Virtual memory

 

Virtual memory

Virtual memory is a memory management technique used by operating systems to give the appearance of a large, continuous block of memory to applications, even if the physical memory (RAM) is limited.

A virtual memory is what its name indicates- it is an illusion of a memory that is larger than the real memory. We refer to the software component of virtual memory as a virtual memory manager. The basis of virtual memory is the noncontigeous memory allocation model. The virtual memory manager removes some components from memory to make room for other components.

The size of virtual storage is limited by the addressing scheme of the computer system and the amount of secondary memory available not by the actual number of main storage locations. 

Purpose  of Virtual Memory

• Running Large Applications: Virtual memory allows the system to run programs that require more memory than is physically available in RAM.
• Multiprogramming: Multiple programs can run simultaneously without conflicts by isolating their address spaces, improving multitasking capabilities.
• Efficient Memory Utilization: Virtual memory allows more efficient use of the system’s RAM by ensuring only frequently used parts of programs are kept in physical memory, while the rest is stored on disk and swapped in as needed.

 

Working of Virtual Memory

It is a technique that is implemented using both hardware and software. It maps memory addresses used by a program, called virtual addresses, into physical addresses in computer memory. 

• All memory references within a process are logical addresses that are dynamically translated into physical address at run time. This means that a process can be swapped in and out of the main memory such that it occupies different places in the main memory at different times during the course of execution.

• A process may be broken into a number of pieces and these pieces need not be continuously located in the main memory during execution. The combination of dynamic run-time address translation and the use of a page or segment table permits this.

If these characteristics are present then, it is not necessary that all the pages or segments are present in the main memory during execution. This means that the required pages need to be loaded into memory whenever required. Virtual memory is implemented using Demand Paging or Demand Segmentation.

Advantages of Virtual Memory

1.    Increased Address Space: Programs can use more memory than is physically available, enabling larger and more complex applications to run.

2.    Multitasking: Multiple applications can run simultaneously with isolated and protected memory spaces, improving system efficiency and stability.

3.    Efficient Memory Use: Only the active parts of a program are loaded into physical memory, leading to more efficient memory utilization.

4.    Flexibility: Virtual memory allows the operating system to allocate memory flexibly and dynamically, adjusting to the needs of different processes.

6. Disadvantages of Virtual Memory

1.    Performance Overhead: Frequent page swapping (thrashing) between RAM and disk can cause significant slowdowns, as disk access is much slower than RAM.

2.    Complexity: The implementation of virtual memory requires sophisticated hardware (MMU) and software (page tables, replacement algorithms).

3.    Limited by Disk Speed: The performance of virtual memory is limited by the speed of the secondary storage device (disk), which is slower than physical RAM.

 

Virtual Memory vs Physical Memory

When talking about the differences between virtual memory and physical memory, the biggest distinction is speed. RAM is much faster than virtual memory, but it is also more expensive.

When a computer needs storage for running programs, it uses RAM first. Virtual memory, which is slower, is used only when the RAM is full.

Feature	Virtual Memory	Physical Memory (RAM)
Definition	An abstraction that extends the available memory by using disk storage	The actual hardware (RAM) that stores data and instructions currently being used by the CPU
Location	On the hard drive or SSD	On the computer’s motherboard
Speed	Slower (due to disk I/O operations)	Faster (accessed directly by the CPU)
Capacity	Larger, limited by disk space	Smaller, limited by the amount of RAM installed
Cost	Lower (cost of additional disk storage)	Higher (cost of RAM modules)
Data Access	Indirect (via paging and swapping)	Direct (CPU can access data directly)
Volatility	Non-volatile (data persists on disk)	Volatile (data is lost when power is off)