z/os and os/390 are both like versions of operating system in Mainframe
and coming to AS/400(Application System 400) it can be said as sister consignee for MAINFRAME machine..
check these link for some more details
Joined: 26 Apr 2004 Posts: 4648 Location: Raleigh, NC, USA
z/OS supports the 64-bit real storage which will improve performance, throughput, and scalability characteristics. 64-bit storage support (z/Architecture) eliminates expanded storage and reduces or eliminates paging, and it also allows the consolidation of LPARs, reducing system overhead.
Enhancements to the Intelligent Resource Director (IRD) and the Workload Manager (WLM). The Intelligent Resource Director provides a new, dynamic facility for managing CPU consumption, Channel Paths, and Channel Subsystem Priorities across multiple system images (LPARs), based on workload priorities. Workload Manager is extended to work across LPARs, and has had a number of self-optimizing advances in workload management.