Explain how it is possible that two programs that are together are much larger than a physical memory can both be executed in a virtual memory?

Answer 1

In a virtual memory system applications are not directly loaded into physical memory. Every process is assigned a virtual and private address space of 2GB. When an application is loaded it is mapped into this virtual space. When the application runs the code is copied from disk to RAM only when it is needed. Most applications contain code that is rarely accessd and there is no need or benefit in having this in RAM all the time. After code has been copied to RAM it will be retained for future use, often even after the application has closed. If this memory is needed for more importan uses it is immediately available as the data it contains can be reloaded from disk if needed. Using this method only frequently accessed data is in memory with the remainder kept on disk. This entire process is handled by the system and is totally transparent to applications. Note: When aplication code must be released from memory it is NOT copied to the pagefile as many uninformed sources have claimed. There is no need for this as this code is hardley ever modified and can simply be reloaded from the original files. Only data that has been modified is copied to the pagefile.