QEMU switched to a data driven representation with a python program to autogenerate the "check bit patterns and extract fields" code, and it's one of the better design overhauls we've done: we started using it mostly for new code but went back and converted some of the old handwritten decoders too. It's much easier to add a new instruction when you only need to add a line like

  USADA8           ---- 0111 1000 rd:4 ra:4 rm:4 0001 rn:4

With hindsight I would have loved to do a proper compiler but that’s undergrad level really. I really recommend it as a toy post-food-coma project for when you’re stuck with the family either next week or at the end of December :)

https://github.com/pts/pts-mips-emulator