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.

Table of contents

1 Overview 2 Common types of RAM 3 Not so common types of RAM 4 RAM packaging 5 Write-Only Memory

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

A PC66
SDRAM module

• SRAM or Static RAM
• DRAM or Dynamic RAM
  • Fast Page Mode DRAM
  • EDO RAM or Extended Data Out DRAM
  • 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.