RAM

 RANDOM ACCESS MEMORY
Random access memory (usually known by its acronym, RAM) is a type of magnetic storage data storage used in computers. It takes the form of integrated circuits that allow the stored data to be accessed in any order — that is, at random and without the physical movement of the storage medium or a physical reading head.
The word "random" refers to the fact that any piece of data can be returned quickly, and in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data. This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.
Uses of RAM
The key benefit of RAM over types of storage which require physical movement is that retrieval times are short and consistent. Short because no physical movement is necessary, and consistent because the time taken to retrieve a piece of data does not depend on its current distance from a physical head; it requires practically the same amount of time to access any piece of data stored in a RAM chip.
Because of this speed and consistency, RAM is used as ‘main memory’ or primary storage: the working area used for loading, displaying and manipulating applications and data. In most personal computers, the RAM is not an integral part of the motherboard or CPU—it comes in the easily upgraded form of modules called memory sticks or RAM sticks about the size of a few sticks of chewing gum, which can be quickly removed and replaced when they become damaged or too small for current purposes. A smaller amount of random-access memory is also integrated with the CPU, but this is usually referred to as "cache" memory, rather than RAM.
The disadvantage of RAM over physically moving media is cost, and the loss of data when power is turned off. For these reasons, nearly all PCs have disc storage as "secondary storage". Small PDAs and music players (up to 8 GiB in Jan 2007) may dispense with disks, but rely on flash memory, to maintain data between sessions of use.
Shadow RAM
Shadow RAM is RAM whose contents are copied from read-only memory (ROM) to allow shorter access times, as ROM is in general slower than RAM. The original ROM is disabled and the new location on the RAM is write-protected. This process is called shadowing.
As a common example, some BIOSes have a feature labeled “use shadow BIOS” or similar in the configuration options. When enabled, functionality that would rely on reading data from the BIOS’s ROM chip instead makes use of the RAM installed in the system. Depending on the system, this may or may not lead to a performance boost for calls to the BIOS.

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