[PopePompus]

What happened if you used auto-decrement addressing with the PC? Did that hang the computer?

[Taniwha]

Yup, as mentioned above the famous:

x: mov -(pc), -(pc)

reads an instruction from x, increments the PC to x+2, then decrements the PC back to x and reads the same instruction, then decrements it again to x-2 and writes it, then it executes from x-2 ......