There's a rumor that there's something embedded in the IBM 5100 which makes it useful for codebreaking.

What is the legitimacy of that claim?

  • 3
    DO you got any reference for that claim? Eventually one with some hint about the area it is originated? Besid that, every computer is a great help for codebreaking compared to doing it manualy :))
    – Raffzahn
    Jul 10, 2018 at 20:53
  • 2
    I used to use a 5100 - I programmed accounting software on it. It was hopelessly slow even for its time. Given the choice now to own today either my accountant client's 5100 or his Selectric Model 50 - I'd choose the typewriter hands down. It would be far more useful and its probably easier to get ribbon cartridges for it than magnetic tape cassettes which were the only storage for the 5100.
    – davidbak
    Jul 11, 2018 at 3:27

2 Answers 2


The 5100 had programmable microcode which could be used to implement crypto-specific opcodes1,2,3. Then there's the whole John Titor4,5 thing...

1 Such as population count

2 I'm unaware of anyone actually having done so.

3 Certain agencies are very interested in such things though.

4 Perhaps it wasn't code-breaking in the traditional sense. If you have software with extra opcodes along with the corresponding microcode for the added instructions (but no description of the CPU's wiring or non-custom microcode) it is conceivable that you would need original 5100 hardware to determine how it would run.

5 No, I don't think he's for real.

  • 1
    LOL, what a great connection :)) I (And a thruput stupid story).
    – Raffzahn
    Jul 10, 2018 at 21:39
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    @Raffzahn I contemplated adding a "tinfoil-hat" tag to this question. Jul 10, 2018 at 21:41
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    The second paragraph of that linked wikipedia article is hilarious for the implication that there was in fact something to be taken seriously about his claims given in the first paragraph but due to the diligent efforts of a few skeptics the conclusion was finally, reluctantly drawn that it was, after all, a hoax. Best writing on wikipedia ever!
    – davidbak
    Jul 11, 2018 at 3:17
  • I appreciate the reference to the John Titor connection. Jun 10, 2019 at 13:27
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    @MauryMarkowitz - this is the version I saw, in particular the sentence "Subsequent closer examination of Titor's assertions provoked widespread skepticism." <== "subsequent examination" because on initial examination everything about his claim to be a time traveler seemed just peachy!
    – davidbak
    Jun 10, 2019 at 15:05

I think the original argument (both in the John Titor hoax and in Steins;Gate) goes like this:

The IBM 5100 can emulate the IBM mainframe ISA (that's true, and that's how the APL running on the IBM 5100 was implemented: They took the mainframe APL implementation, because they didn't have enough time to develop a native APL. That also makes it slow...), and therefore the IBM 5100 can help to break some other unspecified programs that were written for the IBM mainframes.

While that in principle is also true, it's also not particularly clever: An emulator for IBM mainframes can also be written for any other available computer, see e.g. the Hercules emulator.

In the Anime series, one of the characters then goes on to use the IBM 5100 to break into CERN's computers (CERN, or "SERN" in Japanese spelling, gets assigned the role of the evil enemy for some reason). So I guess that could be another source for the "codebreaking" meme. But that's of course entirely unfounded.

I've never heard the argument that the IBM 5100 microcode could be used to implement cryptographic primitives in that connection (though this is also true).

Details on how the IBM 5100 emulated the IBM mainframe ISA can be found in this listing of the "APL executable ROS".

One should also note that the "Microcode" is a 16-bit ISA with 16 registers, not that different from a modern microcontroller ISA, and not at all like the "microcode" on other CPUs with a large numbers of bits used to control the various elements of the CPU.

The original John Titor passage on why he needs an IBM 5100:

I was "sent" to get an IBM computer system called the 5100. It was one the first portable computers made and it has the ability to read the older IBM programming langages in addition to APL and Basic. We need they system to "debug" various lagacy computer programs in 2036. UNIX has a problem in 2038.

  • I've never heard of crypto opcodes being implemented on the 5100 either. It wouldn't surprise me if they were though since certain three-letter agencies were very interested in such systems (e.g. STRETCH/Harvest, Cray I); maybe for field work? Looks like I'm not the only one to have thought that. Jul 11, 2018 at 7:10
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    +1 for mentioning Steins;Gate (although you spelled it incorrectly :P)
    – Pedro A
    Jul 11, 2018 at 12:31
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    @Hamsterrific: I never understood why there is a semicolon... any reason?
    – dirkt
    Jul 11, 2018 at 12:35
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    @dirkt Steins;Gate is a made up name, not referring to anything specific, so there is no "correct english" for it. The semicolon is there to relate it to other works from the same serie (Robotic;Notes, Chaos;Head, Chaos;Child)
    – BgrWorker
    Jul 11, 2018 at 13:59
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    @AlexHajnal - yep, the 5100 had a physical switch on it to select between Basic and APL modes. You'd set the switch and then power cycle it and you were running the appropriate language via the loaded microcode. That wasn't the only computer of its time, BTW, that was natively programmed in a high-level language that was the only language available on the machine: no machine code thus no assembly language. At the same time I was "consulting" (after hours) for an accountant with a 5100 I was working at MAI on their Basic-only multiuser minis.
    – davidbak
    Jul 11, 2018 at 15:50

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