Information Technology Reference
In-Depth Information
the full document already displayed, main memory may con
tain only the current or recently displayed pieces.
As editing of the document proceeds, the computer will have to
work to keep cache, main memory, and virtual memory consistent,
so that any changes made in one small section (perhaps in cache) are
also made in main memory and virtual memory.
When, later on, an updated document is saved, processing again
moves to a new part of the program. Some main steps in the saving
process are as follows:
9.
The “save” part of the program is identified in virtual mem
ory, copied as a section to main memory, and then placed in
cache in pieces as needed.
10.
The updated document from virtual memory is copied back
to a regular file on a disk or other medium.
As this outline suggests, virtual memory contains the entire pro
gram and data for an application as it runs; main memory contains
sections of the program and data set used recently; and cache con
tains the most recent materials used in processing by the CPU.
Movement of program sections and data to the main memory and
to cache occur on an “asneeded” basis, and materials are not
copied until they are actually needed for processing.
This question arises frequently when people are considering the
purchase of a computer. Unfortunately, the only safe answer is
something like “it depends on what you plan to do.” Large soft
ware applications and applications that utilize graphics or sound
may make heavy demands on main memory. Most application
packages make recommendations regarding an appropriate mini
mum size for main memory; however, such minimums may not be
reliable guides for purchasers of new equipment for at least three
reasons:
1.
New versions of software may be larger and require more
memory than previous versions, so when you upgrade your
