Relationship Of Virtual Storage, Direct Access Storage; And Real Storage - IBM System/370 145 Manual

without regard for a specific virtual storage operating system
implementation.
The division of a program and .its data into sections and the transfer
of these sections between direct .access storage and real storage during
program execu·tion is hand1ed entilr:ely by the virtual storage operating
system without any effort by the l~rogrammer. When a planned overlay or
dynamic overlay program structure is used, the programmer is responsible
for dividing the program and its data into phases, determining which
phases can
be
present at the same time in the amount of real storage
available (partition or region), and indicating when phases are to be
loaded into real storage during p1rocessing.
Virtual Storage
,.-----------
.....
--
-
- - - -
Address space
allocated to
executing programs
Tables or an
algorithm used
to map virtual
storage sections
to dire.ct access
storage sections
[) irect Access Storage
Olntents of a portion
of virtual storage
(instructions and
data for executing
programs)
Tables map
virtual storage
sections to real
storage sections
"
Real Storage
Active sections
of executing
programs
Control program Control program
Figure 15.05. 2. Relationship of v'irtual storage, direct access storage,
and real storage
While a virtual storage up to 16 million bytes in size can be
addressed by any System/370 model with OAT hardware, the virtual storage
size "t::hat can be effectively implemented by a given system is affected
by (1) the amount of real storage present,
(2)
the amount of direct
access storage space available to contain the contents of virtual
storage, (3) the speed of the direct access storage devices containing
virtual storage contents, and contention for these devices or the
channels to which they are attached, (4) the speed of the CPU, and (5)
the characteristics of the programs operating concurrently. Hence, the
amount of real storage required to effectively implement a specific
amount of virtual storage can vary by system, depending on the
characteristics of the applications in the workload and the performance
desired, as is discussed in section 15:15.
Once a program section has been loaded into real storage, its virtual
storage addresses can be ~ranslated when they are referenced. Dynamic
address translation hardware is thle mechanism that translates the
virtual storage addresses contained in instructions into real storage
addresses during instruction execuition,. Address translation is
accomplished in System/370 us~ng a hardware-implemented table lookup
procedure that accesses tables corutained in real storage. These tables,
which are maintained by control program routines, (1) define the amount
of virtual storage supported and a:Llocated,
(2)
indicate whether or not
any given program section is currently present in real storage, and (3)
A Guide to the IBM System/370 ModeJL 145
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