Under the Hood of .NET Memory Management - Simple Talk

Chapter 7: The Windows Memory Model
It is important to remember that the allocated memory is virtual, which means it doesn't
directly refer to physical memory. Instead, it refers to a virtual memory address that is
translated by the CPU to a physical memory address on demand.
The reason for using virtual addresses is to allow for a much more flexible memory space
to be constructed; one which is far larger than what is actually available in physical RAM.
To make this possible, a machine's hard drive is used as a kind of secondary memory store
to increase the total amount of memory available to our applications, and we will now
explore that virtual/physical relationship.
Virtual and Physical Memory
Physical memory is simply the amount of physical RAM installed on the machine.
By definition, it has a fixed size that can only be increased by opening the back of the
machine and slotting more inside.
Any resource with a fixed limit causes a problem; once that limit is reached – bang – there
is nowhere to go. In the absence of any other kind of memory, once the physical memory
is all used up, things start to go horribly wrong.
Virtual memory was devised as a way around the problem: using a machine's hard drive
as a kind of alternative memory store of last resort. Physical disk capacity is much greater
than RAM capacity, and so it makes sense to use part of your disk storage as a way of
increasing the amount of available memory. To facilitate this, a large file is created, often
called a "swap" or "page" file, which is used to store physical memory sections that haven't
been accessed for a while. This allows the memory manager (MM) to reuse physical
memory and still have access to "paged" data when requested. I probably don't need to
mention the downside of this cunning solution, but I will, anyway: accessing bits from
RAM is pretty fast; getting them off a disk is really slow.
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