home directories?

8 years, 4 months

14 new TUHS members

by Warren Toomey

All, I've just got back from a few days away to find 14 new subscription requests to the TUHS mailing list. Welcome aboard to you all. Normally I only get one request a month, so I have some concerns about the legitimacy of all these requests, so accept my apologies in advance if there is any off-topic e-mails in the next few days. P.S The mail while I was away was very interesting. Noel, you might also be interested in the B interpreter and Robert Swierczick's B compiler the PDP-7 Unix. The original B compiler doesn't exist, so Robert took the 11/20 C compiler and "undid" the code that does types so that it "became" a B compiler. https://github.com/DoctorWkt/pdp7-unix/tree/master/src/other Cheers all Warren

8 years, 4 months

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