A second advantage of paging is that it can be used to implement virtual memory. If a system has less than the 4 GByte of pages that the 386/486 can handle it is possible to use disk storage to provide additional pages. The way that this works is that the page table contains a bit that can be used to indicate that a page isn’t in memory. In this case the page address can be used to indicate where on disk the page is stored. If a program makes reference to a page that is not in memory, then the operating system is automatically alerted to this and it can take steps to read the page into memory. if there isn’t any available memory then a page can be selected and written out to disk to free a 4 KByte page. This mechanism is called virtual memory and it is supported by advanced operating systems such as OS/2 and Unix. As well as information to help with the implementation of a virtual memory system, the page table contains another layer of protection to supplement those provided by segment addressing.
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