I don't think EX typically accesses writable memory, so that wouldn't be a big problem (and hardware could actually enforce that target of EX is in executable s

I don't think EX typically accesses writable memory, so that wouldn't be a big problem (and hardware could actually enforce that target of EX is in executable storage). It's used as a workaround to have variable portion of instructions that aren't normally variable. The actual non-reentrant code is becoming less common, but it still happens sometimes. 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/OS, though.