Terminology; Operation Of Dynamic Address Translation Hardware - IBM System/370 145 Manual

Summary
As the preceding discussi9niDdicates, a virtual storage environment
is designed primarily to providenev functional capabilities for the
installation as a whole, althougb performance gains are possible for
installations with particu1ar environmental characteristics. The
general functional aims of the IBMrsupplied virtual storage operating
systems are
(1)
to use new hardware features and additional control
program processing to support certain facilities that are not possible
in a nonvirtual storage environment because of real storage restraints
and (2) to hand1e other ftmctions tba.t must be performed by installation
personnel (programmers, operat.ors, and system designers) when virtual
storage and dynamic address t:rans.lation are not used.
It is also important to note tbat while a .virtual storage operating
system permits an installation to be ind.ependent of real storage
restraints to a large degree· and
~enabl.es
real storage to be utilized
more efficiently, the performance of the system and the specific
advantages that can
be
achieved a~e still largely dependent on the
amount of real storage present in the syst.em and on the computing speed
of the CPU, among other things. llience, virtual storage and dynamic
address translation are not
a
substitute for real storage. Rather, they
provide an installation with
gz'ea
1
ter flexibility in the tradeoff between
real storage size and function or performance.
The degree to which a part:.icu1iflr ipstal1ation experiences the
potential benefits of a virtua1 sitorage/dynamic address translation
environment is highly system conf:iguration dependent and application
dependent (number, type, complexiity of applications installed or to be
installed). In addition,
consideJ~ation
must
be given to the system
resources that are specifica1.1y r4equired to support a virtual storage
environment (discussed in seCtion 15:15). Some of the potential
advantages, such as those associ~ted with application maintenance and
operational flexibility and those that result from better management of
real storage, can be experienced iiS soon as a virtual storage operating
system is installed. Others may JtJe aChieved in the future, when new
applications are installed aDdtb~ less restrictive program design
techniques available in a vi~ua1 storage environment are more fully
utilized. In any case, insta11at:ion of a virtual -storage operating
system can make System/370 easier to use and can
be
a major step toward
more rapid installation of appliofltions. Such an operating system can
be of greatest benefit to insta1loiltions desiring to move to or to extend
online operations and thereby atbrln the advantages such an environment
offers.
VIRTUAL STORAGE AND DYNAMIC
ADD~>S
TRANSLATION TERMINOLOGY
For the purpose of presenting 1the concepts of virtual storage and
dynamic address translation, in tbe prev·ious discussion, virtual
storage, programs and data, direc1t. access storage, and real storage were
described as being divided into
~~eas
ca11ed sections. In reality, a
unique term is used to describe eclch of the various sections, namely,
virtual storage page, page. s1ot, and pa.ge frame. In addition, virtual
storage has two levels of
subdivi~iion
in System/370. The following
defines the new terminology actu~lly used by the System/370 virtual
storage operating systems.
Virtual storage in System/370
jis
divided into contiguous segments,
which contain virtual storage pa941~s. A virtual storage semnent, as
implemented in System/370. is a fixed-1ength" consecutive set of
addresses for either 64K or 102ltK bytes that begins on a 64K or 1024K
boundary, respectively. in vi'rtoaJL storage. A virtual storage is
divided into segments all of one !;ize or the other. In general, in
A Guide to the IBM System/370 ModE!! 1115 61