Introduction to Virtual Memory System:
Definition and Explanation:
Virtual memory is a technique to execute processes that may not be completely in main memory. It abstracts main memory into an extremely large storage. This array of storage is mapped on to main memory and its backing store. The backing store is usually an area of the disk outside of the file-system.
We have also seen that:
1. A process can be loaded in separate parts using paging.
2. Logical addresses are translated to physical addresses at run time.
These facts indicate that it is not necessary to load complete process into memory during its execution. The portions or pages which are actually being referenced at any time must be present. The remaining process can be retained on secondary storage i.e. hard disk.
If the flow of execution moves to a page that is not in memory, the operating system has to load the required page from secondary storage into the memory before execution can continue.
The implications of this are:
1. Since processes can be non-contiguous in a paging system, more processes can be sustained simultaneously.
2. Each process can be larger than the available real memory.
The programmer is freed from the limitation of containing his process within the limits of the available memory.
The programmer is presented with the illusion that computer has a large main memory to be used. It is termed virtual memory and is available for every process. The actual physical memory into which the process parts are loaded is referred to as real memory.
