It limits the access which software applications have to the operating system services, and consequently to the internal hardware resources found in a computer, much as a car firewall limits access of heat, or even fire, to the passengers of the vehicle.
The reason that application firewalls are needed in today's internet and data-sharing world is that the other types of firewalls in existence do not control the execution of data, only of the flow of data to the computer's processor.
The computer's hardware resources are essentially: the processor, the RAM, and the hard disk.
Virtual memory is the content of RAM that is temporarily written onto the hard disk in order to free the RAM chips to hold other content or to supply other data for mathematical processing.
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