Virtual memory
Virtual memory is managed by UMMPS with assistance from the Paging Device Processor (PDP) job program. UMMPS responds to requests to allocate and free VM from job programs, allocates VM addresses, allocates real memory, manages segment and page tables, sets storage keys, manages reference and change bits, determines which virtual memory pages should be paged in or out, and communicates with the PDP. New virtual memory pages are initialized to a "core constant" value of x'81' on first reference.