6. DATA SOURCES › 6.7 Memory Measurements › 6.7.1 Real and Virtual Memory

6.7.1 Real and Virtual Memory

Virtual memory systems appear to have more central storage,
or real memory, than they really do.  The operating system
uses solid state and mechanical DASD devices as virtual
memory, and moves 4K pages from the virtual memory devices
into real central storage when they are needed.  Conversely,
as the pages in real memory become inactive, they are
migrated from central storage to the virtual memory DASD
devices.  Using this virtual memory scheme, MVS is able to
support much higher multiprogramming levels than if it had to
rely upon central storage alone.

The storage scheme used to support the virtual memory
environment on the large mainframes running the MVS operating
systems is hierarchical.  There are four levels to this
storage hierarchy, which have data access speeds ranging from
nanoseconds to milliseconds.

HIGH SPEED BUFFER

The fastest and most expensive storage on the mainframe is
the high speed buffer, or cache memory.  It contains the code
and data recently, currently, and soon to be needed by the
instruction/execution elements for processing.  Typical sizes
of cache memory range from 16K to 256K, where K represents
one kilobyte or 1024 bytes.  Access speed is in the
nanosecond range.

CENTRAL STORAGE

Central storage or main storage is the second level in the
hierarchy.  It supports the storage and retrieval needs for
data and instructions in the processor complex.  It is both
slower and less expensive than cache.  It may be viewed
conceptually as a number of 4K page frames.  Typical sizes of
main storage range from 16M to 1024M, where M represents one
megabyte or 1,048,576 bytes.  Access speed is in the
microsecond range.

EXPANDED STORAGE

Expanded storage is the third level in the hierarchy and acts
as a solid state paging device.  It is a high-speed, high-
volume repository for 4K pages.  It is used to reduce paging
and swapping to mechanical DASD, as well as to reduce data
access times to often read or read only data sets.  Typical
sizes of expanded storage range from 64M to 8192M.  Access
speed is in the microsecond range.

AUXILIARY STORAGE

Auxiliary storage is the final level in the hierarchy and is
the slowest and least expensive.  It is made up of high-
performance mechanical DASD devices that have access speeds
in the millisecond range.  It holds all of the page frames
that make up the currently existing address spaces.  MVS
moves 4K pages to and from central and auxiliary storage as
it successfully executes many more tasks than it could if it
had to depend only upon central storage.