back to article Hardware boffin starts work on simulation of an entire IBM S/360 Model 50 mainframe

Hardware guru Ken Shirriff is working on a simulator for the IBM S/360 Model 50 mainframe, launched in April 1964. His program runs the original machine's microcode so it can control and be controlled by an original front panel. Circuitry-wizard Ken has featured on The Reg quite a few times, from reverse-engineering a Sinclair …

  1. Pen-y-gors


    I cut my teeth on a 360/44 at St Andrews in the mid-seventies. Don't suppose there's an Algol-W or Fortran IV compiler available for the simulator?

    1. Zippy´s Sausage Factory

      Re: Wonderful!

      I share your enthusiasm.... I've never used an IBM S/360 myself but I'm excited enough I want to download Hercules and see what I can make it do. (Spoiler: probably not much, if I'm honest)

    2. Warm Braw

      Re: Wonderful!

      The Hercules simulator has been around for a while (though it simulates S/370 and later, not S/360). However, since S/370 is a superset of S/360 it will boot both MVT as well as MVS along with a selection of other IBM OSs.

      There's a list of available operating systems here and a list of the compilers for MVS, MVT and others here.

    3. Down not across

      Re: Wonderful!

      Agreed. I have fondness for the old kit and keeping the history alive.

      I fear that all too soon all that will be lost.

    4. Someone Else Silver badge

      Re: Wonderful!

      A 360/44?!? We had one at my school (New Mexico Tech); doing some research, found it was installed in 1966, making it one of the first installs of that model. No SS instructions (which meant no OS/360 :-( ), but a blinding fast (for its time) floating point processor.

      Ahh, the paper cuts from the punch cards....

      1. Markgeol

        Re: Wonderful!

        I got to Tech when the 360 was being replaced by a DEC-20. For a semester or so it was very impressive to see both machines running side-by-side in the same computer room especially considering the school was only about 1200 students back then (a bit bigger now but still small for an engineering college). I've returned to Tech working as a DBA but no big iron any more--just lots of virtualized servers.

        Cheers to my fellow Techie. :-))

  2. Roland6 Silver badge

    Dev Tools?

    Just need someone to make available the S/360 development toolset; although it wouldn't surprise me if an El Reg reader posts a link to a site where they can be down loaded...

    1. W.S.Gosset Silver badge

      Re: Dev Tools?

      Right on cue, and (now) right above your post...

  3. Doctor Syntax Silver badge

    1. It looked a bit unlikely until I saw just who the guru was.

    2. About the link to It's a bit worrying the website hasn't moved for nearly 2 years.

    3. The original development of the software for the 360 was the inspiration for TMMM.

    1. Liam Proven Silver badge

      [Article author here]

      Pour encourages les autres... "TMMM" is _the Mythical Man Month_ by Fred Brooks.

      It's the seminal text on project management.

      I often use it to explain why multi-core processors are not the wonderful performance benefit they're often taken to be.

      I tell people they should read it to understand about the overheads of resource allocation. But it's quite thick – so they should buy two copies, so they can read it in half the time. Or if they're really short of time, four or even eight copies...

      1. Anonymous Coward
        Anonymous Coward

        I remember coming to my current job and finding my manager even KNEW what TMMM was.

        That's when I realized I was staying for a while.

      2. David 132 Silver badge

        >But it's quite thick

        I like books that are quite thick. I can relate to them.

  4. TheProf



    Teenage Mutant ? ?

    1. Tomislav

      Re: TMMM

      I guess he meant this book?

      1. Roland6 Silver badge

        Re: TMMM

        Whilst I read the original 1975 edition as part of my degree, I would point new readers to the 1995 anniversary edition.

        1. richardcox13

          Re: TMMM

          > I would point new readers to the 1995 anniversary edition.

          Yes the additional chapters are definitely worth it. It preserves the original content, and then reviews it.

          And then you notice the '95 "20th Anniversary" edition is now about 25 years old...

    2. W.S.Gosset Silver badge

      Re: TMMM

      Teenage Mutant Minjer Merkel

  5. Anonymous Coward
    Anonymous Coward

    Keep your eye on the (changing) ball

    I remember reading assembler code for a 360 communications program I was supposed to duplicate onto another architecture. Got very confused as it didn't have near enough booleans and not enough conditional checks against current state. Then noticed code was checking a couple strange values against some very odd addresses.

    The pseudo-booleans were opcodes at those odd _code_ addresses, and the strange values were for NOP vs. BRanch. The program was flipping instructions between fall-through and jump elsewhere, reflecting current 'state'.

    Not your usual Fortran or Cobol code, that.

    1. Christoph

      Re: Keep your eye on the (changing) ball

      Good old self-modifying code - far more efficient, but for some reason frowned on these days. They even stop you writing to program memory!

      1. Someone Else Silver badge

        Re: Keep your eye on the (changing) ball

        And that's a good thing, too, Ollie!

      2. Old Used Programmer

        Re: Keep your eye on the (changing) ball

        The S/360 workaround is the execute instruction.

    2. Old Used Programmer

      Re: Keep your eye on the (changing) ball

      Never ran into the COBOL ALTER GO TO construct, huh?

  6. Dr Paul Taylor


    Cambridge University bought an IBM 360 in the 1970s. Since JCL is not fit for humans, they wrote an operating system called Phoenix, a scheduler called Eagle, a scripting language called Wren and lots of other bird-themed programs. Meanwhile, the top American universities were developing Unix and the FOSS world. I saw Knuth's TeXbook in a bookshop and asked, why can't we have clever software? Of course Phoenix was abandoned in the end. Such a tragedy that all of this intellectual effort went down the drain!

    (Separate story) A friend of mine who had been to school in Moscow had an IBM360 manual in Russian, because the Soviet's had "emulated" it.

    1. Roland6 Silver badge

      Re: Cambridge

      >Cambridge University bought an IBM 360 in the 1970s

      They were a bit late to the game, but probably to be expected.

      From memory the best equipped UK Uni/Poly of the 70's was Hatfield, who seemed to be well into bed with DEC.

    2. maddoxx

      Re: Cambridge

      360 was not just emulated but completely cloned in GDR and USSR.

      see this example - unfortunately only avail in English

      my father sold this machined to the eastern european countries...

  7. Pascal Monett Silver badge

    Oh my God : blinkenlights !

    I wants one !

    My precious !

    1. Sandstone

      Re: Oh my God : blinkenlights !

      Pretty lights and switches... must touch!!!!

      1. Anonymous Coward
        Anonymous Coward

        Re: Oh my God : blinkenlights !

        Not as pretty as the EEC/ICL System 4/70**. A flat panel matrix of coloured squares backlit by an array of pea-bulbs. I have some 8mm film footage of one somewhere.

        Some people would insist that the illuminated buttons of a KDF8 (RCA 501***) console in a darkened room was art in itself.

        ** an IBM 360 compatible based on the RCA Spectra 70/55 paper specification.

        ***The Spectra 7/45 microcode had an option to emulate an RCA 501 - and possibly also another early computer.

  8. keith_w

    What about 360/20's! I know, not really a 360 (totally different architecture) but it still bore the label

    1. Someone Else Silver badge

      The 20 was a 16-bit version of the otherwise 32-bit 360 line. Not sure what OS it ran, but was indeed considered a 360.

      1. VTAMguy

        Princeton University had one in the early 1970s which ran a standalone program called Thor, open for public use, which provided two functions: printing a listing of a program from cards, or duplicating a card deck onto blank cards. That's all it did, but it was very helpful given that all your code was on cards then.

      2. keith_w

        I'm not sure what the O/S was either, even though my first 2 jobs were as operators on that model. Looked it up, according to Wikipedia, it was "Disk Programming System",

  9. H in The Hague

    Front panel as home decoration/art

    I always fancied mounting one of the front panels on the wall as decoration, art even.

    Never got one but love browsing through the pics on Iconic indeed - how many of us grew up thinking that computers look like that. That site also has an interesting discussion of the 555 timer chip.

    1. DougMac

      Re: Front panel as home decoration/art

      I really liked Ben Eater's explanation of the 555 as well

  10. William K Kelley

    S/360-50 microcode held in capacitors

    Curious how he got ahold of the S/360-50 microcode. On several of the S/360 models (40,50,65,67), the microcode was held in arrays of capacitors formed by large plastic sheets with conductors etched on one side of them held between large steel plates. These capacitors were quite sensitive to the pressure applied to them by the steel plates (which were sensitive to the temperature inside the machine's cabinet) and the IBM customer engineer (CE) would occasionally have to re-torque the screws which applied pressure to the plates. A microcode update required demounting the plates, replacing the plastic sheet with the new one, replacing the plates and then re-torquing them until they read properly. Model 30 used plastic 80-column punch cards inserted in slots in the machine for its microcode. Models 44, 75 and 91 were "hard-wired" for best performance and didn't have microcode, which made fixing engineering mistakes more costly. S/370 microcode was held in semiconductor memory and loaded during machine power-up initialization from 8-inch floppy disks which IBM invented for that purpose.

    1. Ittopgun

      Re: S/360-50 microcode held in capacitors

      The Model 30 microcode was as stated on 80 column punch cards with silver contact plates for each of the 80 by 12 location that could be punched out to change the code. They were held against the 'read' plate by air pressure. To update the microcode you had to deflate the bag and then remove and replace the necessary cards. The problems we had as engineers were that over time the silver on the card would stick to the base board which meant a major cleaning job and the other common problem was loss of pressure in the airbags that pressed them against the read plate. The model 25 I think had a loadable microcode from punched cards that had to be reloaded after every power down. The model 40 had TROS which was Transformer Read Only Storage on long flexible tapes stacked around transformer cores.

      1. Liam Proven (Written by Reg staff) Silver badge

        Re: S/360-50 microcode held in capacitors

        > The model 40 had TROS which was Transformer Read Only Storage on long flexible tapes stacked

        > around transformer cores.

        Indeed. Tricky to read these days.

  11. elDog

    First cut my teeth on BAL on 360/40 - 1965?

    World Health Organization (WHO?)

    I learned coding via those 80 column sheets not understanding that they needed to be punched onto cards, read in and stored on tape, sorted, compiled, mashed and reports spewed (days later.) I can almost remember the BALR and other instructions.

    Later I was an operator for a system with real spinning disks but nobody knew how to run sorts using them. Paper tape readers for factory interfaces. Multiple pass card sorts before.

  12. Blackjack Silver badge

    This is amazing, wish someone was doing the same for the Game & Watch clones of my early childhood.

    1. Charles 9

      Some of the Game & Watches are emulated on MAME now. They're increasing their scope to find more things to emulate. I suppose for things like the Seiko Pop Game LCD devices, the main obstacle would be availability.

  13. Anonymous Coward


    At last I can pull the Big Red Button without fear of consequences!

  14. Old Used Programmer

    BRings back memories...

    My first job out of college in 1970 was programming on a S/360-30 with 32K of memory, running DOS, with 4 2311 disk drives (7.25MB each).

    1. ghp

      Re: BRings back memories...

      42311 disk drives ???

    2. chriskno

      Re: BRings back memories...

      My career in IT started as a trainee operator on a 360/30 with 32k memory and 2311 disks. It was running a manufacturing plant in Trafford Park, Manchester (quite close to Old Trafford so very convenient to go and watch United). I progressed through programming (Cobol), leaving millennium bugs all over the place as two bytes were very precious, into management roles - Director of Computer Services, Head of Business Development etc, and now retired. Still remember the 360/30 with affection.

  15. Binraider Silver badge

    In a previous life at a bank, I had the joy of working with a real S390 system. Absolutely solid as a rock; but that perhaps was also a reflection of the quality of the software on it, written with very well known user requirements and acknowledging of the limitations of the communications systems of the time.

    I also keep a model around written originally for S390. Former administrators of the model decided that rather than migrating the Fortran to a modern architecture, that they would instead run it in one of those (rare) commercial emulators... for DOS.

    How many layers of emulation do you want?!? S390 in DOS in VirtualBox (or other VM of choice). It works well in Dosbox though of course such use is definitely against the legal boilerplate of Dosbox.

  16. rafff

    the computer on which virtualisation was invented.

    Oh no it wasn't!

    The Ferranti Atlas got there first and had been running it for years before IBM even thought about it. I remember a face-off at an IFIP conference around 1968 between an iBM-er discussing certain problems that they had discussed while *emulating* virtualisation and paging, and an Atlas man who just said " We have been running it IRL for 10 years now without hitting that problem."

    At my first (commercial) job in 1972 at BP we were just thinking of switching from /360 to /370.

    1. Michael Wojcik Silver badge

      Re: the computer on which virtualisation was invented.

      Citation, please.

      The Atlas was the first machine to have virtual memory, including simple memory protection, but I can't find any reference (aside from your post) that claims it had virtualization. And the Atlas Supervisor paper doesn't say anything about virtualization either.

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