In the early 1940's when computers were still in their infancy,
designers had many hardware and software problems to overcome. Bitter
arguments ensued over design as new and radical prototypes appeared.
One such area of concern was the overall storage system of the machine.
Early computers used the two-level storage system consisting of main
memory and secondary memory. The main memory (RAM) for the first computers
were magnetic cores. The secondary memory (hard disks) were magnetic drums.
Back then, storage was neither inexpensive nor easy to find. Today, 128
megabytes of RAM is a standard in most personal computers. Sixty years ago,
the most sophisticated computer filled up a warehouse and was lucky to have
128 kilobytes, a fraction of what a floppy disk can hold.
This lack of space has serious implications. What happens if program
"grows" while it is being executed? Eventually it will run out of main
memory to use. This was one of the main complaints programmers had. They
had to re-code their program every time they switched machines or added more
It would be nice to have an "unlimited" amount of fast, cheap memory in
order to do some serious computing. AHA! Main memory is not the only type
memory we have.
Although a significant amount slower, we have the non-volatile hard
drive. Now, when we run out of main memory we could use the hard drive's
memory to store data and code. Because the hard drive is so slow we would
like to keep most of the current program in our fast main memory and
create the illusion of an unlimited amount of fast available memory.
In order to do this, a special automatic set of hardware and software was
needed. Now we can treat all memory as being on the same level. Thus, the
concept of virtual memory was born.
All new and great ideas usually come into the world on unfriendly
terms. Virtual memory was no different. When the idea came up it was
considered too radical for the conservative computer profession. The first
virtual memory machine was developed in 1959. They called it the one level
storage system. Although it was heavily criticized, it spurred many new
prototypes during the early 1960's.
Before virtual memory could be regarded as a stable entity, many
models, experiments, and theories had to be developed to overcome the
numerous problems with virtual memory. Specialize hardware had to be
developed that would take a "virtual" address and translate it into an
actual physical address in memory (secondary or primary). Some worried
that this process would be expensive, hard to build, and take too much
processor power to do the address translation.
The final debate was laid to rest in 1969. The question was: Could
virtual memory controlled systems perform better than manual programmer
strategies? The IBM's research team, lead by David Sayre, showed that the
virtual memory overlay system worked consistently better than the best
By the late 1970's the virtual memory idea had been perfected
enough to use in every commercial computer. Personal computers were a
different matter. Developers thought that their computers would not be
subject to the problems of large-scale commercial computers. Consequently,
early PC's didn't include virtual memory. Ironically enough, they
discovered the same problems that their predecessors discovered during
the 50's and 60's. In 1985 Intel offered virtual memory and cache in the
386 microprocessor and Microsoft offered multiprogramming in Windows 3.1.
Others finally followed and virtual memory found its place in our everyday