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. RAM is usually used for primary storage in computers to hold actively-used and actively-changing information, although some devices use certain types of RAM to provide long term secondary storage.

Overview

Computers use RAM to hold the program code and data during execution. One defining characteristic of RAM is that its accesses to different memory locations are almost always completed at about the same speed, in contrast to some other technologies that required a certain delay time for a bit or byte to “come around.”

Early vacuum tube-based systems behaved much like modern RAM, even though the devices failed much more regularly. Core memory, which used wires attached to small ferrite electromagnetic cores, also had roughly equal access time (the term “core” is still used by some programmers to describe the RAM at the heart of a computer). The basic ideas behind tube and core memory are still used in modern RAM implemented with integrated circuits.

Alternative primary storage mechanisms usually involved a non-uniform delay for memory access. Delay line memory used a sequence of sound wave pulses in mercury-filled tubes to hold a series of bits. Drum memory acted much like the modern hard disk, storing data magnetically in continuous circular bands. (See primary storage for a greater discussion of these alternatives and others.)

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. 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.

An interesting use of RAM is allocating parts of it as a partition, effectively acting as a hard drive, only much faster. It is usually referred to as a ramdisk.

Common types of RAM

• 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. See description under Dynamic random access memory.
• WRAM
• MRAM
• FeRAM

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)