Single-level store - Revision history

Jnc: +cat - not the best, but doesn't really have a perfect one

2018-12-16T00:10:39Z

+cat - not the best, but doesn't really have a perfect one

Jnc: Emphasize SLS and paging are logically unconnected

2017-10-06T13:03:45Z

Emphasize SLS and paging are logically unconnected

Jnc: /* Initial meaning */ rm dup link

2017-09-21T16:54:22Z

‎Initial meaning: rm dup link

Jnc: Pretty decent coverage, IMO

2017-09-16T22:26:01Z

Pretty decent coverage, IMO

New page

A '''single-level store''' ('''SLS''') or '''single-level memory''' has had two meanings; they are related in that in both, information may be in [[main memory|primary storage]] (usually [[Random Access Memory|RAM]]) or in [[secondary storage]] (usually [[disk]]), and the [[operating system]] automatically moves information between them, as needed, without the active involvement of the user.

==Initial meaning==

It originally meant what is now usually called [[virtual memory]], introduced in 1962 by the [[Atlas]] system at Manchester. In this, memory is divided into fixed-size [[page]]s, and the current actual physical location of a page is unimportant to a [[process]]; the operating system automagically arranges to bring pages into primary storage from secondary storage when a [[process]] needs them.

Since secondary storage is usually much larger than primary storage (or, to emphasize the converse, primary storage is usually much smaller than secondary storage), everything cannot simmply be brought into primary storage; it is necessary to manage what is in primary storage, and move things out to make room, as needed.

The operating system takes on the responsibility of locating pages and making them available for processing: if a needed page is in primary storage, it is immediately available; if a page is in secondary storage, a [[page fault]] occurs and the operating system brings the page into primary storage.

==Contemporary meaning==

The term SLS now usually refers to an [[architecture]] of a computing system in which there are no [[file]]s, only persistent objects (often called [[segment]]s), which are mapped into processes' [[address space]]s (which usually consist entirely of a collection of mapped objects).

The entire storage system of the computer, including both primary and secondary storage (the distinction between which is no longer visible to the users) can be thought of as a single two-dimensional plane, with segment numbers on one axis, and addresses within a segment on the other.

Although in theory entire segments could be moved between primary and secondary storage, it is usually implemented as virtual memory, using pages, for a number of practical reasons: e.g. the latency when a missing segment is needed will be higher if entire segments are being moved; the [[main memory]] allocation issue is simpler when it is all allocated in fixed-size pages; etc.

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, often to make room in primary storage for other pages.

===History===

The persistent object / single-level store concept was first introduced by [[Multics]] in the mid-1960s, where the actual physical implementation included a number of levels of storage types. Multics initially had three levels: main memory, a high-speed [[drum]], and disks.

← Older revision		Revision as of 00:10, 16 December 2018
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	The persistent object / single-level store concept was first introduced by [[Multics]] in the mid-1960s, where the actual physical implementation included a number of levels of storage types. Multics initially had three levels: main memory, a high-speed [[drum]], and disks.		The persistent object / single-level store concept was first introduced by [[Multics]] in the mid-1960s, where the actual physical implementation included a number of levels of storage types. Multics initially had three levels: main memory, a high-speed [[drum]], and disks.
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 The entire storage system of the computer, including both primary and secondary storage (the distinction between which is no longer visible to the users) can be thought of as a single two-dimensional plane, with segment numbers on one axis, and addresses within a segment on the other.
-Although in theory entire segments could be moved between primary and secondary storage, it is usually implemented as virtual memory, using pages, for a number of practical reasons: e.g. the latency when a missing segment is needed will be higher if entire segments are being moved; the [[main memory]] allocation issue is simpler when it is all allocated in fixed-size pages; etc.
+Although in theory entire segments could be moved between primary and secondary storage, this kind of system is usually implemented as virtual memory, using pages, for a number of practical reasons. (E.g. the latency when a missing segment is needed will be higher if entire segments are being moved; the [[main memory]] allocation issue is simpler when it is all allocated in fixed-size pages; etc.)
 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, often to make room in primary storage for other pages.

@@ Line 3: / Line 3: @@
 ==Initial meaning==
-It originally meant what is now usually called [[virtual memory]], introduced in 1962 by the [[Atlas]] system at Manchester. In this, memory is divided into fixed-size [[page]]s, and the current actual physical location of a page is unimportant to a [[process]]; the operating system automagically arranges to bring pages into primary storage from secondary storage when a [[process]] needs them.
+It originally meant what is now usually called [[virtual memory]], introduced in 1962 by the [[Atlas]] system at Manchester. In this, memory is divided into fixed-size [[page]]s, and the current actual physical location of a page is unimportant to a [[process]]; the operating system automagically arranges to bring pages into primary storage from secondary storage when a process needs them.
 Since secondary storage is usually much larger than primary storage (or, to emphasize the converse, primary storage is usually much smaller than secondary storage), everything cannot simmply be brought into primary storage; it is necessary to manage what is in primary storage, and move things out to make room, as needed.