Yikes! I would love sipping coffee watching the chief architect chew up whoever suggested that. That sounds awful even on a microcontroller.

On s390 the memory at address 0 (low core) has all sorts of important stuff. Of course s390 has paging enabled pretty much always but still...

AVR’s registers are mapped to address 0. So reading and writing NULL is actually modifying r0.

AVR’s r0 is also a totally normal register, unlike most other RISC which typically have r0 == 0.

Thanks for saving me a search, because I was expecting r0 to be hardcoded to zero.

Sometimes hardware is designed with insufficient input from software folks and the result is something asinine like that. That, or some people like watching the world burn.