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Single-level storage (SLS) is a computer storage term which has had two meanings. The two meanings are related in that in both, pages of memory may be in primary storage (RAM) or in secondary storage (disk); however, the current actual physical location of a page is unimportant to a process. It originally meant what is now usually called virtual memory, introduced in 1962 by the Atlas system at Manchester.〔''One-Level Storage System'', T. Kilburn, D.B.G. Edwards, M.J. Lanigan, F.H. Sumner, IRE Trans. Electronic Computers April 1962 (Accessed 2014-Aug-07 )〕 It now usually refers to the organization of a computing system in which there are no files, only persistent objects (sometimes called segments), which are mapped into processes' address spaces (which consist entirely of a collection of mapped objects). The entire storage of the computer is thought of as a single two-dimensional plane of addresses (segment, and address within segment). The persistent object concept was first introduced by Multics in the mid-1960s, in a project shared by MIT, General Electric and Bell Labs.〔''Virtual Memory, Processes, and Sharing in Multics'', Robert C. Daley, Jack B. Dennis (Accessed 2014-Aug-07 )〕 It also was implemented as virtual memory, with the actual physical implementation including a number of levels of storage types. (Multics, for instance, had three levels: main memory, a high-speed drum, and disks.) SLS is now most often associated with IBM i (formerly known as ''i5/OS'' or ''OS/400''), the operating system of the IBM System i, although IBM first implemented SLS in 1978 in the System/38 and its Control Program Facility (CPF) operating system, the predecessor to IBM i. == Design == 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 and 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Single-level store」の詳細全文を読む スポンサード リンク
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