RANDOM ACCESS MEMORY (RAM)

Random access memory (usually known by its acronym, RAM) is a type of computer data storage. 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. RAM is a volatile memory as the information or instructions stored in it will be lost if the power is switched off.

The word "random" refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.^[1] 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.

Originally, RAM referred to a type of solid-state memory, and the majority of this article deals with that, but physical devices which can emulate true RAM (or, at least, are used in a similar way) can have "RAM" in their names: for example, DVD-RAM.

RAM is usually writable as well as readable, so "RAM" is often used interchangeably with "read-write memory". The alternative to this is "ROM", or Read Only Memory. Most types of RAM lose their data when the computer powers down. "Flash memory" is a ROM/RAM hybrid that can be written to, but which does not require power to maintain its contents. RAM is not strictly the opposite of ROM, however. The word random indicates a contrast with serial access or sequential access memory.

"Random access" is also the name of an indexing method: hence, disk storage is often called "random access" because the reading head can move relatively quickly from one piece of data to another, and does not have to read all the data in between. However the final "M" is crucial: "RAM" (provided there is no additional term as in "DVD-RAM") always refers to a solid-state device.

Many CPU-based designs actually have a memory hierarchy consisting of registers, on-die SRAM caches, DRAM, paging systems, and virtual memory or swap space on a hard-drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the "random access" term in RAM. Even within a hierarchy level such as DRAM, the specific row/column/bank/rank/channel/interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable.

info : wikipedia