IBM System/370 145 Manual page 75

long-term fixed. The resident portion of an operating system control
program is never paged and, therE~fore its pages are marked long-term
fixed. Pages that must be fixed for only a portion of the time they are
present in real storage aremarkE~ short-term fixed. For example, a
page containing an I/O buffer is marked short-term fixed prior to the
initiation of the I/O operation t:hat ref.erences the buffer. After the
I/O operation completes, the pagE~ is unfixed and i t becomes eligible for
a page-out. Pages should be
mar~~ed
fixed only when necessary since page
fixing reduces the amount of real storage that can be shared by
concurrently executing paged pro~~ams, (that which is available to be
allocated to the nonfixed pages) and can, therefore, affect system
performance.
As
indicated previously, the supervisor in aOOS/VS environment, and
a portion of the control program in OS/VS1 and OS/VS2 environments, are
resident in real storage. That i.s, their pages' are marked fixed and ~re
not placed in external page stora.ge (because they are not paged) even
though they are allocated space in virtual storage. In OS/VS2 and
OS/VS1, certain other portions of the control program are pageable and
are made resident in virtual storage, which means they are contained in
external page storage during system operation. During system
initialization, these pageable control program routines are allocated
virtual storage and loaded into real storage from system libraries by
the program fetch routine. These routines will be written in external
page storage as a result of normal paging activity in OS/VSl
environments and as a result of specific page-out requests in OS/vS2.
Control program routines that are resident in virtual storage are
brought into real storage from ex·ternal page storage, instead of from a
system library, when they are required during system operation.
JU,st as control program routinles can be fixed or pageable, problem
programs operate in one of two modes in OS/VS1 and OS/vS2 environments:
paged mode or nonpaged mode. The latter is also sometimes called
virtual equals real (V=R) mode.. lNhen a problem program operates in
paged mode, which is called virtual mode in a DOS/VS environment, it is
resident in virtual storage and pageable. A pageable prO<Jram operates
in a contiguous area of virtual s
1
torage (partition or region) and is
assigned available real storage on a demand paged basis. Hence, virtual
storage addresses must be transla1ted into real storage addresses. The
real storage dynamically allocated to programs operating in paged. mode
need not be'contiguous, and such programs normally can operate with less
real storage than their design point (virtual storage) amount
dynamically a110cat.ed to them. This is the mode of operation described
in section 15:05.
Paged (virtual) mode is the nOl:;'mal mode of operation of programs in a
virtual storage environment. HowHver, certain programs cannot operate
correctly in this mode and must rlm in nonpaged (V=R) mode, which is
called real mode in a DOS/VS envil:;'onment. In general, a program must
operate in nonpaged (real) mode if it:
• Contains a channel program that is modified while the channel
program is active (Section 15:: 10 discusses the reason)
• Is highly time dependent (involves certain testing operations on I/O
devices, for example)
• Must have all of its pages in real storage when i t is executing (for
performance reasons, for example)
Other characteristics that require a program to be executed in
nonpaged mode and that are operati.ng system dependent are listed in the
programming systems supplements,
~rhich
also discuss steps that can be
taken to avoid executing a program in nonpaged mode.
A Guide to the IBM System/370 Model 145
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