Memory-mapped files (MMFs) offer a unique memory management feature that allows applications to access files on disk in the same way they access dynamic memory—through pointers. With this capability you can map a view of all or part of a file on disk to a specific range of addresses within your process's address space. And once that is done, accessing the content of a memory-mapped file is as simple as dereferencing a pointer in the designated range of addresses.
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http://msdn.microsoft.com/library/en-us/dngenlib/html/msdn_manamemo.asp?frame=true