Virtual memory

VIRTUAL MEMORY

What is virtual memory?

Virtual memory is a memory management technique where secondary memory can be used as if it were a part of the main memory. Virtual memory is a common technique used in a computer’s operating system (OS).

Virtual memory uses both hardware and software to enable a computer to compensate for physical memory shortages, temporarily transferring data from random access memory (RAM) to disk storage. Mapping chunks of memory to disk files enables a computer to treat secondary memory as though it were main memory.

Virtual memory is important for improving system performance, multitasking and using large programs. However, users should not overly rely on virtual memory, since it is considerably slower than RAM. If the OS has to swap data between virtual memory and RAM too often, the computer will begin to slow down – this is called thrashing.

Virtual memory was developed at a time when physical memory – also referenced as RAM – was expensive. Computers have a finite amount of RAM, so memory will eventually run out when multiple programs run at the same time. A system using virtual memory uses a section of the hard drive to emulate RAM. With virtual memory, a system can load larger or multiple programs running at the same time, enabling each one to operate as if it has more space, without having to purchase more RAM.

How virtual memory works

Virtual memory uses both hardware and software to operate. When an application is in use, data from that program is stored in a physical address using RAM. A memory management unit (MMU) maps the address to RAM and automatically translates addresses. The MMU can, for example, map a logical address space to a corresponding physical address.

If, at any point, the RAM space is needed for something more urgent, data can be swapped out of RAM and into virtual memory. The computer’s memory manager is in charge of keeping track of the shifts between physical and virtual memory. If that data is needed again, the computer’s MMU will use a context switch to resume execution.

While copying virtual memory into physical memory, the OS divides memory with a fixed number of addresses into either pagefiles or swap files. Each page is stored on a disk, and when the page is needed, the OS copies it from the disk to main memory and translates the virtual addresses into real address.

What are the limitations of using virtual memory?

Although the use of virtual memory has its benefits, it also comes with some tradeoffs worth considering, such as:

Applications run slower if they are running from virtual memory.

Data must be mapped between virtual and physical memory, which requires extra hardware support for address translations, slowing down a computer further.

The size of virtual storage is limited by the amount of secondary storage, as well as the addressing scheme with the computer system.

Thrashing can occur if there is not enough RAM, which will make the computer perform slower.

It may take time to switch between applications using virtual memory.

It lessens the amount of available hard drive space.

What are the benefits of using virtual memory?

The advantages to using virtual memory include:

It can handle twice as many addresses as main memory.

It enables more applications to be used at once.

It frees applications from managing shared memory and saves users from having to add memory modules when RAM space runs out.

It has increased speed when only a segment of a program is needed for execution.

It has increased security because of memory isolation.

It enables multiple larger applications to run simultaneously.

Allocating memory is relatively inexpensive.

It does not need external fragmentation.

CPU use is effective for managing logical partition workloads.

Data can be moved automatically.

Pages in the original process can be shared during a fork system call operation that creates a copy of itself.

Process in a blocking state. For the execution to proceed the OS must bring the required page into the memory.

The OS will search for the required page in the logical address space.

The required page will be brought from logical address space to physical address space. The page replacement algorithms are used for the decision-making of replacing the page in physical address space.

The page table will be updated accordingly.

The signal will be sent to the CPU to continue the program execution and it will place

the process back into the ready state.