Comment by sacnoradhq

This statement would be technically legal on its own in x86 real mode if the compiler didn't do null pointer checks. However it would set the divide-by-zero IRQ handler to itself 0000:0000, and when the next division by zero happened, the machine run into UB (likely a reset or halt) because it would jump there, do 4x ADD byte ptr [BX + SI], AL (or ADD byte ptr [EAX], AL) followed by running the remaining interrupt vectors as instructions.