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Build Technology

Rebuilding the PDP-8 With a Raspberry Pi 92

braindrainbahrain writes: Hacker Oscarv wanted a PDP-8 mini computer. But buying a real PDP-8 was horribly expensive and out of the question. So Oscarv did the next best thing: he used a Raspberry Pi as the computing engine and interfaced it to a replica PDP-8 front panel, complete with boatloads of fully functional switches and LEDs.
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Rebuilding the PDP-8 With a Raspberry Pi

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  • that you can wire up more MSI TTL to add instructions or other features. That's the charm of the old-school PDP-8. (Okay, not the really old-school DTL version, but the version I remember in a friend's dorm room...)

  • I programmed a PDP8 in Fortran. In the good old days....

    • We had a PDP 8/I that the EE guys built a high speed paper tape reader for. One I/O addresss made it go forward, the other backward. Watching various sort algorithms run against data on the tape were educational in a unique way.

      We also had a paper tape based 4K 2Pass Algol compiler that worked, it waited until you reloaded the freshly punched tape of intermediate format to start the next pass and gave you an loadable paper table on the final pass.

      Not bad for a machine that had 8 Opcodes.

      • Oh, the paper tape... When I was a Comsomol member there were FS-1500 tape readers made in Chechoslovakia. They were really high speed - 1500 bytes per second. The tape just flew through them nonstop. When the first Western readers arrived (made in Poland by US license), they were slow as snails. But the Western tape punchers were really good.

        • Interesting; things apparently regressed before they could progress. The first paper tape reader for a computer (Colossus []) read at 5000 characters/s in normal operation, and could be cranked up to 9700 char/s (85 km/h), but the tape wasn't strong enough to survive that speed for long.
          Of course, the Official Secrets Act made sure the Colossus design wasn't available on the open market.

  • I don't know anything about the PDP-8, but isn't using a Raspberry PI completely overkill? Wouldn't a much more basic ATmega328P be enough for the task?

    • He should have gone for a PDP-11! As an aside, simh is a really awesome piece of software.
    • Ya, it's kind of a non-story really. Ok, he used a replica panel, and you can't just buy those online easily. But a raspberry pi running an emulator is just decidedly not geeky. I can run Unix version 1 and 6 and BSD 2.9 on my Mac and PC, but I don't tell people I rebuilt a PDP-7 or PDP-11 or VAX.

      Meanwhile there ARE people out there who have built real computers and CPUs from scratch as a hobby, without any emulators behind the scenes. Check out the [] web ring. Those are

  • FPGAs (Score:4, Interesting)

    by jones_supa ( 887896 ) on Friday March 27, 2015 @02:18PM (#49356127)

    We really should be preserving old computers in HDL in a form as loyal as possible to the original. Then we could always reimplement them in FPGA and make "real" hardware cheaply enough until the sun burns out.

    It's doable, although these are big efforts.

    There is already this Japanese guy who has done it for the SNES [].

    • I was about to post this. In fact, I bet the resulting HDL code for this particular computer can be implemented in a technology that's cheaper than FPGA, like perhaps commercial flash PLD. Also things seem to be moving towards OpenCL which is behavioural and C-like, which may help people who are used to that paradigm, like people who do MCUs including the Raspberry Pi.

  • Fond Memories (Score:5, Interesting)

    by Gim Tom ( 716904 ) on Friday March 27, 2015 @02:21PM (#49356155)
    My first "personal computer" was a PDP-8i at Georgia Tech in the late 1960's. The ISy school had one in a small room in the basement with an ASR TTY (33 I think). There was another room with at least one more TTY with punch and you would code on that machine and after signing up for time on the PDP-8i you would take your paper tape in and after toggling in the boot sequence and loading the BIN tape then the Assembler you would run your tape to punch out your assembled program to run on the machine. I may be leaving out a number of steps since that was a while back.

    in any case that was my first taste of writing any code in a machines assembly language and even then I dreamed of having my very own PDP-8.

    This is a cool project and even for an Old Man I can fully relate to why it was done. I think this experience led to a life long career working with computers ranging from Big Iron mainframes to PC's networks and a variety of internal and Internet facing Servers. Yes, even though retired, I have a couple of Arduinos and Raspberry Pi's around to play with! Learning new things has kept me going all these years.
  • by cruff ( 171569 ) on Friday March 27, 2015 @02:43PM (#49356419) Homepage
    The SBC6120 uses a Harris 6120 CPU chip which is a PDP-8/e-like microprocessor. It has a companion FP6120 front panel with switches and lights, which is a scaled down version of an older modle rack mount PDP-8 front panel. You built them from kits, loads of fun for those who like that sort of thing. Mine has a CF card for the hard drives (a whole whopping 2 MB each under OS/8!). You may be able to find an unbuilt kit, as the maker of the kit, Spare TIme Gizmos, will not be making any new ones going forward.
    • by g4ugm ( 4054675 )
      Even the SBC6120 is getting hard to find , I waited for two years before one became available and that's the one with no front panel. But it is a great beast...
  • The 8 was a great system but the 11 was far better.

    Just checking ebay, this guy selling the 8E is smoking something []. He thinks it's a mainframe.

    However this PDP-11 [] can be had for a reasonable price.

    The point being, you can run emulation software [] on commodity hardware but I guess as the TFA indicates he wanted the nostalgia look. He could have easily just mounted an LEDs behind the panel with small pattern generator circuit instead of using the Pi.

    • I remember helping to install one of those RX01 dual floppys took two of us to lift it - those little PDP 03/s where built like a brick out house
  • I first learned machine language on a PDP-5, which was similar to the PDP-8, but limited to 4KB of memory. Mostly I just used it to toggle in small programs through the console switches, but I think we got the FOCAL [] interpreter running on it at one point. Those were the days. To think now there is a generation of programmers who have known nothing but JavaScript.

  • My cellphone has more storage and processing power. You would think a PDP8 would be worth little more than scrap at this point.
    • by tomhath ( 637240 )

      You would think a PDP8 would be worth little more than scrap at this point.

      Which is why almost all of them were scrapped years ago. And anyone who is still running something on one really, really, needs spare parts.

      • by AteEm ( 4054471 )
        As an owner of a real PDP8/m, I can assure you that spare parts are available. The beauty is, you can actually troubleshoot down to part that can be purchased off the shelf; today's systems are fixed by board swapping, lest you waste your time finding out the problem is in some custom IC. Of course, I'm not sure anyone does that kind of troubleshooting anymore :(
    • Absent interference in the market by governments and/or corporations, price is determined by supply and demand, not capability. I can't think of any rational reason for anybody to interfere with the market for PDP8s, so I'm going to assume it's a free market. Although economic theory with its neat little graphs might give one the impression that it's some kind of science, the actual shape of the supply and demand graphs (and thus the equilibrium price) are determined by emotional "ugly bags of mostly wate

    • by SeaFox ( 739806 )

      My cellphone has more storage and processing power. You would think a PDP8 would be worth little more than scrap at this point.

      You know, at some point things stop being "old toys", "old cars", and "old computers that aren't powerful enough to do anything moden on", and become antique, and collectable.

    • Dusenbergs are expensive now, too. So are Pierce Arrows.

      Even though you can get a used Dodge Neon for a lot less.

  • Are old PDP8s really expensive? But I bet no one saved the boxes they came in...

  • Wait, no, that was the boot loader address for the PDP-11. Never mind.
  • For people who want to build a real hardware silicon PDP-8 computer, there exists an LSI version of it, the Harris/Intersil 6100 processor []. It's a standard 40-pin package integrated circuit.

    It's a static CMOS processor that can be clocked down to zero hertz if you like (the registers don't need 'refreshing' so it can be clocked as slow as you like) and it's a 12 bit processor and implements the PDP-8 Instruction set.

    They haven't been made for years but they exist in NOS (new old stock) quantities and can b

  • I'd like to make a BESM-6 emulator with PIC18. But nobody knows it's privileged instructions for now which means that it's impossible to recreate it fully.

  • Oscar is exhibiting his PiDP-8, along with 10 or so other people showing * real * PDP-8s, at next month's Vintage Computer Festival East [] -- and they'll all be up-and-running, including an original 1965 "Straight-8". Slashdot published a video about the event just a few weeks ago [].
  • Whilst buying one may be out of the question, it is possible to build a PDP-8 "clone" from raw TTL. I know this was done on at least one college course, and there are books out there to support it...

"If it's not loud, it doesn't work!" -- Blank Reg, from "Max Headroom"