Layout Of Virtual Storage, External Page Storage, And Real Storage - IBM System/370 145 Manual

Virtual Storage
Segment N
(pages 0 to 15 or 31)
~~
Virtual
storage
pages
within
segments
~~
Segment 1
(pages 0 to 15 or 31)
Segment 0
(pagesOto 150r31)
~~
--
~~
Tables or an
algorithm
map pages
and slots
Paged area
Nonpaged area
External
Page Storage
Slots
(containing
pages of
instructions
and data)
Contents of
pageable
virtual storage
Tables map
virtual storage
pages and
page frames
Page-out
Page-in
Page frames
(containing active
pages of executing
programs)
Control
program
- -
_.
- - - -
-- - - -
-
-'----------
Address space for
programmers use
Figure 15.05.4. Layout of virtual storage, external page storage, and
real storage
Basic to the implementation of virtual storage using direct access
storage and DAT hardware is the
mE~thod
of determining when pages are to
be brought into real storage and, therefore, when real storage is
allocated to pages. The method supported by IBM-supplied virtual
storage operating systems, that
01:
bringing a page into real storage
only when it is needed by an executing program, is called a demand
paging technique. Since programs execute on a priority basis in DOS/VS,
OS/VS1, and OS/vS2 environments, as they do in OS (MFT and MVT) and in
DOS (Versions 3 and 4) environmen1:s, .real storage is, in effect, still
allocated on a priority basis.
A request for a page-in is· genE~rated by the occurrence of a ~
exception or a ~ translation exception, a condition that is also
called a
~
fam t.
An
interrupt:ion occurs during the execution of an
instruction when DAT hardware attE~mpts to translate a virtual storage
address into a real storage addresis and the appropriate page table
indicates that the page is not cuz:rently present in real storage. A
page fault condition causes an int:erruption in order to alert the
control program to the fact that ~L page frame must be allocated.
A Guide to the IBM System/370 Mode!! 145 63