As as been pointed out elsewhere S/38 AS/400 OS is a SASOS (Single Address Space Operating System). However, the other really new innovation was the capability based security system. This was based on the 9 bit byte. Object pointers had to have the 9th bit set throughout to be legal pointers. Only a limited number of instructions could produce a validated pointer. It was physically impossible to construct a pointer except through the security system. I very much doubt that an AS/400 has ever been hacked.