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Why is that possible in this fictional reality but stockpiling an RPi isn't?

Because it doesn't fit the given narrative: it's all about scavenging/recycling.

Your reality does make better sense, but doesn't make a good story.

There are vastly more Z80's in circulation that Raspberry Pis.

Also, in a worse-case scenario, it's much easier to build a Z80 by hand from individual transistors and/or logic gates¹ (which would still be absurdly difficult, but not impossible) than any ARM CPU (including the ones in any generation of RPi). In a slightly-less-than-worse-case scenario, it's much easier to build a Z80-based computer by hand (i.e. with a Z80 you pulled off some other piece of hardware) than your average ARM-based computer (including any RPi).

¹ "logic gates" not necessarily being transistor-based, either; one could take a cue from the guy building a 32-bit RISC-V machine with vacuum tubes: https://www.ludd.ltu.se/~ragge/vtc/

A Z80 has over 8500 transistors, so even though it's much simpler than a Raspberr Pi, I don't think you're going to solder together an 8000 transistor processor in a post apocalyptic world. Even the old 4004 has around 2300.

I'd guess that those 8500 transistors would be better used to build thousands of much simpler logic controllers to help automate infrastructure that's lost its computer control systems.


There are simpler "transistor computers" that might be more feasible to build from discrete components:


There are examples elsewhere in these comments (linked by myself and others) of CPUs with similar (if not greater) transistor counts soldered together by hand out of TTL chips or even out of individual discrete transistors.

There are multiple ways to skin a cat, though (perhaps literally; this would be the post-apocalypse, after all!), and you're right that there are numerous ways to put transistors to use besides building full-blown CPUs. One of the key advantages of a general-purpose CPU is that it's general purpose and can be made to do all sorts of different things, but there are certainly plenty of cases where that ain't necessary and you'd need a fraction of that capability at most.

Still, they'll probably go hand-in-hand. "Chips" are just discrete components, whether crafted from a single chunk of silicon or itself built out of discrete components and treated as a single discrete unit. Building a whole general-purpose CPU from individual transistors is much easier when those transistors are already arranged on a little board you can plug into your bigger board. Chances are that no matter if someone's building a whole CPU or something more special-purpose and limited, that someone will be doing so in terms of already-assembled-and-composable gates rather than transistors directly, if only for the sake of one's own sanity.


[1] https://en.wikipedia.org/wiki/ARM_architecture#Acorn_RISC_Ma...

says about 30.000 gates, which is 10.000 less than Motorola 68000 and arguably faster.

[2] https://en.wikipedia.org/wiki/Zarch

at least made us Amiga and AtariST geeks envious.

Furthermore there is 'Microsequencer' like described there:

[3] https://en.wikipedia.org/wiki/Microsequencer

and following up from there to for example

[4] http://www.microcorelabs.com/mcl65.html

which is a 6502 softcore with microsequencing applied

Alas... There are many options to choose from according to the available technology, tools & knowledge. One does not have to make an exact copy of something which made sense for arbitrary reasons, which don't necessarily apply when doing it from scratch under different circumstances.

> says about 30.000 gates, which is 10.000 less than Motorola 68000 and arguably faster.

The Z80's at 9,000 transistors (not sure how many gates, but almost certainly a fraction of that transistor count), so even the Acorn would be heavy in comparison. Still doable, though; just takes more time.

In terms of speed, it has less to do with transistor count and more about how close together you can get the transistors. Big, hand-wired CPUs tend to be slower than small single-chip ones just from the sheer latency differences between components.

> One does not have to make an exact copy of something which made sense for arbitrary reasons, which don't necessarily apply when doing it from scratch under different circumstances.

True, and I ain't saying one does. If we're at the point where we have to hand-wire replacements, though, it helps to have at least some degree of compatibility with the thing we're replacing. There are at least some schematics out there for building 8-bit CPUs from TTL chips¹², and I'd imagine those would all be viable candidates if we have to re-bootstrap our computational power and run out of other CPUs to tide us over in the meantime.

Ideally we should be working on CollapseOS equivalents/ports for as many CPUs as possible, so that we know that no matter what we're stuck with, there's always a way to repurpose it. Just as importantly, though, we should be hoarding copies of pinout/wiring diagrams, hardware manuals, etc. to make sure we have the knowhow on the hardware side, too.

¹ http://cpuville.com/Kits/8-bit-processor-kit.html - happens to be bus-compatible with the Z80, though not ISA-compatible as far as I can tell.

² http://mycpu.thtec.org/www-mycpu-eu/index1.htm - more "modern" features like Ethernet and VGA out, so a more likely candidate for general purpose computing if we really do run out of Z80s to scavenge

The flash in the Pi will eventually die. Other high density ICs may also die from the laws of thermodynamics.

I don’t think it’s any coincidence that this collapse brings computing down to an era which is many people’s personal computing heyday, and not a decade before or after.

I’d hazard a guess that 8-bit machines played a part in the author’s young life - first computer, first job, happiest childhood summer, last computer they felt in control of before they got annoyingly complex - something like that. And therefore a collapse ending right when the author would have useful skills but things wouldn’t be too hard, is the most fun one to imagine.

Computing was around 40 years before the 1980s and electricity for a hundred years, but who wants to try and rebuild room sized punched card machines for ballistic trajectory calculations, get greasy fingers on mechanical parts, or deal with HT electrical power supplies safely, yawn, no fun there. rPi the same - by then everyone can do it and author isn’t special, so whatever. It’s not different enough from right now.

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