Katy – 68000 Linux on a Solderless Breadboard (2014) | Hacker News Reader

Katy – 68000 Linux on a Solderless Breadboard (2014)

bigmessowires.com

197 pointscaned31 comments

Heh, Using a 555 timer for preemptive scheduling in Linux is one of the biggest flexes I've seen in a while.

For me, the whole thing is an impressive flex, but I know next to nothing about electronics. Can you elaborate what’s interesting about using 555 timers in particular?

The 555 timer is considered one of the most successful chip designs in electronics. It has a simple, well understood, design. The timing on the 555 can be configured to generate a periodic signal using just a few resistors and capacitors. The 555 is often used as a timer in electronic circuits. The Linux scheduler is usually configured to respond to interrupts generated by a cpu. To replace those cpu interrupts by a simple 555 timer is impressive due to the unconventional nature of the setup.

Also, the 555 is an ancient chip, that pretty much appeared as the basic chip (along with it's friend, the 741 general purpose op-amp), being the two of them the protagonists of pretty much 70% of the hobby circuits in the 80s.

There's a bit of a meme among electronics hobbyists that for so many projects with an Arduino, somebody in the comments says "I could've done that with a 555”. It's a very common chip with a wide range of uses from arcade sound generation to the timer on a toaster to light/motion activated alarms.

As someone that grew up reading Elektor articles, that pretty much sums up most of the gadgets I see on Maker magazines.

The big difference is that is much easier to program a microcontroller in software, than mastering electronics, specially when analogue circuits are evolved, and the only debugging tools are a multimeter, pen and paper.

I've seen it often on Hackaday over the years, and while they probably could do it all with a 555, I certainly couldn't... My specialties are software and mechanical stuff, for everything in-between I shove in an Arduino and hope it works!

There is something about the 555 that triggers love from people

Yeah most chips don't get stuff like a discrete version [1] or a furniture recreation [2].

1: https://shop.evilmadscientist.com/productsmenu/652

2: https://www.google.com/amp/s/www.instructables.com/Giant-555...

I see what you did there :->

I don't think it's a flex per se, but more of a nod to where many people start off in their electronics careers. The LM555 has been used for four(?) decades to teach electronics to students.

A cheap $1 8-pin microcontroller (attiny, avr8, pic) could do the job of an LM555 without the LM555's required capacitors/resistors. But that's not as fun.

HN 2014: https://news.ycombinator.com/item?id=8622720

Video showcasing it: https://www.youtube.com/watch?v=SRdLlaUmmpM by it's original designer: Steve Chamberlin. :)

Follow up with the real board: https://www.bigmessowires.com/68-katy/

>it was unstable when tested at 4 MHz

I'd be willing to bet that sprinkling in handful of bypass capacitors on the rails, and keeping those leads short, would get everything up to 4 Mhz. I don't think those leads are long enough, or have enough capacitance, to be the limiting factor.

The breadboard itself has a bit of capacitance too doesn't it?

absolutely, and the springs are cursed

pushing breadboards with all loose components like this MHz deep sucks

So I assume with UCLinux on a 68K, there's nothing to stop a rogue program from destabilizing the system? (Mind, not a hostile program, just a buggy program.)

On an 8086, you can get a modicum of protection by limiting your processes to the Tiny or Small memory model (thus a rogue 16 bit address can't easily "leak" out of their respective segments and clobber the kernel).

Yes but probably the only program is you one or two executables

So it's possible to run Linux on any 8 bit CPU?

The MC68008 has an external 8bit data bus. Internal registers are 32bit wide. Typically one means CPUs with 8bit wide registers when one talks about a 8bit CPU, e.g. 6502, 8080 etc.

The Linux kernel requires a MMU for virtual memory support. Here a severly stripped-down version, μClinux, is used which doesn't support virtual memory and (hence) doesn't require a MMU, so any CPU with a 32bit linear address space (implying 32bit wide address registers) could be supported. At the very least a C compiler (supporting GNU extensions) for that CPU is needed.

Said that, there is ELKS (a subset of Linux functionality for 8086) and there is even a (memory-enhanced) C64 running a Linux VM (very, very slowly though).

My bad, I thought registers are 8 bit wide in MC68008. What you are saying totally makes sense.

So, a Unix running on a Sinclair QL, which is based on 68008 would be possible, theoretically!

That was Linus's first computer! I wonder if he'd be interested in writing such a kernel, to revisit an old friend...

You could, if you'd emulate a 32b cpu on that 8-bitter.

The real limitation here is memory. Imho it says a lot about modern day software bloat that even an ancient 2.0 series kernel needs ~600KB (ROM/RAM combined). For what, really? That's already a lot of features & almost all hw drivers stripped from it, right? What's that remaining ~600KB code+data doing then?

640kb “should be enough for everyone”

I would be remiss if I didn’t post this classic hack - Linux running on an ATmega via emulation, bit-banging a DIMM directly. [1]

[1] https://dmitry.gr/?r=05.Projects&proj=07.%20Linux%20on%208bi...

This post is really making me want to order one of Ben Eater's 8 bit computer kits

I ordered but never open the box. For Covid hotel q 2 week project. They changed the rule and hence no 2 week isolation to do that sadly.

(2014)

But can it run Doom?

It seems so: https://github.com/krikzz/doom-68k

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