IBM System/370 145 Manual page 62

contain the addresses of real storage sections allocated to the program
sections that are currently .present in real storage.
During the execution of each instruction, address translation is
performed on any virtual storage address in the instruction that refers
to data or to an instruction. Translation occurs after the 24-bit
effective virtual storage address has been computed by adding base,
displacement, and, if any, index values together, as usual. The result
of the address translation is a 24-bit real storage address designating
the location containing the data or instruction referenced by the
virtual storage address in the instruction. The virtual storage
addresses in channel programs (CCW lists) a:re not translated by channel
hardware during channel program execution and programmed translation is
required prior to initiation of a channel operation.
In reality, OAT hardware provides dynamic relocation of the sections
of a program during its execution. This capability is not provided
by
DOS Version 4, OS MFT, and OS MVT. DOS Version 4 supports program
relocation only at link-edit time. MFT and MVT support program
relocation at program load time as well as at link-edit time. Once a
program has been loaded into an area· of real storage by the program
fetch routine, these operating systems cannot relocate the program to
another area of real storage during its execution. Thus, an entire
program or a portion of a program cannot be written on direct access
storage during execution and later reloaded. into different real storage
locations to continue execution. Once loaded, therefore, a program is
bound during its execution to its initially allocated real storage
addresses. In a virtual storage environment, a program is bound only to
the virtual storage addresses it was assigned during loading.
The dynamic relocation provided by OAT hardware eliminates, for most
programs, the need for allocating and dedicating a contiguous area of
real storage to an entire program for the duration of its execution, a
requirement for all programs in DOS Version 4, OS MFT, and OS .MVT.
(As
discussed later in this subsection, some programs cannot operate
correctly in the manner being described, that is, with .sections
transferred only as required between direct access storage and real
storage.) In a virtual storage environment., real storage is no longer
divided into contiguously addressed partitions or dynamically allocated
regions that can contain one executing job step (program) at a time.
Further, when real storage is allocated toa section of an executing
program, the real storage is not dedicated to that program section for
the duration of program execution. Concurrently executing programs can
dynamically share the same real storage sections. That is, in general,
the real storage available for allocation to executing programs can be
allocated to any program section
as
needed. When a section of an
executing program must be loaded, any available section of real storage
can be assigned (subject to certain restrictions' imposed by operating-
system-dependent real storage organizations). When the program section
is no longer required, i t can be written in direct access storage, if it
has been altered, and the real storage assigned to i t can be made
available for allocation to another section of the same program or to a
section of another program.
The assignment of real storage sections is handled entirely by the
operating system, which keeps account of which sections of concurrently
operating programs are the most active. The operating system does not
attempt to allocate a given amount of real storage to each executing
program. It merely allocates real storage to those sections it
determines are the most active, without taking into account the
particular program to which the active section belongs.
OAT hardware, therefore, provides more than translation from address
space (virtual storage) to real storage space. It provides the
52 A Guide to the IBM System/310 Model 145