Names And Location Of Instructions And Data In A Virtual; Storage Environment - IBM System/370 145 Manual

Consecutive
addresses
o
to 16,777,215
maximum in
System/370
Virtual Storage
r-------l
I
I
Address space
available to
programmers
I
I
I
I
I
I
I
Address space a lIocated
I
to the control program
I
that is always present
in real storage
I
L _ - - - ___
J
Names of instructions
and data
-mapped
Contains
virtual storage
addresses
Executable program
Direct Access Storage
Contents of a portion
of virtual storage
(instructions and data)
Location of data
and instructions
Figure 15.05.1. Names and location of instructions and data in a virtual
storage environment
For the purpose of resource management in a virtual storage
environment, virtual storage and its contents, direct access storage
used to contain a portion of the contents of virtual storage, and real
storage are divided into contiguous, fixed-length sections of equal
size. Once a program has been fetched from a program library and
initiated, instructions and data within a program are transferred
between real storage and direct access storage, a section at a time,
during program execution. A section of an executing program is brought
into a real storage section only when it is required, that is, only when
a virtual storage address in the section is referenced by the executing
program.
A
program section that is present in real storage is written
back in a direct access storage section only when the real storage
assigned to i t is required by another program section and only if it has
been changed.
A virtual storage operating system control program monitors the
activity of the sections of all executing programs and attempts to keep
the most active sections in real storage, leaving the least active
sections in direct access storage. Figure 15.05.2 illustrates the
relationship of virtual storage, direct access storage, and real storage
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A Guide to the IBM System/370 Model 145