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Random access memory
Random access memory or RAM is a type of computer storage whose contents can
be accessed in any order. This is in contrast to sequential memory devices
such as magnetic tapes, discs and drums, in which the mechanical movement of
the storage medium forces the computer to access data in a fixed order. It
is usually implied that RAM can be both written to and read from, in
contrast to read-only memory or ROM.
Overview
Computers use RAM to hold the program code and data during execution. In the
first electronic computers, RAM was built from vacuum tubes, and later
magnetic cores. The term "core" is still used by some programmers to
describe the RAM at the heart of a computer.
Many types of RAM are volatile, which means that unlike some other forms of
computer storage such as disk storage and tape storage, they lose their data
when the computer is powered down.
Throughout the history of computing, a variety of technologies have been
used for RAM, and usually more than one in the same computer, with
high-memories constructed out of the same technology as the logic, and
slower, cheaper technologies used for bulk storage. Some early computers
used mercury delay lines, in which a series of acoustic pulses were sent
along a tube filled with mercury. When the pulse reached the end of the
tube, the circuitry detected whether the pulse represented a binary 1 or 0
and caused the oscillator at the beginning of the line to repeat the pulse.
Other early computers stored RAM on high-speed "magnetic drums".
Later designs used arrays of small ferrite electromagnets, known as core
memory.
Modern RAM generally stores a bit of data as either a charge in a capacitor,
as in dynamic RAM, or the state of a flip-flop, as in static RAM.
Common types of RAM
* SRAM or Static RAM
* DRAM or Dynamic RAM
o Fast Page Mode DRAM
o EDO RAM or Extended Data Out DRAM
o SDRAM or Synchronous DRAM
+ DDR SDRAM or Double Data Rate Synchronous DRAM
+ RDRAM or Rambus DRAM
Not so common types of RAM
* Dual-ported RAM
* Video RAM, a dual-port memory with one random access port and one
sequential access port.
RAM packaging
Semiconductor RAM is produced as integrated circuits (ICs). RAM ICs are
often assembled into plug-in modules. Some standard module types are:
* single-in-line memory module (SIMM)
* dual-in-line memory module (DIMM)
Write-Only Memory
In 1972 some engineers at the Signetics Corporation published a spoof data
sheet for a write-only memory. This was a type of RAM with no read facility
and therefore no outputs. The full title was Fully Encoded, 9046×N,
Random Access Write-Only Memory and the part number was 25120.
This content from Wikipedia is licensed under the GNU Free Documentation License.
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