Virtual Memory

Virtual Memory

• Virtual memory is a method of using hard disk space to provide extra memory. It simulates additional main memory.

• In Windows operating system, the amount of virtual memory available equals the amount of free main memory plus the amount of disk space allocated to the swap file.

• Fig. 4.9.1 shows logical view of virtual memory concept.

• A swap file is an area of your hard disk that is set aside for virtual memory. Swap files can be either temporary or permanent.

• Virtual memory is stored in the secondary storage device. It helps to extend additional memory capacity and work with primary memory to load applications. The virtual memory will reduce the cost of expanding the capacity of physical memory. The implementations of virtual memory will different for operating systems to operating system.

• Each process address space is partitioned into parts that can be loaded into primary memory when they are needed and written back to secondary storage otherwise. Address space partitions have been used for the code, data and stack identified by the compiler and relocating hardware. The portion of the process that is actually in main memory at any time is defined to be the resident set of the

• The logical addressable space is referred to as virtual memory. The virtual address space is much larger than the physical primary memory in a computer system. The virtual memory works with the help of secondary storage device and its speed is low compared to the physical storage location.

• Virtual memory uses two types of addresses Virtual address and physical address.

• Virtual address is referred by process and physical address is actual address of the main memory.

• Whenever a process accesses a virtual address, the system must translate it to a physical address. Virtual memory system uses special purpose hardware. This hardware is called as memory management unit.

• Most virtual memory system use a technique called paging. The virtual address space is divided into fixed size units called pages. The corresponding units in the physical memory are called page frames. The pages and page frames are normally the same size. The area on a hard disk that stores page frames is usually called the paging file or the swap file.

Fig. 4.9.2 shows concept of virtual memory.

• When the system is ready to run a process, the system loads the process code and Jerite data from secondary storage into primary memory. Only a small portion of these on needs to be in primary memory at once for the process to execute. The success of implementing virtual memory system is how to map virtual addresses to physical addresses. Because processes reference only virtual addresses.

• Virtual memory can be implement in one of two ways:

1. Paging

2. Segmentation

• Following are the situations, when entire program is not required to load fully.

1. User written error handling routines are used only when an error occurs in the data or computation.

2. Certain options and features of a program may be used rarely.

3. Many tables are assigned a fixed amount of address space even though only a small amount of the table is actually used.

4. Many routines are commonly used at mutually exclusive times during a run.

• Virtual memory makes the task of programming much easier. Virtual memory is bily commonly implemented by demand paging. Virtual memory mechanism bridges d the size and speed gaps between the main memory and secondary storage and is usually implemented in part by software technique.

Benefits

1. Less I/O needed to load/swap a process.

2. Degree of multiprogramming can be increased.

3. A program's logical address space can be larger than physical memory.