"With a single-level storage the entire storage of a computer is thought of as a single two-dimensional plane of addresses, pointing to pages. Pages may be in primary storage (RAM) or in secondary storage (disk); however, the current location of an address is unimportant to a process. The operating system takes on the responsibility of locating pages and making them available for processing. If a page is in primary storage, it is immediately available. If a page is on disk, a page fault occurs and the operating system brings the page into primary storage. No explicit I/O to secondary storage is done by processes: instead, reads from secondary storage are done as the result of page faults; writes to secondary storage are done when pages that have been modified since being read from secondary storage into primary storage are written back to their location in secondary storage."
IOW this is classic VM behaviour. Which bit of the wiki article is revelatory?
(NB. I've actually used multics).