If I do a read of a &#x27;null&#x27; pointer on the Cortex ARM I&#x27;m using it&#x27;s the initial stack pointer. If you try to write to that without some special magic involving the flash controller one gets a bus fault interrupt. Behavior depends on what the ISR does.<p>AVR Processors don&#x27;t have bus faults. Some machines I&#x27;ve worked on have RAM based at address zero.