2. PERFORMANCE REPORTING ANALYSIS › 2.2 Real Storage Management Analysis › 2.2.4 MVS Concepts › 2.2.4.1 Virtual Storage Concepts

2.2.4.1 Virtual Storage Concepts

This section introduces concepts common to virtual memory
systems.  A virtual storage system (or virtual memory system)
is a system where executing (active) programs reside in
auxiliary storage (i.e., on disk) and selected portions of
the program are placed in real storage as needed.

The need for memory management arises from the development of
hardware and software architecture to support multiprocessing
and time-sharing systems.  The earliest computer systems ran
one job at a time in serial order.  Thus, there was no
sharing of resources.  As computers and peripheral devices
became faster, the desire to use these resources more
efficiently led to the introduction of multiprogramming
systems.  A multiprogramming system allows several jobs to be
in memory concurrently.  This allows one job to use CPU
resources while another is waiting on I/O and vice versa.

The next level of development occurred as people costs
started to approach hardware costs.  The desire to increase
programmer productivity led to the introduction of time-
sharing systems where many users shared memory for short
periods of time.  An interactive time-sharing system gave
individual programmers access to computing power through
terminals when they needed it.  When a given terminal was not
active, the system could occupy itself with other users or
other tasks.

The development of virtual systems stemmed from the creation
of these systems.  Memory was a very expensive resource and
the memory demands of the time-sharing systems greatly
exceeded the capacity of the existing computer systems.  This
began the attempts to deal with the issue of real storage
management.

The type of address resolution used by the hardware was the
major factor determining what type of memory sharing scheme
was used for managing real storage.  The three types of
addressing used by the various systems and some of the design
implications for sharing memory are briefly discussed in
Section 2.2.4.1.1. The purpose of describing these early
systems is to develop the logical progression of these
concepts for the reader not familiar with virtual systems.

Two critical issues arise from the time-sharing of real
memory that are only mentioned in this chapter.  They are
data security and system integrity.  Early systems largely
ignored these subjects.  In all time-shared systems, the real
storage management methodology must have data security as a
primary goal.

The following sections discuss paging systems to give you a
foundation for the discussion of the MVS paging algorithms in
Section 2.2.4.1.2 and for the memory management schemes in
Section 2.2.4.3.

    1 - Address Resolution Schemes
    2 - Paging Concepts