New Gigatron architecture idea

[tocksin]

It's been a long time since I've looked at the Gigatron, but it keeps calling me back. I want to make a version that removes the things I didn't really like about it, but wanted to hear what people here thought.

I want a von Neumann architecture so I could do native assembly executing out of RAM (no vCPU reliance). This opens the possibility of expansion slots for hardware with their own memory space. I also didn't like counting lines for racing the beam in native assembly. So I want to include an interrupt and an external timer. Then you shove the video drawing into a interrupt service routine, and don't ever have to worry about counting lines again. And of course, I still want to get a native keyboard (no pluggy).

I laid out a design, but it adds 10 more chips to the design. It keeps the same instructions with two exceptions: instead of the input register being put on the databus, it's now a peripheral in the memory space. Instead the Y register can be put on the databus. This is required for saving the register values during an interrupt anyway. And then I have to add a return from interrupt instruction.

Of course this means software is not really compatible anymore. It might not take a lot to convert some software, but I just wanted to make native assembly more fun to write. What do people here think about this? Do you just prefer to write in vCPU or BASIC? Is it worth the 10 extra chips?

[monsonite]

I read your post, I understand your wishes.

I met Marcel in September 2019 in Cambridge UK, for a vintage computer festival. We presented the Gigatron to the UK computer enthusiasts.

Marcel and I worked on a superfast Gigatron 12.5 MHz.

IMHO. Please don't try to re-invent the Gigatron. Leave it, as it is - as a lasting memorial to it's creator Marcel van Kervinck.

If you have ambitious ideas, then post them here, but leave the Gigatron for what it is, so eveyone can enjoy it, and Marcel's talent

[tocksin]

I agree. The Gigatron and Marcel are a great inspiration. And they are not the only ones that I draw inspiration from. Even the Gigatron includes ideas from other designs. For example, the ALU design did not originate with Marcel and other CPU designs share the same ALU. I think it's which ideas we choose to draw from which create something new and unique. The Novasuar, the Kobold, the Isetta, Eater's CPU - they all are very similar because they're all gate-level CPU designs with graphics outputs. But they are still all very different. I don't have a name for mine just yet, but I'll let you know when I do.

[Hans61]

Are you familiar with this project Minimal-64x4:

https://github.com/slu4coder/Minimal-64x4-Home-Computer

https://www.youtube.com/watch?v=L1oECH6rPvs

[qwertyface]

I think that for me a lot of what is interesting about the Gigatron, and would thus make something "Gigatron-like" is exactly the sort of thing you might be trying to get away from.

I feel Marcel found a point of balance, where he could use a relatively small amount of logic and yet get a machine that was relatively fast and powerful. I imagine that most small changes would result in something that was both bigger and worse. Quite how much luck or judgement were involved, I don't know.

Programming the Gigatron often involves quite a lot of "Sudoku solving", but I think the quirky architecture throws up solutions as often as problems. A good example of this is the way that the combination of the Harvard architecture, pipeline without hazard detection and segmented memory map allows for the right-shift table, which would be much slower and more space intensive otherwise. Apparent shortcomings that actually go a long way to compensate for another apparent shortcoming (no right-shift instruction).

A good point for discussion might be what the essential Gigatron traits are, and which are incidental. For example, I definitely consider the Harvard architecture an essential feature, but the XOUT register is definitely an implementation detail.