Virtual-Memory Terminology

Backing Store A location on disk where the contents of a memory page are stored. This can be in a pagefile for pages whose contents have been modified. It can also be the original location of the data, such as a page in an image (program) file, for unmodified pages. The backing store is used to refill the contents of a virtual page if it has been removed from and later restored to physical memory.

Working Set This is the set of pages in the process virtual address space actually in physical memory. It includes pages dedicated to memory management and pages that describe the working set itself. It does not include pages whose contents exist only in a pagefile or other backing store.

Free Page List A list of pages available for allocation. It consists mainly of pages that have been removed from process working sets. If a process makes a reference to a page that has been removed from its working set and placed on the free page list, it can be moved immediately back into the working set of the process. Once a page has been used for another purpose, this is no longer possible.

Modified Page List A list of pages that have been removed from process working sets after being modified, but have not yet been saved to backing store. This means that they cannot be reused until the contents of the page have been copied to backing store. If the process that most recently owned a page makes a reference to it, it can be placed immediately back into the working set of that process. Periodically, OpenVMS flushes modified pages to backing store and moves the associated pages to the free page list.

Pagefile A special file on disk used to hold memory contents temporarily. When the system must free a page of physical memory that has no current backing store, its contents are copied to a block in a pagefile before the memory page is used for another purpose. OpenVMS supports the simultaneous use of multiple pagefiles.