Custom firmware for emulating a Soviet mpi bus rom

[Max120l]

Hi! I purchased an Elektronika uknc 0511, which is a pdp11 clone used in Soviet schools in the late 80s. It uses a bus similar to qbus with shared address/data lines, and one of the 4 ROMs that live on this bus was dead. So with a lot of help from Claude (don't hate me too much for this), we came up with a firmware that emulates the original ROMs and also replaced the 4 ROMs with only one. It also has a ram tester and other diagnostic tools. There's also the possibility of serving software from it ( haven't explored it yet though). Here's the repo and I hope it can be of use : https://github.com/Max120l/onerom-1801re2

[piersfinlayson]

Thanks for the post and delayed response - I will take a look at this soon!

[Max120l]

You're welcome! Looking forward to hear what you think about it!

[piersfinlayson]

I've had time to look at this now - it's an interesting project. While I had come across ROMs with muxed address/data pins and latched address lines before, I don't remember them using the "ROM is ready to serve" signal.

I see the approach you've had to take - read a 24 bit wide span of GPIOs and then turn that into the 16-bit word lookup address - as a result of the Fire 24 GPIO/physical pin mapping not be able support "One ROM style" direct PIO->DMA->PIO serving. So, instead you've got a CPU core doing the address re-construction.

I can also see the diagnostics tooling you've built into One ROM to be able to monitor activity on the host.

Very clever!

Given the pin-out is so different from a regular One ROM, I think my approach here would be to introduce a shim board, to map the data/addr pins from the original ROM to be more similar to other ROMs supported by One ROM. Some thought would be required to ensure 2 x contiguous 8-bit blocks for the 16 data lines. And this would require a custom set of serving algorithms. This would remove the requirement for CPU involvement in the serving.

I have in mind an enhancement to allow an arbitrary set of PIO instructions to be programmed to One ROM as part of writing its configuration - in addition to the pre-defined serving algorithms. This could take advantage of that.

This would also mean that the diagnostics could probably be moved to be self-contained with a plugin, not requiring core firmware changes.

Thanks for sharing!

[Max120l]

Thank you for looking at it! It's been working flawlessly so far, and saved me from being stuck with sourcing something completely unfindable. I have a million other projects going on, but when I'll have time I'll look at building up the diagnostics. Thank you for the suggestions and creating such a malleable piece of hardware