IBM System/370 145 Manual page 72

OS/VS1 and OS/VS2 environments, a segment is the unit of virtual storage
allocation.
Each segment of virtual storage is divided into contiguous, fixed-
length, consecutive sets of addresses called virtual storage pages.
Each segment in the virtual storage contains the same number of virtual
storage pages, each of which is the same size. A virtual storage page,
as implemented in System/310, can be either 2K or 4K bytes and is
located on a 2K or 4K virtual storage boundary, respectively, within a
segment.
The contents of virtual storage--instructions and data--are divided
(by the operating system) into fixed-length contiguous areas called
pages, corresponding in size to the virtual storage page size chosen,
either 2K or 4K bytes. The addresses associated with a virtual storage
page refer to the contents of a page.
The direct access storage used to contain the portion of the total
contents of virtual storage that is not always present in real storage
is called external
~
storage. Direct access space within external
page storage is divided into physical records called slots, which are of
page size, either 2K or 4K bytes. A slot, therefore, can contain one
page at a time. A virtual storage page that is allocated and that
actually has contents usually has a slot in external page storage
associated with i t to contain these contents (depending on the nature of
the contents and how external page storage is managed by the operating
system).
Instructions and data are transferred between external page storage
and real storage as needed on a page basis. This transfer process is
called paging, and a direct access device that contains external page
storage is called a paging device. A slot in external page storage is
associated with a particular virtual storage page by means of an
algorithm or via tables that are maintained by the control program.
Real sto~age also is divided into fixed-length, consecutively
addressed areas called ~ frames, -nich are always the same size as
the page being used, either 2K or 4K bytes,. Page frames are located on
2K or 4K real storage boundaries. A page frame is a block of real
storage that can contain one page. Hence, a page of data and/or
instructions occupies a slot when i t is in external page storage and a
page frame when it is in real storage. Whether or not a page is present
in real storage, a program addresses the contents of the page using
virtual storage addresses.
The act of transferring a page from external page storage into real
storage is called a page-in. This action may also be described as the
loading of
~~.
The reverse act, transferral of a page contained in
real storage to a slot in external page storage, is called a page-out.
Figure 15.05.4 illustrates the relationship of virtual storage, external
page storage, and real storage that was conceptually shown in Figure
15.05.2. (Note that the terms swap-in, swap-out, and working set have a
specific meaning in a time-sharing environment and are defined in
OS/Virtual storage
l
Features Supplement under wTime Sharing Option w .)
As previously indicated, OAT hardware uses tables to perform address
translation. These tables are the segment -table and ~ tables. One
segment table and a set of page tables are required to perform address
translation for one virtual storage. The segment table defines the
virtual storage size, indicates allocated virtual storage, and points to
the real storage location of the page tables. The page tables indicate
which pages are currently in real storage and contain the real storage
addresses of these pages. As pages are paged in and out, the control
program makes changes to the page tables as required.
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A Guide to the IBM System/310 Model 145