A very simple (but also messy) CPU I built in logisim-evolution: https://github.com/reds-heig/logisim-evolution
Features:
- Entire one-level design fits on my screen
- von Neumann architecture
- 32-bit data path
- 24-bit address space / 64 MB memory
- tty output
- all operations can be conditional (though most are useless!)