Surprised no one has mentioned that the Lisp Machine operating system worked this way. Maybe it doesn't count because there was no protection against intention

Surprised no one has mentioned that the Lisp Machine operating system worked this way. Maybe it doesn't count because there was no protection against intentional reading or writing of memory belonging to other apps or the OS. But the protections against unintentional writing were good enough to make for a remarkably robust system, in a time when nobody cared about security.