Single-level store

Single-level storage (SLS) or single-level memory 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.

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. 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: originally, 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.

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