Virtual Memory

1. What is Virtual Memory?
     Virtual Memory is a feature of an operating system that enables a process to use a memory (RAM) address space that is independent of other processes running in the same system, and use a space that is larger than the actual amount of RAM present, temporarily relegating some contents from RAM to a disk, with little or no overhead.

2.How virtual memory works?
     Virtual memory is used by the operating system in order to enhance the storage capacity of the working memory, without requiring the installation of additional Random Access Memory modules.
The disadvantage of the virtual memory it that it resides on the hard drive and as we all known it, hard drives are significantly slower when it comes to read/write operations. Thus if you have low capacity RAM modules and a significant amount of virtual memory, your computer will still work relatively well, but with a noticeable drop in processing speed.
-When a computer is running, many programs are simulataneously sharing the CPU. Each running program, plus the data structures needed to manage it, is called a process.
Each process is allocated an address space. This is a set of valid addresses that can be used. This address space can be changed dynamically. For example, the program might request additional memory (from dynamic memory allocation) from the operating system. If a process tries to access an address that is not part of its address space, an error occurs, and the operating system takes over, usually killing the process (core dumps, etc).
-How does virtual memory play a role? As you run a program, it generates addresses. Addresses are generated (for RISC machines) in one of three ways:
A load instruction
A store instruction
Fetching an instruction
Load/store create data addresses, while fetching an instruction creates instruction addresses. Of course, RAM doesn't distinguish between the two kinds of addresses. It just sees it as an address. Each address generated by a program is considered virtual. It must be translated to a real physical address. Thus, address tranlation is occuring all the time. As you might imagine, this must be handled in hardware, if it's to be done efficiently. You might think translating each address from virtual to physical is a crazy idea, because of how slow it is. However, you get memory protection from address translation, so it's worth the hardware needed to get memory protection.

3.How virtual memory works in Windows?
     Normal memory access techniques used by 32-bit Windows programs use standard linear byte addressing, and so are limited to 4GiB of addressable memory, whether it is real or virtual.On Windows, the amount of this 4GiB space that can actually occupy physical RAM is halved to 2GiB per process. Windows uses the other 2GiB of virutal address space as a per-process overhead for the kernel, and to speed up paging. This is really dumb because it means that if your process allocates > 2GiB heap memory, then Windows must page some to virtual memory, even if you have > 2GiB of actual RAM! Sort of like the DOS 640KiB limit reborn!
To overcome this dumb design, Windows has a memory addressing scheme called the AWE API, which allows a process to allocate up to 3GiB of memory and have that memory reside on chips. To use this, the program must be specially written to use the AWE API.
There’s also another virtual memory technology in Windows called PAE. This is not useful to application programs — it is how the Windows kernel can use > 4GiB of real memory to allocate physical memory to all processes on the system. Each process is still limited to the 4GiB address space each, with 2GiB mapped to RAM (or 3GiB, if the program uses the AWE API) and the rest having to be virtual (paged to disc). PAE just lets Windows keep more than one of these big processes in memory at once even if their combined total heap is more than 4GiB (and assuming you have more than 4GiB of RAM of course).

4.How virtual memory works in Linux?
     On Linux, if I'm not mistaken, the virtual memory is called 'swap' and occupies a separate partition on the hard drive.In addition to swapping, virtual memory is used to manage all pages of memory, which are required for file caching, process isolation, and even network communication. Anything that queues data, you can be assured, traverses the virtual memory system. Depending on a server’s role, virtual memory functionality may not be optimal. An administrator can dramatically improve overall system performance by adjusting certain virtual memory manager settings.

5.How virtual memory works in OSX or MacOSX?
     Mac OS X, like Linux, supports both swap partitions and the use of swap files, but the default and recommended configuration is to use multiple swap files.