Memory System Design II - Fundamentals of Computer Organization and Architecture - page 142

Figure 7.8 A 1024 1024 memory array organization
7.2. VIRTUAL MEMORY
The concept of virtual memory is in principle similar to that of the cache memory
described in Section 6.2. A virtual memory system attempts to optimize the use
of the main memory (the higher speed portion) with the hard disk (the lower
speed portion). In effect, virtual memory is a technique for using the secondary sto-
rage to extend the apparent limited size of the physical memory beyond its actual
physical size. It is usually the case that the available physical memory space will
not be enough to host all the parts of a given active program. Those parts of the pro-
gram that are currently active are brought to the main memory while those parts that
are not active will be stored on the magnetic disk. If the segment of the program con-
taining the word requested by the processor is not in the main memory at the time of
the request, then such segment will have to be brought from the disk to the main
memory. The principles employed in the virtual memory design are the same as
those employed in the cache memory. The most relevant principle is that of keeping
active segments in the high-speed main memory and moving inactive segments back
to the hard disk.
Movement of data between the disk and the main memory takes the form of
pages. A page is a collection of memory words, which can be moved from the
disk to the MMwhen the processor requests accessing a word on that page. A typical
size of a page in modern computers ranges from 2K to 16K bytes. A page fault
occurs when the page containing the word required by the processor does not
exist in the MM and has to be brought from the disk. The movement of pages of pro-
grams or data between the main memory and the disk is totally transparent to the
application programmer. The operating system is responsible for such movement
of data and programs.