In most operating systems virtual memory is used as a temporary storage area to swap what is held in RAM to and from the hard drive. For instance, if you were to run an application that would consume 512MB of memory in order to run you would need at least that much free space in RAM to run it. What if the machine only had 256MB of RAM? The OS would have to "simulate" RAM on the hard drive in order to get the application to run.

Therefore when you are running a 64-bit versus a 32-bit application you can effectively double the amount of memory needed by the application. This is primarily because of the need to store the same amount of data in a larger container. The simplest example is, if your application needed to store a number in memory a 32-bit application would at least need a 32-bit location to store it. Whereas a 64-bit application would require at least a 64-bit location to store it. Add up all this and you can see an increase in the required memory needed to run your application.

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