Editing Memory paging (section)

=={{Anchor|SWAP-SPACE}}History==
In the 1960s, swapping was an early virtual memory technique. An entire program or entire [[Memory segmentation|segment]] would be "swapped out" (or "rolled out") from RAM to disk or drum, and another one would be ''swapped in'' (or ''rolled in'').<ref>{{Cite book
|contribution  = Operating systems
|title         = Encyclopedia of computer science and technology
|editor1-last  = Belzer
|editor1-first = Jack
|editor2-last  = Holzman
|editor2-first = Albert G.
|editor3-last  = Kent
|editor3-first = Allen
|publisher     = CRC Press
|volume        = 11
|page          = 442
|year          = 1981
|url           = https://books.google.com/books?id=uTFirmDlSL8C
|isbn          = 0-8247-2261-2
|url-status       = live
|archive-url    = https://web.archive.org/web/20170227051057/https://books.google.com/books?id=uTFirmDlSL8C&printsec=frontcover
|archive-date   = 2017-02-27
}}</ref><ref>{{Cite book
|first       = Harvey G.
|last        = Cragon
|title       = Memory Systems and Pipelined Processors
|publisher   = Jones and Bartlett Publishers
|page        = 109
|year        = 1996
|url         = https://books.google.com/books?id=q2w3JSFD7l4C
|isbn        = 0-86720-474-5
|url-status     = live
|archive-url  = https://web.archive.org/web/20170227205647/https://books.google.com/books?id=q2w3JSFD7l4C
|archive-date = 2017-02-27
}}</ref> A swapped-out program would be current but its execution would be suspended while its RAM was in use by another program; a program with a swapped-out segment could continue running until it needed that segment, at which point it would be suspended until the segment was swapped in.

A program might include multiple [[Overlay (programming)|overlays]] that occupy the same memory at different times. Overlays are not a method of paging RAM to secondary storage{{efn|name=secondary}} but merely of minimizing the program's RAM use. Subsequent architectures used [[memory segmentation]], and individual program segments became the units exchanged between secondary storage and RAM. A segment was the program's entire code segment or data segment, or sometimes other large data structures. These segments had to be [[contiguous data storage|contiguous]] when resident in RAM, requiring additional computation and movement to remedy [[Fragmentation (computer)|fragmentation]].<ref>{{Cite book
|contribution  = Virtual memory systems
|title         = Encyclopedia of computer science and technology
|editor1-last  = Belzer
|editor1-first = Jack
|editor2-last  = Holzman
|editor2-first = Albert G.
|editor3-last  = Kent
|editor3-first = Allen
|publisher     = CRC Press
|volume        = 14
|page          = 32
|year          = 1981
|url           = https://books.google.com/books?id=KUgNGCJB4agC
|isbn          = 0-8247-2214-0
|url-status       = live
|archive-url    = https://web.archive.org/web/20170227081754/https://books.google.com/books?id=KUgNGCJB4agC&printsec=frontcover
|archive-date   = 2017-02-27
}}</ref>

[[Ferranti]]'s [[Atlas (computer)|Atlas]], and the [[Atlas Supervisor]] developed at the [[University of Manchester]],<ref>{{cite web|url=http://www.chilton-computing.org.uk/acl/technology/atlas/p019.htm|title=The Atlas Supervisor|first1=T|last1=Kilburn|author-link1=Tom Kilburn|first2=R B|last2=Payne|first3=D J|last3=Howarth|date=1962}}</ref> (1962), was the first system to implement memory paging. Subsequent early machines, and their operating systems, supporting paging include the [[IBM M44/44X]] and its MOS operating system (1964),<ref>{{cite conference|author=R. W. O'Neill|title=Experience using a time sharing multiprogramming system with dynamic address relocation hardware|conference=Proc. AFIPS Computer Conference 30 (Spring Joint Computer Conference, 1967)|pages=611–621|doi=10.1145/1465482.1465581}}</ref> the [[SDS 940]]<ref>{{cite book|url=http://bitsavers.org/pdf/sds/9xx/940/900640A_940_RefMan_Aug66.pdf|title=Scientific Data Systems Reference Manual, SDS 940 Computer|pages=8–9|date=1966}}</ref> and the [[Berkeley Timesharing System]] (1966), a modified [[IBM System/360 Model 40]] and the [[IBM CP-40|CP-40]] operating system (1967), the [[IBM System/360 Model 67]] and operating systems such as [[TSS/360]] and [[CP/CMS]] (1967), the [[RCA Spectra 70#Model 70/46|RCA 70/46]] and the [[Time Sharing Operating System]] (1967), the [[GE 645]] and [[Multics]] (1969), and the [[PDP-10]] with added [[Raytheon BBN|BBN]]-designed paging hardware and the [[TENEX (operating system)|TENEX]] operating system (1969).

Those machines, and subsequent machines supporting memory paging, use either a set of [[page address register]]s or in-memory [[page table]]s{{efn|Some systems have a global page table, some systems have a separate page table for each process, some systems have a separate page table for  each segment{{efn|E.g., [[Multics]], [[OS/VS1]], [[OS/VS2]], [[VM/370]]}} and some systems have cascaded page tables.{{efn|E.g.,[[z/OS]].}}}} to allow the processor to operate on arbitrary pages anywhere in RAM as a seemingly contiguous [[logical address]] space. These pages became the units exchanged between secondary storage{{efn|name=secondary}} and RAM.