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?
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.
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.