8 ms·

Thats an interesting alternative history thought experiment. I wonder how early one could capture the complexity of the full PDP-11 microarchitecture on a sing

by Aloha 3y ago

Thats an interesting alternative history thought experiment. I wonder how early one could capture the complexity of the full PDP-11 microarchitecture on a single chip? Would it have been affordable? What about the support hardware?

WalterBright 3y ago
If Intel could do it, DEC could do it.
- xxs 3y ago
  DEC Alpha had the weakest memory model, which while cool to write for with its all memory barriers/concurrency - it's quite annoying in the real world, a major reason while DEC failed.
- xxs 3y ago
  DEC Alpha had the weakest memory model, which while cool to write for with its all memory barriers/concurrency - it's quite annoying to work with in the real world, a major reason while DEC failed.
  - jacquesm 3y ago
    Compared to the madness that was x86 memory models the Alpha was quite sane. I'm not sure what you base your 'weakest' on (it suggests a comparison with others) but when Alpha was released there wasn't much to compare it to besides the R4000 and that just as hard if not harder to source than the Alpha machines were. I had a bunch of them (SGI boxes of various plumage) and the OS hardly took advantage of the chip, but on the Alpha it all just worked at 64 bits out of the box.
    - xxs 3y ago
      >I'm not sure what you base your 'weakest' Linux kernel has memory barriers, Alpha makes an exception with needing virtually all of them - with other architectures (esp. the total store order ones) some (most) of the barriers are nop. Yet, overall Alpha is famous for how crazy the memory model is. There is nothing like any longer (personally I am happy with Java Memory Model). Just to make sure it's understood "weak" attributed to :memory model: just means how concurrent with regards to reads and writes it is. A quote[0]: AND THEN THERE'S THE ALPHA -------------------------- The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that, some versions of the Alpha CPU have a split data cache, permitting them to have two semantically-related cache lines updated at separate times. This is where the address-dependency barrier really becomes necessary as this synchronises both caches with the memory coherence system, thus making it seem like pointer changes vs new data occur in the right order. [0]: https://www.kernel.org/doc/Documentation/memory-barriers.txt https://www.kernel.org/doc/Documentation/memory-barriers.txt
      - jacquesm 3y ago
        That's what allows the Alpha to be multi-processor. And if you like the 'Java Memory Model' I'm not sure what your point really is, you're comparing a virtual machine with actual hardware.
        xxs 3y ago
        >you're comparing a virtual machine with actual hardware. The memory model is meant for developers - and how many&different memory barriers they have to issue.
        jacquesm 3y ago
        Your typical Java developer has zero knowledge about this and on the UNIX versions that shipped with the Alpha (for me, at the time that was RedHat) fairly elegantly hid that complexity to the point that you could just forget about it unless you cared about extreme performance. In my case the ability to address large amounts of memory and a filesystem that wasn't limited by the 32 bit limitation was the key factor and while it took another decade for x86 to catch up I was happily shipping.
        xxs 3y ago
        >Your typical Java developer has zero knowledge about this That's bit much of stereotyping, depends I guess - "Java Concurrency in Practice" is one of the most sold books (when it comes to Java), and it covers some parts quite well. I have met quite a few folks that understand the matters pretty well, admittedly part of jsr-166. It was the "double-checked idiom" vs the original Java memory model is effectively what brought sane memory models even to C++. >unless you cared about extreme performance That's the whole purpose of the weak memory models. Albeit, they did fail to deliver - very error prone, close to impossible to debug, effectively outclassed by x86-64 and Sun Sparc (both being total store order), even arm-64 became "stronger".
        jacquesm 3y ago
- Aloha 3y ago
  Making the microprocessor is just one challenge - you have to produce a workable system that someone wants to buy (aka, is cheaper or more attractive than the other options. It does appear that DEC could have made a single chip PDP-11 in 1980, and probably could have made one with an expanded address space to get around the memory limitations in the PDP-11. Intel did not do that, IBM in this case managed to - and it had the cachet and name recognition to sell an product that was largely middling in a technical level, over other options that were often superior technically and cheaper. Intel made a decent enough microprocessor, yes it was kludgy (even the 8086 was), but it performed well enough (I've never seen a claim that the 68000 significantly outperformed the 8086), and was available in quantity. The rest of the things that made the IBM PC a system that stormed the world were COTS parts assembled in an attractive package. Now, could DEC have done all of that with the PDP-11, yes, absolutely - but the winds were prevailing against it because of the nature of DEC management - I fundamentally do not believe that DEC would not have allowed itself to ship something as 'flawed' as the IBM PC was.
  - gumby 3y ago
    > I've never seen a claim that the 68000 significantly outperformed the 8086... I think you made a typo and meant 6800. Too late to edit though. I make this typo too because the 68k was such a significant chip in history.
    - Aloha 3y ago
      No, I meant 68000 - what I'm trying to say is that while the 68000 is a significantly more elegant microarchitecture, had more functionality - and it did get to better performance faster, its not really any better in performance when judged on a clock for clock basis to the 8086.
- gumby 3y ago
  Except the difference was Grove vs Olsen. Both were engineers, both built great hardware, but Grove feared his competitors and his customers' fickleness while Olsen was about beating / carving off business from IBM. In the early 80s I moved from the MIT/128 ecosystem to the Stanford/101 ecosystem and even to my business-ignorant eyes (I was still in research at the time) and though I didn't understand why for years, it was like night and day.
nxobject 3y ago
Hmm – the smallest, most highly-integrated PDP-11 (-compatible) package ever made was the QFP 1806VM2, with around ~135k transistors; it integrated MMU, UART, parallel interface, keyboard controller, etc, but did floating-point instructions in interpretive microcode. I think that same transistor count was reached by Motorola on the 68020, which would've been around 1984, but would have needed the peripheral controllers mentioned here.
- WalterBright 3y ago
  The FPU for the 8086 was a separate chip, and the IBM PC still required a board full of chips to make a working computer. (If I recall correctly, it wasn't until the 486 that the FPU was incorporated.)
  - Aloha 3y ago
    The IBM PC had an advantage in that most of the support parts were IIRC largely jellybean 74 series logic - could a PC-PDP have used similar COTS parts? The larger issue with DEC was a strong NIH trend (almost as strong as IBM), I dont know if they could have bucked that trend to successfully launch a market winning PC. It probably would have looked like the DEC Professional, which I think looks like market failure. So the dreams of a 64 bit extension to the PDP-11 might be stillborn ;-)
    - Gibbon1 3y ago
      I read an essay by a guy that worked on project to produce a DEC minicomputer using ECL logic. And there was yet another group working on the DEC Alpha. At the time the main group was using most of the companies resources betting the company on dethroning IBM in the main frame arena. And was trying to stab the Alpha and Minicomputer group in the back. So probably not. Suspect IBM's skunk work project was done as a hedge and anti-trust reasons. Personal Computers were going to take some business away from them they wanted it to be their product not someone else. Anti-trust also meant it needed to use off the shelf stuff.
      - 3y ago
SomeoneFromCA 3y ago
Soviet home computer BK https://en.wikipedia.org/wiki/Electronika_BK https://en.wikipedia.org/wiki/Electronika_BK was the smallest pdp-11, but it was not a single chip one. Also MSP430 is a cutdown pdp-11.

by Aloha 3y ago

WalterBright 3y ago

If Intel could do it, DEC could do it.

xxs 3y ago
DEC Alpha had the weakest memory model, which while cool to write for with its all memory barriers/concurrency - it's quite annoying in the real world, a major reason while DEC failed.
xxs 3y ago
DEC Alpha had the weakest memory model, which while cool to write for with its all memory barriers/concurrency - it's quite annoying to work with in the real world, a major reason while DEC failed.
- jacquesm 3y ago
  Compared to the madness that was x86 memory models the Alpha was quite sane. I'm not sure what you base your 'weakest' on (it suggests a comparison with others) but when Alpha was released there wasn't much to compare it to besides the R4000 and that just as hard if not harder to source than the Alpha machines were. I had a bunch of them (SGI boxes of various plumage) and the OS hardly took advantage of the chip, but on the Alpha it all just worked at 64 bits out of the box.
  - xxs 3y ago
    >I'm not sure what you base your 'weakest' Linux kernel has memory barriers, Alpha makes an exception with needing virtually all of them - with other architectures (esp. the total store order ones) some (most) of the barriers are nop. Yet, overall Alpha is famous for how crazy the memory model is. There is nothing like any longer (personally I am happy with Java Memory Model). Just to make sure it's understood "weak" attributed to :memory model: just means how concurrent with regards to reads and writes it is. A quote[0]: AND THEN THERE'S THE ALPHA -------------------------- The DEC Alpha CPU is one of the most relaxed CPUs there is. Not only that, some versions of the Alpha CPU have a split data cache, permitting them to have two semantically-related cache lines updated at separate times. This is where the address-dependency barrier really becomes necessary as this synchronises both caches with the memory coherence system, thus making it seem like pointer changes vs new data occur in the right order. [0]: https://www.kernel.org/doc/Documentation/memory-barriers.txt https://www.kernel.org/doc/Documentation/memory-barriers.txt
    - jacquesm 3y ago
      That's what allows the Alpha to be multi-processor. And if you like the 'Java Memory Model' I'm not sure what your point really is, you're comparing a virtual machine with actual hardware.
      - xxs 3y ago
        >you're comparing a virtual machine with actual hardware. The memory model is meant for developers - and how many&different memory barriers they have to issue.
        jacquesm 3y ago
        Your typical Java developer has zero knowledge about this and on the UNIX versions that shipped with the Alpha (for me, at the time that was RedHat) fairly elegantly hid that complexity to the point that you could just forget about it unless you cared about extreme performance. In my case the ability to address large amounts of memory and a filesystem that wasn't limited by the 32 bit limitation was the key factor and while it took another decade for x86 to catch up I was happily shipping.
        xxs 3y ago
        >Your typical Java developer has zero knowledge about this That's bit much of stereotyping, depends I guess - "Java Concurrency in Practice" is one of the most sold books (when it comes to Java), and it covers some parts quite well. I have met quite a few folks that understand the matters pretty well, admittedly part of jsr-166. It was the "double-checked idiom" vs the original Java memory model is effectively what brought sane memory models even to C++. >unless you cared about extreme performance That's the whole purpose of the weak memory models. Albeit, they did fail to deliver - very error prone, close to impossible to debug, effectively outclassed by x86-64 and Sun Sparc (both being total store order), even arm-64 became "stronger".
        jacquesm 3y ago
Aloha 3y ago
Making the microprocessor is just one challenge - you have to produce a workable system that someone wants to buy (aka, is cheaper or more attractive than the other options. It does appear that DEC could have made a single chip PDP-11 in 1980, and probably could have made one with an expanded address space to get around the memory limitations in the PDP-11. Intel did not do that, IBM in this case managed to - and it had the cachet and name recognition to sell an product that was largely middling in a technical level, over other options that were often superior technically and cheaper. Intel made a decent enough microprocessor, yes it was kludgy (even the 8086 was), but it performed well enough (I've never seen a claim that the 68000 significantly outperformed the 8086), and was available in quantity. The rest of the things that made the IBM PC a system that stormed the world were COTS parts assembled in an attractive package. Now, could DEC have done all of that with the PDP-11, yes, absolutely - but the winds were prevailing against it because of the nature of DEC management - I fundamentally do not believe that DEC would not have allowed itself to ship something as 'flawed' as the IBM PC was.
- gumby 3y ago
  > I've never seen a claim that the 68000 significantly outperformed the 8086... I think you made a typo and meant 6800. Too late to edit though. I make this typo too because the 68k was such a significant chip in history.
  - Aloha 3y ago
    No, I meant 68000 - what I'm trying to say is that while the 68000 is a significantly more elegant microarchitecture, had more functionality - and it did get to better performance faster, its not really any better in performance when judged on a clock for clock basis to the 8086.
gumby 3y ago
Except the difference was Grove vs Olsen. Both were engineers, both built great hardware, but Grove feared his competitors and his customers' fickleness while Olsen was about beating / carving off business from IBM. In the early 80s I moved from the MIT/128 ecosystem to the Stanford/101 ecosystem and even to my business-ignorant eyes (I was still in research at the time) and though I didn't understand why for years, it was like night and day.

nxobject 3y ago

Hmm – the smallest, most highly-integrated PDP-11 (-compatible) package ever made was the QFP 1806VM2, with around ~135k transistors; it integrated MMU, UART, parallel interface, keyboard controller, etc, but did floating-point instructions in interpretive microcode. I think that same transistor count was reached by Motorola on the 68020, which would've been around 1984, but would have needed the peripheral controllers mentioned here.

WalterBright 3y ago
The FPU for the 8086 was a separate chip, and the IBM PC still required a board full of chips to make a working computer. (If I recall correctly, it wasn't until the 486 that the FPU was incorporated.)
- Aloha 3y ago
  The IBM PC had an advantage in that most of the support parts were IIRC largely jellybean 74 series logic - could a PC-PDP have used similar COTS parts? The larger issue with DEC was a strong NIH trend (almost as strong as IBM), I dont know if they could have bucked that trend to successfully launch a market winning PC. It probably would have looked like the DEC Professional, which I think looks like market failure. So the dreams of a 64 bit extension to the PDP-11 might be stillborn ;-)
  - Gibbon1 3y ago
    I read an essay by a guy that worked on project to produce a DEC minicomputer using ECL logic. And there was yet another group working on the DEC Alpha. At the time the main group was using most of the companies resources betting the company on dethroning IBM in the main frame arena. And was trying to stab the Alpha and Minicomputer group in the back. So probably not. Suspect IBM's skunk work project was done as a hedge and anti-trust reasons. Personal Computers were going to take some business away from them they wanted it to be their product not someone else. Anti-trust also meant it needed to use off the shelf stuff.
    - 3y ago

SomeoneFromCA 3y ago

Soviet home computer BK https://en.wikipedia.org/wiki/Electronika_BK https://en.wikipedia.org/wiki/Electronika_BK was the smallest pdp-11, but it was not a single chip one. Also MSP430 is a cutdown pdp-11.