Virtual Memory Explained
If you have spent any length of time using windows you have probably heard the term Virtual Memory. Virtual Memory is exactly what it says, Memory that is Virtual not real.
Your computer does not work from the hard drive where all your programs and data are stored. When you open a file or program it loads itself into the memory and works from there.
Your computer only has so much memory and sooner or latter would get filled up by all the files and programs you had opened. This is where Virtual Memory (AKA Swap File) comes in.
As you open more things on your computer the items that are used the least are temporarily put back to your hard drive into the Swap File until needed again. They information does not go to its original location on the hard drive until it is saved.
This is a simple explanation of virtual memory. Unfortunately it isn’t very accurate. Equating virtual memory with the pagefile is incorrect. This might help in having a superficial understanding of virtual memory but will seriously impede having a real understanding.
Virtual memory is essentially a system using both physical memory and the hard disk. A pagefile is important to a properly functioning virtual memory system but it isn’t strict necessary. All modern computer systems are based on virtual memory and this will remain true even if the pagefile is disabled.
Virtual memory combines physical memory and the hard disk in such a way as to take advantage of the strengths of both and minimize the weaknesses. This is done in a way that is completely transparent to applications. Applications always acccess physical memory but only theough the memory management hardware of the processor.
Central to virtual memory is the concept of paging. Paging is the act of copying memory to or from the harddisk. Rarely used data is generally kept on disk while frequently used data is kept in memory. This is a dynamic process which changes as the situation changes. This is an on-going process, it does not wait till physical memory is exhausted.
Even a running program is only partially loaded into memory. Portions are brought into memory as needed and kept there until the memory is needed for more important uses. Only modified data is copied to the pagefile.
Physical memory is essentially a window into the programs and data stored on disk and should be used to the fullest possible extent. Unused memory is wasted and will impair performance.
Virtual memory is not a slow extension to physical memory, it is a system to optimize it’s use. In practical use it’s performance approaches that of a much larger memory size.
In most cases the default settings controlling virtual memory are optimum and “tweaks” are generally counter productive.
Larry Miller
Microsoft MCSA