Random Access Memory

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RAM redirects here. For other meanings of the word ram see Ram (disambiguation).

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 frequently. 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.

Currently, there are several types of non-volatile RAM under development, which will preserve data while powered down. Technologies that are being used include carbon nanotube technology and magnetic tunnel effect.

In the summer of 2003, a 128 Kib Magnetic RAM chip was introduced, which was manufactured with 0.18 micrometre technology. The core technology of MRAM is based on the magnetic tunnel effect. In June of 2004, Infineon unveiled a 16-Mib prototype based on 0.18 µm technology once again.

In 2005, Compu-Technics Inc. (http://www.compu-technics.com) presented a 256 GiB/4GHz non-volatile Magnetic RAM array, as well as a notebook using this chip, SG220.

As for carbon nanotube memory, a high-tech startup Nantero (http://www.nantero.com/) has built a functioning prototype 10 GiB array in 2004.

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
• NV-RAM or Non-Volatile RAM
• DRAM or Dynamic RAM
  • Fast Page Mode DRAM
  • EDO RAM or Extended Data Out DRAM
  • XDR DRAM
  • SDRAM or Synchronous DRAM
    • DDR SDRAM or Double Data Rate Synchronous DRAM
    • RDRAM or Rambus DRAM

Uncommon types of RAM

• Dual-ported RAM
• Video RAM, a dual-port memory with one random access port and one sequential access port. This is becoming more popular as people need more and more video memory. 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 Pin Package (SIP)
• Dual in-line Package (DIP)
• Single in-line memory module (SIMM)
• Dual in-line memory module (DIMM)
• Rambus modules are actually DIMMs, but are often referred to (by Rambus themselves and others) as RIMMs due to their proprietary slot.
• Small outline DIMM (SO-DIMM). Smaller version of the DIMM, used in laptops. Comes in versions with 72 (32 bit), 144 (64 bit), 200 (72 bit) pins
• Small outline RIMM (SO-RIMM)

Buffering of RAM modules

Larger RAM modules have significantly higher capacity loads for their signal lines. Buffering reduces these loads, but increases latency.

• Unregistered RAM
• Registered RAM: Address and control lines are buffered, data lines are unbuffered
• Fully buffered RAM: All the lines are buffered

Memory wall

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In today's computers memory access is becoming very slow when compared to CPU cycles since most computers use cheap, but comparatively slow, DRAM for the main memory. Hence, the memory access, like hard disk access, might become the term that bounds computation speed. This is another important bound on fast computations.

Shadow RAM

Shadow RAM is the part of RAM with its contents copied from ROMs from where it will run much faster [1] (http://hardwarehell.com/articles/shadowram.htm). ROMs are slower to use than RAMs. The original ROM is disabled and the new location on the RAM is write protected. This process is also called shadowing.

External links

• Definition (http://searchmobilecomputing.techtarget.com/sDefinition/0,,sid40_gci214255,00.html)
• What is RAM? (http://webopedia.com/TERM/R/RAM.html)
• Article "How RAM Works (http://www.howstuffworks.com/ram.htm)" by Jeff Tyson and Dave Coustan
• Memory eXpansion Technology (http://mxt.sourceforge.net/)
• Corsair Memory Basics Presentation (http://www.corsairmemory.com/memory_basics/153707/index.html) (flash required)ar:رام

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