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What is Virtual Memory?

by Daryl Jones / November 3, 2004 5:17 AM PST

The concept of virtual memory or virtual storage has been around since the 60?s. The definition applies then as it does today.

?The term virtual storage is normally associated with the ability to address a storage space much larger than that available in the primary storage of a particular computer system. Virtual storage is not a new concept. It first appeared in the Atlas computer system constructed at the University of Manchester in England in 1960. The widespread use of virtual storage systems is relatively recent.?

From ?An Introduction to Operating Systems? by Harvey M. Deitel, copyright 1984, Chapter 8, page 181, section 8.1 Introduction, first paragraph.

A virtual memory system allows several large address space programs to run on hardware with small amount of memory configuration. This is accomplished by virtual addressing. Each CPU has a hardware definition of virtual address: 8, 16, 32, 64 bit unsigned integer. This means each program on a 32-bit addressable computer can access up to 4-billion-bytes of address space. This exceeds the physical memory of many computer systems. Therefore, the 4-billion-bytes of address space is virtual address space.

Memory management for virtual memory computer systems maps the active part of virtual address to the physical memory (RAM) and the inactive part of the virtual address space to the external storage media (Disk and/or Tape). If the external storage media is a disk, then a Pagefile can be used.

In most cases, virtual memory is mapped by virtual address, which is mapped to RAM and the Pagefile. From all of this: when you get the message of ?Virtual Memory is low? you can add more memory or increase the Pagefile size. By adding more memory, programs will run faster that?s universal. That goes for PCs to Mainframes.

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