As far back as we can remember, there have always been hacks, exploits, and just curiosity about undocumented CPU instructions. The Z80 had them. Even the HP41C calculator had some undocumented codes. The HCF (Halt and Catch Fire) instruction was apocryphal, but we always heard the old video controller chips could be coaxed into blowing up certain monitors. You don’t hear too much about things like that lately, perhaps because fewer people are working in assembly language.
[Sergi Àlvarez i Capilla] not only works in assembly language, he was writing an ARM assembler when he noticed something funny. Instructions are built in a regular pattern and some of the patterns were missing. What to do? [Sergi] lost no time trying them out.
His result? He found at least one instruction that causes a Raspberry Pi to lock up dead. All by itself, that’s not a big deal, but the instruction isn’t protected so any user program could potentially lock up a Pi. A reset will set things right, of course. No combustion occurs. The newer models, by the way, seem to be immune and [Sergi] tested some phones and other devices with ARM processors to no effect.
The practical implications? Probably not much? But if you were putting a Pi in a critical system, you might find this result interesting. Maybe the best defense against this kind of hack is only having one instruction to start with. The 6502, by the way, was well known for having a large number of useful undocumented op codes. There’s some talk about that in the video below (the undocumented part starts around 6:00).