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Random access memory
From Wikipedia, the free encyclopedia
Random access memory (usually known by its acronym, RAM) is a type of data store used in computers. It takes the form of integrated circuits that allow the stored data to be accessed in any order — that is, at random and without the physical movement of the storage medium or a physical reading head.
The word "random" refers to the fact that any piece of data can be returned quickly, and in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data. This contrasts with storage mechanisms such as tapes, magnetic disks and optical disks, 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.
Originally, RAM referred to a type of solid-state memory, and the majority of this article deals with that - but physical devices which can emulate true RAM (or, at least, are used in a similar way) can have "RAM" in their names: for example, DVD-RAM.
RAM is usually writeable as well as readable, so "RAM" is often used interchangeably with "read-write memory". The alternative to this is "ROM", or Read Only Memory. Most types of RAM lose their data when the computer powers down. "Flash memory" is a ROM/RAM hybrid that can be written to, but which does not require power to maintain its contents. RAM is not strictly the opposite of ROM, however. The word random indicates a contrast with serial access or sequential access memory.
"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.
Uses of RAM
The key benefit of RAM over types of storage which require physical movement is that retrieval times are short and consistent. Short because no physical movement is necessary, and consistent because the time taken to retrieve a piece of data does not depend on its current distance from a physical head - it costs practically the same time to access any piece of data stored in a RAM chip.
Because of this speed and consistency, RAM is used as 'main memory' or primary storage: the working area used for loading, displaying and manipulating applications and data. In most personal computers, the RAM is not an integral part of the motherboard or CPU - it comes in the easily upgraded form of modules called memory sticks or RAM sticks about the size of a few sticks of chewing gum, which can be quickly removed and replaced when they become damaged or too small for current purposes. A smaller amount of random-access memory is also integrated with the CPU, but this is usually referred to as "cache" memory, rather than RAM.
The disadvantage of RAM over physically moving media is cost, and the loss of data when power is turned off. For these reasons, nearly all PCs have disc storage as "secondary storage". Small PDAs and music players (up to 8 GiB in Jan 2007) may dispense with disks, but rely on flash memories, rather than RAM, to maintain data between sessions of use.
Computers use RAM to hold the program code and data during computation. A defining characteristic of RAM is that all memory locations can be accessed at almost the same speed. Most other technologies have inherent delays for reading a particular bit or byte.
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 all 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.
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.
Some types of RAM can detect or correct random faults called memory errors in the stored data, using RAM parity and error correcting codes.
A four-megabyte RAM card measuring about twenty-two by fifteen inches (55.8 by 38.1 centimeters); made for the VAX 8600 minicomputer (circa 1986). Dual in-line package (DIP) Integrated circuits populate nearly the whole board; the RAM chips are in the majority located in the rectangular areas to the left and right.Early main memory systems built from vacuum tubes, such as the Williams tube, behaved much like modern RAM, except that they failed frequently. Core memory, which used wires threaded through small ferrite electromagnetic donuts, was highly reliable and non-volitile, with access times of a few microseconds. The term “core” is still used by some programmers to describe the RAM main memory of a computer. The basic concepts of tube and core memory are 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 circular tracks, but with a read/write head for each track. (See primary storage for a greater discussion of these alternatives and others.)
Currently, several types of non-volatile RAM are under development, which will preserve data while powered down. The technologies used include carbon nanotubes and the magnetic tunnel effect.
In summer 2003, a 128 KiB magnetic RAM chip was introduced, which was manufactured with 0.18 µm technology. The core technology of MRAM is based on the magnetic tunnel effect. In June of 2004, Infineon Technologies unveiled a 16 MiB prototype again based on 0.18 µm technology.
As for carbon nanotube memory, a high-tech startup Nantero built a functioning prototype 10 GiB array in 2004.
The memory wall
The term "memory wall", first officially coined in Hitting the Memory Wall: Implications of the Obvious (PDF), refers to the growing disparity between CPU and memory speed. From 1986 to 2000, CPU speed improved at an annual rate of 55% while memory speed only improved at 10%. 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 due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel summarized these causes in their Platform 2015 documentation (PDF): "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). Intel's new Tri-Gate could solve this problem. 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, 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 Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures which projects a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. The data on Intel Processors clearly shows a slowdown in performance improvements in recent processors. However Intel's new processors, Core 2 (codenamed Conroe) shows a significant improvement over previous Pentium 4 processors.
Shadow RAM is RAM whose contents are copied from read-only memory (ROM) to allow shorter access times , as ROM is in general slower than RAM. The original ROM is disabled and the new location on the RAM is write-protected. This process is called shadowing.
As a common example, some BIOSes have a feature labeled “use shadow BIOS” or similar in the configuration options. When enabled, functionality that would rely on reading data from the BIOS’s ROM chip instead makes use of the RAM installed in the system. Depending on the system, this may or may not lead to a performance boost for calls to the BIOS.
For economical reasons, the large (main) memories found in personal computers, workstations, and non-handheld game-consoles (such as playstation and xbox) normally consists of dynamic RAM (DRAM). Other parts of the computer, such as cache memories and data buffers in hard disks, normally use static RAM (SRAM).
General DRAM packaging formats
Dynamic random access memory (DRAM) is produced as integrated circuits (ICs) bonded and mounted into plastic packages with metal pins for connection to control signals and buses. Today, these DRAM packages are in turn often assembled into plug-in modules for easier handling. Some standard module types are:
DRAM chip (Integrated Circuit or IC)
Dual in-line Package (DIP)
DRAM (memory) modules
Single In-line Pin Package (SIPP)
Single In-line Memory Module (SIMM)
Dual In-line Memory Module (DIMM)
Rambus In-line Memory Module (RIMM), technically DIMMs but called RIMMs due to their proprietary slot.
Small outline DIMM (SO-DIMM). Smaller version of the DIMM, used in laptops. Comes in versions with:
72 pins (32-bit)
144 pins (64-bit)
200 pins (72-bit)
Small outline RIMM (SO-RIMM). Smaller version of the RIMM, used in laptops.
Stacked v. non-stacked RAM modules
Stacked RAM chips use two RAM wafers that are stacked on top of each other. This allows large module (like a 512mb or 1Gig SO-DIMM) to be manufactured using cheaper low density wafers. Stacked chip modules draw more power.
Common DRAM modules
Common DRAM packages as illustrated to the right, from top to bottom:
DIP 18-pin (DRAM chip, usually pre-FPRAM)
SIPP (usually FPRAM)
SIMM 30-pin (usually FPRAM)
SIMM 72-pin (so-called "PS/2 SIMM", usually EDO RAM)
DIMM 168-pin (SDRAM)
DIMM 184-pin (DDR SDRAM)
DIMM 240-pin (DDR2 SDRAM) - (DRAM not pictured to the right.)