I don&#x27;t think EX typically accesses writable memory, so that wouldn&#x27;t be a big problem (and hardware could actually enforce that target of EX is in executable storage). It&#x27;s used as a workaround to have variable portion of instructions that aren&#x27;t normally variable.<p>The actual non-reentrant code is becoming less common, but it still happens sometimes.<p>Although I think it could be possible to implement that writable pages cannot be executed via a special bit in PSW. Not sure about practical implications to z&#x2F;OS, though.