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I'm also noodling on an emulator, shooting for cycle accuracy (to the point where you'd be able to run vapor lock demos). Mine passes the 6502 functional tests but not yet the Tom Harte suite. To add to the difficulty, I want to run it on an RP2040 and bit-bang the video. Is yours up in an open source repo? Mine isn't (yet) but I'm open to joining forces.

I started mine about 6 years ago, put it on the shelf, and am getting back to it. Overall I think Rust is working well for it (and for pico DVI out), but I can also see the advantages of Zig for something this low-level and specialized.

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133Mhz, that's 133 cycles per instruction for a 6502 (assuming you emulate only the 6502). Seems doable but then there's the BCD stuff and that's rather tough emulate (you'll definitely need a lot of cycles to reproduce that)...

But that sounds real cool. Would you share you project ?

It's a bit tricky, but my decimal adc is 19 lines of code, all of which is fairly straightforward, mostly bit fiddling stuff that I expect will compile to a few instructions each.

I need to go through an open source releasing process but I'll get that going. Thanks for your interest!

As a followup, here it is:

https://github.com/raphlinus/r6502