At the opposite end of the spectrum, we find
PicoRV32, a minimal 32-bit RISC-V implementation
- Small (750-2000 LUTs in 7-Series Xilinx Architecture)
- High fmax (250-450 MHz on 7-Series Xilinx FPGAs)
Is that maybe even smaller than Arlet's 6502?? It isn't far off, I think.
(Part of the point of RISC-V is that it describes a family of related, and in some ways compatible, ISAs, from 32 bit embedded to 128 bit full-on. Extensions such as floating point and compressed instruction set are defined but optional.)
(There's
an efficient 128-bit emulator written by the wizard Fabrice Bellard, available in C and in-browser, running a small 64-bit Linux. With a couple of tweaks I was able to build and run it on OSX. It's MIT-licensed.)