Virtual Memory: The Implementation Of Virtual Computing And The Memory Management Unit

Abstract:

UNIX has a technique called Virtual Memory to accommodate for the shortage of memory space. Virtual memory increases the available memory the computer has by enlarging the "address space," or places in memory where data can be stored. This paper describes concepts like swapping, demand paging, page fault, page replacement algorithm, thrashing, some virtual memory allocation algorithms and the Memory Management Unit (MMU).

Introduction:

Each program running on the machine is called a process and each process has its own address space that the process uses to store its data and code segments. Sometimes there is not enough memory to hold the entirety of large programs or when too many programs are running together on the machine. As a get around, UNIX uses a technique called Virtual Memory. Virtual memory is the use of space on the hard disk to simulate additional RAM [1]. In order to free up space in memory, the UNIX operating system

Implementation of virtual memory:

The virtual address space is divided into same sized blocks of contiguous memory address called pages that are used to store data. The technique employed by the operating system to retrieve data from the secondary storage (pages) for use in main memory is called paging.

Demand paging is a paging technique in which a page is brought back to the main memory only when an attempt is made to access it and the page is not currently in main memory. In this technique, the process may begin with none of its pages (data) in main memory, slowly loading all its pages into the RAM. This is known as lazy loading. It follows that pages are to be loaded in memory only when they are required by the