I think it’s fair enough in this case to say “modern” probably means “modern programming languages can compile to and run on it”

There’s some question in the comments here about how much of the ISA is actually implemented but it should theoretically be possible to write Rust code and run it on this thing, for example. There are many other toy CPU designs out there which are much more limited in terms of what can compile to run on them.

Architecture-wise, even a regular modern CPU isn't that hard. For example, Berkeley has the BOOMv3 core[0] which is performance-competitive[1] with commercial chips taped out in the last few years. I think commercial chips are faster because of improvements in analog design, and not some super special architecture sauce (although I'm sure there's some special sauce -- it's probably not the defining factor).

[0]: https://boom-core.org

[1]: https://carrv.github.io/2020/papers/CARRV2020_paper_15_Zhao....

A modern CPU needs two things:

1. Privilege levels with preemption.

2. Hardware support for virtual memory.

At that point, it can run OSes broadly similar to what most people are using in practice.

I disagree. Modern refers to using the learnings and improvements that have been made over the last few decades of designing CPUs. To me including 2 features doesn't necessarily make a design modern.

My point is to separate the absolute toys (CPUs which are on a level with the 6502) from the somewhat more complicated designs that can be used to demonstrate fundamental concepts beyond just decoding and executing instructions.

Ah, yes, those two things from the sixties to run that OS from the seventies.

Right because a RISC-V CPU isn't modern.

You can make an outdated CPU design with any ISA.

They put modern in quotes. What does that tell us?

That they are building their design using discrete parts which is not a modern way thing to synthesize logic to.