Virtual Memory Manager (VMM) Overview

System Management Concepts: Operating System and Devices

Virtual Memory Manager (VMM) Overview

The Virtual Memory Manager (VMM) provides the virtual memory facilities that are used by the other parts of the system to implement the following:

Virtual address space of processes
Sharing of executables
Shared memory segments
Mapped files

The VMM implements virtual memory, allowing the creation of segments larger than the physical memory available in the system. The segments are divided into fixed-size units called pages. Each page in a segment can be in physical memory or stored on disk until it is needed. When a process accesses a page that is not present in physical memory, the VMM reads the page into memory; this is called a PageIn. When physical memory is not available, the VMM writes pages to disk; this is called a PageOut or PageSteal.

The following are some of the segment types:

Working storage	Segments are used to implement the data areas of processes and shared memory segments. The pages for working storage segments are stored in the paging spaces configured in the system.
Persistent storage	Segments are used to manipulate files and directories. When a persistent storage segment is accessed, the pages are read and written from its file system.
Client storage	Segments are used to implement some virtual file systems like Network File System (NFS) and the CD-ROM file system. The storage for client segment pages can be in a local or remote computer.