Virtual Storage And Dynamic Address Translation Concepts - IBM System/370 145 Manual

Given the existing processor storage restraints on application design
and development and the storage requirements that are becoming
increasingly more characteristic of many of the new types of
applications, it becomes advantageous to allow programmers to design and
code applications for a larger address space than they currently have.
That is, programmers should be able to use as much address space as an
application requires so that special program structures and techniques
are not required to fit the application into a given storage size.
Programmers can then concentrate more on the application and less on the
techniques of programming. In addition, the size of the address space
provided should not be determined by processor storage size, as it is in
DOS Versions 3 and 4, OS MFT, and OS MVT, so that the address space can
be larger than the processor storage available.
A larger address space should be provided, therefore, by a means
other than making processor storage as large as the address space
desired. This requirement can be satisfied by providing programmers
with an address space (called virtual storage) that is supported using
online direct access storage and dynamic address translation hardware.
This approach also offers the advantage of supporting a larger address
space for a lower cost than if larger processor storage is used, since
direct access storage continues to be significantly less expensive per
bit than processor storage. In addition, dynamic address translation
hardware offers functional capabilities that large processor storage
alone cannot provide.
VIRTUAL STORAGE Am> DYNAMIC ADDRESS TRANSLATION CONCEPTS
Virtual storage is an address space the maximum size of which is
determined by the addressing scheme of the computing system that
supports it rather than by the actual number of physical processor
storage locations present in the computing system. In System/370; for
example, which uses a 24-bit binary address, a virtual storage as large
as 16,777,216 bytes can be supported. When virtual storage is
implemented, the storage that can be directly accessed by the CPU,
normally called processor or main storage, is referred to as real
storaqe.
The concept of virtual storage is made possible by distinguishing
between the names of data and instructions and their physical location.
In a virtual storage environment. there is a distinction between address
space and real storage space. Address space (virtual storage) is a set
of identifiers or names (virtual storage addresses) that can be used in
a program to refer to data and instructions. Real storage space is a
set of physical storage locations in the computer system in which
instructions and data can be placed for processing by the CPU. The
number of addresses in the two spaces need not be the same, although
both spaces begin with address zero and have consecutive addresses. The
programmer refers to data and instructions
,by
name (virtual storage
address) without knowing their physical location.
When virtual storage is not implemented, there is, in effect, no
differentiation between address space and real storage space. The
address space that can be used in programs is identical in size to the
real storage space available and the address in an instruction
represents both the name and the location of the information it
references.
In a virtual storage enVironment, therefore, the address space
available to programmers is that provided by the virtual storage size
implemented by a given system--not the address space provided by the
real storage available in the given system configuration. In DOS/VS,
OS/VS1, and OS/VS2, virtual storage rather than real storage is divided
into consecutively addressed partitions or dynamically allocated regions
48
A Guide to the IBM System/370 Model 145