Random access memory

"RAM" redirects here.

Example of writable volatile random access memory: Synchronous Dynamic RAM modules, primarily used as main memory in personal computers, workstations, and servers.

Computer memory types

Volatile

DRAM, e.g. DDR SDRAM SRAM Upcoming Z-RAM TTRAM Historical Delay line memory Selectron tube Williams tube

Non-volatile

ROM PROM EPROM EEPROM Flash memory Upcoming FeRAM MRAM CBRAM PRAM SONOS RRAM Racetrack memory NRAM Historical Drum memory Magnetic core memory Plated wire memory Bubble memory Twistor memory

<<MENTION "SRAM" IN INTRO.>>

Random access memory (also hyphenated as random-access memory), usually known by its acronym RAM, is a device for data storage used in computers, such that the stored data can be accessed in any order (that is, "at random"). Today it takes the form of integrated circuits.

• The word random thus 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.

<<THIS DEFINITION OF RAM IS DISPUTED HERE: RAM is a Misnomer.>>

Other dfns:

• "A memory device in which information can be accessed in any order."
• "In computer use, device whereby any location in memory can be found, on average, as quickly as any other location. A computer's RAM is its main memory where it can store data, so the size of the RAM, measured in kilobytes or megabytes, is an important indicator of the capacity of the computer."
• "Computer's main memory where programs, application software, and data are stored. The size of the RAM (measured by kilobytes) is an important indicator of the capacity of the computer; also called read/write memory."
• "Computer main memory in which specific contents can be accessed (read or written) directly by the CPU in a very short time regardless of the sequence (and hence location) in which they were recorded. Two types of memory are possible with random-access circuits, static RAM (SRAM) and dynamic RAM (DRAM). A single memory chip is made up of several million memory cells. In a SRAM chip, each memory cell stores a binary digit (1 or 0) for as long as power is supplied. In a DRAM chip, the charge on individual memory cells must be refreshed periodically in order to retain data. Because it has fewer components, DRAM requires less chip area than SRAM; hence a DRAM chip can hold more memory, though its access time is slower."

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.

The word RAM is mostly associated with volatile types of memory (such as dynamic random access memory (DRAM) memory modules), where the information is lost when the power is switched off. (Strictly speaking, modern types of DRAM are therefore not truly (or technically) random access, as data are read in bursts; the name DRAM has stuck however.) Many other types of memory can be classified as RAM as well, including most types of ROM and a kind of flash memory called NOR-Flash.

• Quotes from "Types of RAM Random Access Memory. entryboot.com":
  • "Older computers use magnetic core memory while the present day ones use semiconductor memory. The core memory is non-volatile; the contents are not lost when power is removed unlike semiconductor memory which is volatile. Semiconductor memory is of two types: static memory (SRAM) and dynamic memory (DRAM). Static memory preserves the contents of all the locations as long as the power supply is present. Dynamic memory can retain the content of any location only for a few milliseconds. Within that period, each location must be written again with the same contents. This is known as refreshing."
  • "In a Random Access Memory (RAM), access time is the same for all locations. It does not make any difference whether the location is top-most, the lower-most or an intermediate one. Any location can be accessed without any relation to other locations. Core memory and semiconductor memory are RAMs."
  • "In a sequential access memory, the read or write access is sequential. The read/write mechanism encounters the locations physically one by one. The time taken for accessing the first location is the shortest and the time taken for accessing the last location is the longest. Thus the access time of an address varies with the physical position of the address. The magnetic tape is a sequential memory. In floppy and hard disks, a combination of random access and sequential access is followed."

• Also see: How RAM Works, howstuffworks.com.

History

An early type of widespread writable random access memory was the magnetic core memory, developed in 1949-1951, and subsequently used in most computers up until the development of the static and dynamic integrated RAM circuits in the late 1960s and early 1970s. Before this, computers used relays, delay lines or various kinds of vacuum tube arrangements to implement "main" memory functions (that is, hundreds or thousands of bits), some of which were random access, some not. Latches built out of vacuum tube triodes, and later, out of discrete transistors, were used for smaller and faster memories such as registers and (random access) register banks. Prior to the development of integrated ROM circuits, permanent (or read-only) random access memory was often constructed using semiconductor diode matrices driven by address decoders.

Overview

Types of RAM

Top L-R, DDR2 with heat-spreader, DDR2 without heat-spreader, Laptop DDR2, DDR, Laptop DDR.

1 Megabit chip.

Modern types of writable RAM generally store a bit of data in either the state of a flip-flop, as in SRAM (static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM, EEPROM and Flash. Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits or error correction codes. RAM of the read-only type, ROM, instead uses a metal mask to permanently enable/disable selected transistors, instead of storing a charge in them.

As both SRAM and DRAM are volatile, other forms of computer storage, such as disks and magnetic tapes, have been used as "permanent" storage in traditional computers. Many newer products instead rely on flash memory to maintain data between sessions of use: examples include PDAs, small music players, mobile phones, synthesizers, advanced calculators, industrial instrumentation and robotics, and many other types of products; even certain categories of personal computers, such as the OLPC XO-1, Asus Eee PC, and others, have begun replacing magnetic disk with so called flash drives (similar to fast memory cards equipped with an IDE or SATA interface).

There are two basic types of flash memory: the NOR type, which is capable of true random access, and the NAND type, which is not; the former is therefore often used in place of ROM, while the latter is used in most memory cards and solid-state drives, due to a lower price.

Memory hierarchy

Many computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external 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, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable. (Generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom.)

In many modern personal computers, the RAM comes in an easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or too small for current purposes. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system. The overall goal of using a memory hierarchy is to obtain the higher possible average access performance while minimizing the total cost of entire memory system.

Swapping

If a computer becomes low on RAM during intensive application cycles, the computer can perform an operation known as "swapping". When this occurs, the computer temporarily uses hard drive space as additional memory. Constantly relying on this type of backup memory is called thrashing, which is generally undesirable because it lowers overall system performance. In order to reduce the dependency on swapping, more RAM can be installed.

Other uses of the "RAM" term

Other physical devices with read/write capability can have "RAM" in their names: for example, DVD-RAM. "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.

RAM disks

Software can "partition" a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a RAM disk. Unless the memory used is non-volatile, a RAM disk loses the stored data when the computer is shut down. However, volatile memory can retain its data when the computer is shut down if it has a separate power source, usually a battery.

Shadow RAM

Sometimes, the contents of a ROM chip are copied to SRAM or DRAM to allow for shorter access times (as ROM may be slower). The ROM chip is then disabled while the initialized memory locations are switched in on the same block of addresses (often write-protected). This process, sometimes called shadowing, is fairly common in both computers and embedded systems.

As a common example, the BIOS in typical personal computers often has an option called “use shadow BIOS” or similar. When enabled, functions relying on data from the BIOS’s ROM will instead use DRAM locations (most can also toggle shadowing of video card ROM or other ROM sections). Depending on the system, this may or may not result in increased performance. On some systems the benefit may be hypothetical because the BIOS is not used after booting in favor of direct hardware access. Of course, somewhat less free memory is available when shadowing is enabled.

Recent developments

Several new types of non-volatile RAM, which will preserve data while powered down, are under development. The technologies used include carbon nanotubes and the magnetic tunnel effect. In summer 2003, a 128 KB (128 × 2¹⁰ bytes) magnetic RAM (MRAM) chip manufactured with 0.18 µm technology. In June 2004, Infineon Technologies unveiled a 16 MB (16 × 2²⁰ bytes) prototype again based on 0.18 µm technology. Nantero built a functioning carbon nanotube memory prototype 10 GB (10 × 2³⁰ bytes) array in 2004. Whether some of these technologies will be able to eventually take a significant market share from either DRAM, SRAM, or flash-memory technology, however, remains to be seen.

Since 2006, "Solid-state drives" (based on flash memory) with capacities exceeding 642 gigabytes and performance far exceeding traditional disks have become available. This development has started to blur the definition between traditional random access memory and "disks", dramatically reducing the difference in performance. Also in development is research being done in the field of plastic magnets, which switch magnetic polarities based on light.

Memory wall

The "memory wall" is the growing disparity between the speed of the central processing unit (CPU) and that of the memory outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries. From 1986 to 2000, CPU speed improved at an annual rate of 55 percent, while memory speed improved at only 10 percent. Given these trends, it was expected that memory latency would become an overwhelming bottleneck in computer performance.

Currently, CPU speed improvements have slowed significantly, partly because of major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel summarized these causes in its white paper titled "A Platform 2015 Workload Model," as follows:

“First of all, as chip geometries shrink and clock frequencies rise, the transistor leakage current increases, leading to excess power consumption and heat (more on power consumption below). Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called Von Neumann bottleneck), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.”

The RC delays in signal transmission were also noted in a report titled "Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures," which projects a maximum of 12.5 percent average annual CPU performance improvement between 2000 and 2014. The data on Intel Processors clearly show a slowdown in performance improvements in recent processors. However, Intel's new processors, Core 2 Duo (codenamed Conroe), show a significant improvement over previous Pentium 4 processors; due to a more efficient architecture, performance has increased while clock rate actually decreased.

Security concerns

Contrary to simple models (and perhaps common belief), the contents of modern SDRAM modules aren't lost immediately when the computer is shutdown; instead, the contents fade away, a process that takes only seconds at room temperatures, but which can be extended to minutes at low temperatures. It is therefore possible to get hold of an encryption key if it was stored in ordinary working memory (i.e. the SDRAM modules). This is sometimes referred to as a cold boot attack.

See also

• Computer
• Computer science
• Electronics
• Read-only memory

External links

• Types of RAM Random Access Memory. entryboot.com. Retrieved January 14, 2009.
• Tyson, Jeff, and Dave Coustan. How RAM Works. howstuffworks.com. Retrieved January 14, 2009.
• What kind of Memory Do I Have? What Kind of RAM Do I Need? Darrell's computer. Retrieved January 14, 2009.
• RAM is a Misnomer. Find By Click. Retrieved January 14, 2009.

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