TUHS

tuhs@tuhs.org

34 participants
6418 discussions

by Johnny Billquist

On 2018-06-18 14:17, Noel Chiappa wrote: > > The "separate" bus for the semiconductor memory is just a second Unibus > > Err, no. :-) There is a second UNIBUS, but... its source is a second port on > the FASTBUS memory, the other port goes straight to the CPU. The other UNIBUS > comes out of the CPU. It _is_ possible to join the two UNIBI together, but > on machines which don't do that, the _only_ path from the CPU to the FASTBUS > memory is via the FASTBUS. Ah. You and Ron are right. I am confused. So there were some previous PDP-11 models who did not have their memory on the Unibus. The 11/45,50,55 accessed memory from the CPU not through the Unibus, but through the fastbus, which was a pure memory bus, as far as I understand. You (obviously) could also have memory on the Unibus, but that would be slower then. Ah, and there is a jumper to tell which addresses are served by the fastbus, and the rest then go to the Unibus. Thanks, I had missed these details before. (To be honest, I have never actually worked on any of those machines.) Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt(a)softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol

6 years, 7 months

Re: [TUHS] core

by jnc＠mercury.lcs.mit.edu

> From: Clem Cole > My experience is that more often than not, it's less a failure to see > what a successful future might bring, and often one of well '*we don't > need to do that now/costs too much/we don't have the time*.' Right, which is why I later said a "successful architect has to pay _very_ close attention to both the 'here and now' (it has to be viable for contemporary use, on contemporary hardware, with contemporary resources)". They need to be sensitive to the (real and valid) concerns of people about who are looking at today. By the same token, though, the people you mention need to be sensitive to the long-term picture. Too often they just blow it off, and focus _solely_ on today. (I had a really bad experience with someone like that just before I retired.) They need to understand, and accept, that that's just as serious an error as an architect who doesn't care about 'workable today'. In retrospect, I'm not sure you can fix people who are like that. I think the only solution is to find an architect who _does_ respect the 'here and now', and put them in control; that's the only way. IBM kinda-sorta did this with the /360, and I think it showed/shows. > I can imagine that he did not want to spend on anything that he thought > was wasteful. Understandable. But see above... The art is in finding a path that leave the future open (i.e. reduces future costs, when you 'hit the wall'), without running up costs now. A great example is the QBUS 22-bit expansion - and I don't know if this was thought out beforehand, or if they just lucked out. (Given that the expanded address pins were not specifically reserved for that, probably the latter. Sigh - even with the experience of the UNIBUS, they didn't learn!) Anyway.. lots of Q18 devices (well, not DMA devices) work fine on Q22 because of the BBS7 signal, which indicates an I/O device register is being looked at. Without that, Q18 devices would have either i) had to incur the cost now of more bus address line transceivers, or ii) stopped working when the bus was upgraded to 22 address lines. They managed to have their cake (fairly minimal costs now) and eat it too (later expansion). > Just like I retold the Amdahl/Brooks story of the 8-bit byte and Amdahl > thinking Brooks was nuts Don't think I've heard that one? >> the decision to remove the variable-length addresses from IPv3 and >> substitute the 32-bit addresses of IPv4. > I always wondered about the back story on that one. My understanding is that the complexity of variable-length address support (which impacted TCP, as well as IP) was impacting the speed/schedule for getting stuff done. Remember, it was a very small effort, code-writing resources were limited, etc. (I heard, at the time, from someone who was there, that one implementer was overheard complaining to Vint about the number of pointer registers available at interrupt time in a certain operating system. I don't think it was _just_ that, but rather the larger picture of the overall complexity cost.) > 32-bits seemed infinite in those days and no body expected the network > to scale to the size it is today and will grow to in the future Yes, but like I said: they failed to ask themselves 'what are things going to look like in 10 years if this thing is a success'? Heck, it didn't even last 10 years before they had to start kludging (adding A/B/C addresses)! And ARP, well done as it is (its ability to handle just about any combo of protocol and hardware addresses is because DCP and I saw eye-to-eye about generality), is still a kludge. (Yes, yes, I know it's another binding layer, and in some ways, another binding layer is never a bad thing, but...) The IP architectural concept was to carry local hardware addresses in the low part of the IP address. Once Ethernet came out, that was toast. >> So, is poor vision common? All too common. > But to be fair, you can also end up with being like DEC and often late > to the market. Gotta do a perfect job of balance on that knife edge - like an Olympic gymnast on the beam... This is particularly true with comm system architecture, which has about the longest lifetime of _any_ system. If someone comes up with a new editor or OS paradigm, people can convert to it if they want. But converting to a new communication system - if you convert, you cut yourself off. A new one has to be a _huge_ improvement over the older gear (as TCP/IP was) before conversion makes sense. So networking architects have to pay particularly strong attention to the long term - or should, if they are to be any good. > I think in both cases would have been allowed Alpha to be better > accepted if DEC had shipped earlier with a few hacks, but them improved > Tru64 as a better version was developed (*i.e.* replace the memory > system, the I/O system, the TTY handler, the FS just to name a few that > got rewritten from OSF/1 because folks thought they were 'weak'). But you can lose with that strategy too. Multics had a lot of sub-systems re-written from the ground up over time, and the new ones were always better (faster, more efficient) - a common even when you have the experience/knowledge of the first pass. Unfortunately, by that time it had the reputation as 'horribly slow and inefficient', and in a lot of ways, never kicked that: http://www.multicians.org/myths.html Sigh, sometimes you can't win! Noel

6 years, 7 months

Re: [TUHS] core

by jnc＠mercury.lcs.mit.edu

> From: "Theodore Y. Ts'o" > To be fair, it's really easy to be wise to after the fact. Right, which is why I added the caveat "seen to be such _at the time_ by some people - who were not listened to". > failed protocols and designs that collapsed of their own weight because > architects added too much "maybe it will be useful in the future" And there are also designs which failed because their designers were too un-ambitious! Converting to a new system has a cost, and if the _benefits_ (which more or less has to mean new capabilities) of the new thing don't outweigh the costs of conversion, it too will be a failure. > Sometimes having architects being successful to add their "vision" to a > product can be worst thing that ever happened A successful architect has to pay _very_ close attention to both the 'here and now' (it has to be viable for contemporary use, on contemporary hardware, with contemporary resources), and also the future (it has to have 'room to grow'). It's a fine edge to balance on - but for an architecture to be a great success, it _has_ to be done. > The problem is it's hard to figure out in advance which is poor vision > versus brilliant engineering to cut down the design so that it is "as > simple as possible", but nevertheless, "as complex as necessary". Absolutely. But it can be done. Let's look (as an example) at that IPv3->IPv4 addressing decision. One of two things was going to be true of the 'Internet' (that name didn't exist then, but it's a convenient tag): i) It was going to be a failure (in which case, it probably didn't matter what was done, or ii) it was going to be a success, in which case that 32-bit field was clearly going to be a crippling problem. With that in hand, there was no excuse for that decision. I understand why they ripped out variable-length addresses (I was just about to start writing router code, and I know how hard it would have been), but in light of the analysis immediately above, there was no excuse for looking _only_ at the here and now, and not _also_ looking to the future. Noel

6 years, 7 months

Re: [TUHS] core

by jnc＠mercury.lcs.mit.edu

> From: Tony Finch <dot(a)dotat.at> > Was this written down anywhere? Alas, no. It was a presentation at a group seminar, and used either hand-drawn transparencies, or a white-board - don't recall exactly which. I later tried to dig it up for use in Nimrod, but without success. As best I now recall, the concept was that instead of the namespace having a root at the top, from which you had to allocate downward (and then recurse), it built _upward_ - if two previously un-connected chunks of graph wanted to unite in a single system, they allocated a new naming layer on top, in which each existing system appeared as a constituent. Or something like that! :-) The issue with 'top-down' is that you have to have some global 'authority' to manage the top level - hand out chunks, etc, etc. (For a spectacular example of where this can go, look at NSAP's.) And what do you do when you run out of top-level space? (Although in the NSAP case, they had such a complex top couple of layers, they probably would have avoided that issue. Instead, they had the problem that their name-space was spectacularly ill-suited to path selection [routing], since in very large networks, interface names [adddresses] must have a topological aspect if the path selection is to scale. Although looking at the Internet nowadays, perhaps not!) 'Bottom-up' is not without problems of course (e.g. what if you want to add another layer, e.g. to support potentially-nested virtual machines). I'm not sure how well Dave understood the issue of path selection scaling at the time he proposed it - it was very early on, '78 or so - since we didn't understand path selection then as well as we do now. IIRC, I think he was mostly was interested in it as a way to avoid having to have an asssignment authority. The attraction for me was that it was easier to ensure that the names had the needed topological aspect. Noel

6 years, 7 months

Re: [TUHS] core

by jnc＠mercury.lcs.mit.edu

> From: Dave Horsfall > idiots keep repeating those "quotes" ... in some sort of an effort to > make the so-called "experts" look silly; a form of reverse > jealousy/snobbery or something? It really pisses me off You've just managed to hit one of my hot buttons. I can't speak to the motivations of everyone who repeats these stories, but my professional career has been littered with examples of poor vision from technical colleagues (some of whom should have known better), against which I (in my role as an architect, which is necessarily somewhere where long-range thinking is - or should be - a requirement) have struggled again and again - sometimes successfully, more often, not. So I chose those two only because they are well-known examples - but, as you correctly point out, they are poor examples, for a variety of reasons. But they perfectly illustrate something I am _all_ too familiar with, and which happens _a lot_. And the original situation I was describing (the MIT core patent) really happened - see "Memories that Shaped an Industry", page 211. Examples of poor vision are legion - and more importantly, often/usually seen to be such _at the time_ by some people - who were not listened to. Let's start with the UNIBUS. Why does it have only 18 address lines? (I have this vague memory of a quote from Gordon Bell admitting that was a mistake, but I don't recall exactly where I saw it.) That very quickly became a major limitation. I'm not sure why they did it (the number of contact on a standard DEC connector probably had something do with it, connected to the fact that the first PDP-11 had only 16-bit addressing anyway), but it should have been obvious that it was not enough. And a major one from the very start of my career: the decision to remove the variable-length addresses from IPv3 and substitute the 32-bit addresses of IPv4. Alas, I was too junior to be in the room that day (I was just down the hall), but I've wished forever since that I was there to propose an alternate path (the details of which I will skip) - that would have saved us all countless billions (no, I am not exaggerating) spent on IPv6. Dave Reed was pretty disgusted with that at the time, and history shows he was right. Dave also tried to get a better checksum algorithm into TCP/UDP (I helped him write the PDP-11 code to prove that it was plausible) but again, it got turned down. (As did his wonderful idea for bottom-up address allocation, instead of top-down. Oh well.) People have since discussed issues with the TCP/IP checksum, but it's too late now to change it. One place where I _did_ manage to win was in adding subnetting support to hosts (in the Host Requirements WG); it was done the way I wanted, with the result that when CIDR came along, even though it hadn't been forseen at the time we did subnetting, it required _no_ hosts changes of any kind. But mostly I lost. :-( So, is poor vision common? All too common. Noel

6 years, 7 months

Re: [TUHS] core

by Johnny Billquist

On 2018-06-18 04:00, Ronald Natalie <ron(a)ronnatalie.com> wrote: >> Well, it is an easy observable fact that before the PDP-11/70, all PDP-11 models had their memory on the Unibus. So it was not only "an ability at the lower end", b > That’s not quite true. While the 18 bit addressed machines all had their memory directly accessible from the Unibus, the 45/50/55 had a separate bus for the (then new) semiconductor (bipolar or MOS) memory. Eh... Yes... But... The "separate" bus for the semiconductor memory is just a second Unibus, so the statement is still true. All (earlier) PDP-11 models had their memory on the Unibus. Including the 11/45,50,55. It's just that those models have two Unibuses. Johnny -- Johnny Billquist || "I'm on a bus || on a psychedelic trip email: bqt(a)softjar.se || Reading murder books pdp is alive! || tryin' to stay hip" - B. Idol

6 years, 7 months

In Memorium: Alan Turing

by Dave Horsfall

We lost the Father of Computing, Alan Turing, on this day when he suicided in 1954 (long story). Just imagine where computing would've been now... Yes, there are various theories surrounding his death, such as a jealous lover, the FBI knowing that he knew about Verona and could be compromised as a result of his sexuality, etc. Unless they speak up (and they ain't), we will never know. Unix reference? Oh, that... No computable devices (read his paper), no computers... And after dealing with a shitload of OSs in my career, I daresay that there is no more computable OS than Unix. Sorry, penguins, but you seem to require a fancy graphical interface these days, just like Windoze. -- Dave Horsfall DTM (VK2KFU) "Those who don't understand security will suffer."

6 years, 7 months

Re: [TUHS] core

by jnc＠mercury.lcs.mit.edu

> From: Derek Fawcus <dfawcus+lists-tuhs(a)employees.org> > my scan of it suggests that only the host part of the address which were > extensible Well, the division into 'net' and 'rest' does not appear to have been a hard one at that point, as it later became. > The other thing obviously missing in the IEN 28 version is the TTL Yes... interesting! > it has the DF flag in the TOS field, and an OP bit in the flags field Yeah, small stuff like that got added/moved/removed around a lot. > the CIDR vs A/B/C stuff didn't really change the rest. It made packet processing in routers quite different; routing lookups, the routing table, etc became much more complex (I remember that change)! Also in hosts, which had not yet had their understanding of fields in the addresses lobotomized away (RFC-1122, Section 3.3.1). Yes, the impact on code _elsewhere_ in the stack was minimal, because the overall packet format didn't change, and addresses were still 32 bits, but... > The other bit I find amusing are the various movements of the port > numbers Yeah, there was a lot of discussion about whether they were properly part of the internetwork layer, or the transport. I'm not sure there's really a 'right' answer; PUP: http://gunkies.org/wiki/PARC_Universal_Packet made them part of the internetwork header, and seemed to do OK. I think we eventually decided that we didn't want to mandate a particular port name size across all transports, and moved it out. This had the down-side that there are some times when you _do_ want to have the port available to an IP-only device, which is why ICMP messages return the first N bytes of the data _after_ the IP header (since it's not clear where the port field[s] will be). But I found, working with PUP, there were some times when the defined ports didn't always make sense with some protocols (although PUP didn't really have a 'protocol' field per se); the interaction of 'PUP type' and 'socket' could sometimes be confusing/problemtic. So I personally felt that was at least as good a reason to move them out. 'Ports' make no sense for routing protocols, etc. Overall, I think in the end, TCP/IP got that all right - the semantics of the 'protocol' field are clear and simple, and ports in the transport layer have worked well; I can't think of any places (other than routers which want to play games with connections) where not having ports in the internetwork layer has been an issue. Noel

6 years, 7 months

Re: [TUHS] core

by jnc＠mercury.lcs.mit.edu

> From: Derek Fawcus > Are you able to point to any document which still describes that > variable length scheme? I see that IEN 28 defines a variable length > scheme (using version 2) That's the one; Version 2 of IP, but it was for Version 3 of TCP (described here: IEN-21, Cerf, "TCP 3 Specification", Jan-78 ). > and that IEN 41 defines a different variable length scheme, but is > proposing to use version 4. Right, that's a draft only (no code ever written for it), from just before the meeting that substituted 32-bit addresses. > (IEN 44 looks a lot like the current IPv4). Because it _is_ the current IPv4 (well, modulo the class A/B/C addressing stuff). :-) Noel

6 years, 7 months

Re: [TUHS] maybe off-topic: Unix on a microcontroller

by jnc＠mercury.lcs.mit.edu

> From: Johnny Billquist > It's a separate image (/netnix) that gets loaded at boot time, but it's > run in the context of the kernel. ISTR reading that it runs in Supervisor mode (no doubt so it could use the Supervisor mode virtual address space, and not have to go crazy with overlays in the Kernet space). Never looked at the code, though. Noel

6 years, 7 months

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TUHS