Now available on Mastodon

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As it happens, I set up my Mastodon account some time ago, long before Elon Musk’s ego decided it needed a new toy to destroy. I just hadn’t used it much because … well, I didn’t use Twitter much either. However, all the kerfuffle over Twitter has prompted me to look again at Mastodon. You can find/follow me at: @speculatrix@mastodon.online… Read more »

Zolatron 64 6502-based homebrew computer: mission accomplished

Okay, so I’m going to call it. This project is finished. Well, not finished exactly. No project is ever finished. You can always do more. And so it is with the Zolatron – I have ideas and plans that will keep me happily tinkering away for years. But I’ve met my main objective – which is to better understand how… Read more »

Reviving a Centurion

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Something that has been providing me with plenty of inspiration for my Zolatron homebrew project lately is the Usagi Electric YouTube channel, particularly the series of videos on restoring a Centurion mini computer. For one thing, it’s comforting to know there are people more nuts than you. I can just image the reaction I’d get from my More Significant Other… Read more »

What’s better than a 6502 homebrew computer?

Yep, you guessed it – two of them. One of the side effects of ordering PCBs from the likes of JLCPCB is that you have to order at least five copies of each board. This doesn’t quite mean that I have enough PCBs to make five Zolatron 64s. For one thing, each of the machines above uses two of the… Read more »

Zolatron 64: Writing an OS for a 6502 homebrew computer

I’ve said it before and I’ll say it again: I have no idea how you write an operating system. Even though I’ve sort-of written one. I’m calling it ZolOS. The Zolatron 64 6502 homebrew computer project started as idle curiosity that just got out of hand. I figured I’d be happy if I could get as far as writing ‘Hello… Read more »

Zolatron 64: memory expansion

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Things are continuing apace with the Zolatron 64 6502 homebrew computer. In fact, progress has been so rapid that I haven’t really had much time to talk about it. So here’s a quick refresher – and details of the new memory expansion board. Serial killer The 6551 ACIA serial board is dead. Well, it’s not dead, but I’ve stopped using… Read more »

Elliott 405 – basic principles

This selection from ‘Big Doug’ Selway’s trove of documents on the 1950s Elliott 405 mainframe computer takes us right back to the basics. First up is a ‘functional block diagram’ of the system. And what I find interesting about this is: How basic it is. Computers were really quite crude beasts back then. How it was felt you needed to… Read more »

A newbie’s introduction to CUPL and CPLDs

When you’re dealing with computer logic, things can get very complicated very fast. This is especially true when it comes to address decoding. You can easily find yourself tangled in a messy web of AND, OR, NAND and NOR gates. And there’s a problem you can run into when stringing together a bunch of logic gates – propagation delay. It… Read more »

Zolatron 64: a bug hunter’s diary

It’s always a good day when you get a package from the fab. There’s something simultaneously exciting and daunting about unwrapping a shiny new batch of PCBs. On the one hand, there are the possibilities promised by the new device. On the other, you’re wondering whether you made some kind of mistake in the design and if the thing will… Read more »

Zolatron 64 roundup

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This is the 300th blog post on this site, and I thought it would be appropriate to mark the occasion with a summary of what’s happening with my Zolatron 64 6502-based homebrew computer project. You see, the thing is, I’m feel like I’m about to announce ‘mission accomplished’ for this project. And that’s not something I get to say very… Read more »

Zolatron 64: 6502 bare metal random

There’s a problem when you build your own computer and therefore have to write your own operating system. All the things you rely on when coding in other environments – such as maths functions – are not there. You have to write them yourself. The Zolatron 64, like most homebrew computers, is effectively a bare metal project. I’m writing all… Read more »

Zolatron 64: being persistent

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In an earlier post, I described how I can now load software on to the Zolatron 64 6502 homebrew computer. The LOAD command was pretty basic – you couldn’t specify a file. Instead it just loaded the same file (zd.bin) which I’d have to overwrite if I wanted to load different code. Well, those days are over, and the Zolatron… Read more »

Casio fx-590 – reviving an old friend

It was about to go in the recycling pile when I thought, “I wonder if I can save it.” There’s no telling how old this lovely Casio fx-590 calculator is, because in spite of deploying my best Google-fu I can find very little information on it. One source suggested that maybe it came out at the beginning of the 1990s…. Read more »

Zolatron 64: More adventures in level shifting

In a previous post, I detailed how I’m having to shift voltage levels on signals running between a 65C22 VIA chip and the GPIO pins of a Raspberry Pi. The solution I settled on was to go for simple voltage dividers using pairs of resistors. And that’s been working fine. I mean, you’d expect it to work with the 5V… Read more »

Zolatron 64: The start of an operating system

Let’s be clear about this, I have no idea how you write an operating system. I just know you need to have one. When I first started writing ROM code for the Zolatron 64 (Z64) 6502-based homebrew computer all that really mattered was getting things to work. And I was amazed when they did. I was following in the footsteps… Read more »