Random-access memory (RAM) is a form of computer data
storage. A
random-access device allows stored data to be accessed in very nearly the same amount
of time for any storage location, so data can be accessed quickly in any random
order. In contrast, other data storage media such ashard disks, CDs, DVDs and magnetic tape, as well as early primary memory types such
as drum memory, read and write data only in a predetermined
order, consecutively, because of mechanical design limitations. Therefore the
time to access a given data location varies significantly depending on its
physical location.
Today, random-access memory takes the form of integrated
circuits. Strictly
speaking, modern types of DRAM are not random access, as data is read in
bursts, although the name DRAM / RAM has stuck. However, many types of SRAM, ROM, OTP, and NOR flash are still random access even in a strict sense. RAM is often
associated with volatile types of memory (such as DRAM memory modules), where its stored information is lost if
the power is removed. Many other types of non-volatile memory are RAM as well,
including most types of ROM and a type of flash memory called NOR-Flash. The first RAM modules to come into the
market were created in 1951 and were sold until the late 1960s and early 1970s.
History
Early computers used relays, or delay lines for "main" memory functions. Ultrasonic delay lines could
only reproduce data in the order it was written. Drum memory could be
expanded at low cost but retrieval of non-sequential memory items required
knowledge of the physical layout of the drum to optimize speed. Latches built
out of vacuum tube triodes, and later, out of discrete transistors, were used for smaller and faster memories such as random-access register
banks and registers. Such registers were relatively large, power-hungry and too
costly to use for large amounts of data; generally only a few hundred or few thousand
bits of such memory could be provided.
The first practical form of random-access memory was the Williams tube starting in
1947. It stored data as electrically charged spots on the face of a cathode ray tube. Since the electron beam of the CRT could read and write the spots on the
tube in any order, memory was random access. The capacity of the Williams tube
was a few hundred to around a thousand bits, but it was much smaller, faster,
and more power-efficient than using individual vacuum tube latches.
Magnetic-core memory, invented in 1947 and developed up until the mid 1970s, became a
widespread form of random-access memory. It relied on an array of magnetized
rings; by changing the sense of magnetization, data could be stored, with each
bit represented physically by one ring. Since every ring had a combination of
address wires to select and read or write it, access to any memory location in
any sequence was possible.
Magnetic core memory was the standard form of memory system until displaced
by solid-state memory in integrated circuits, starting in the early
1970s. Robert H. Dennard invented dynamic random-access memory (DRAM) in 1968; this allowed replacement of a 4 or 6-transistor latch
circuit by a single transistor for each memory bit, greatly increasing memory
density at the cost of volatility. Data was stored in the tiny capacitance of
each transistor, and had to be periodically refreshed in a few milliseconds
before the charge could leak away.
Prior to the development of integrated read-only memory (ROM) circuits, permanent (or read-only)
random-access memory was often constructed using diode matrices driven by address decoders, or specially wound core rope memory planes.
Types of RAM
The two main forms of modern RAM are static RAM (SRAM) and dynamic RAM (DRAM). In SRAM, a bit of data is stored using the state of a flip-flop. This form of RAM is more expensive to
produce, but is generally faster and requires less power than DRAM and, in
modern computers, is often used as cache memory for the CPU. DRAM stores a bit of data using a
transistor and capacitor pair, which together comprise a memory cell. The
capacitor holds a high or low charge (1 or 0, respectively), and the transistor
acts as a switch that lets the control circuitry on the chip read the
capacitor's state of charge or change it. As this form of memory is less
expensive to produce than static RAM, it is the predominant form of computer
memory used in modern computers.
Both static and dynamic RAM are considered volatile, as their state is lost or reset when power
is removed from the system. By contrast, Read-only
memory (ROM) stores data by permanently enabling or
disabling selected transistors, such that the memory cannot be altered. Writeable
variants of ROM (such as EEPROM and flash memory) share properties of both ROM and RAM,
enabling data to persist without power and to be updated without
requiring special equipment. These persistent forms of semiconductor ROM
include USB flash drives, memory cards for cameras and
portable devices, etc. As of 2007, NAND flash has begun to replace older forms of
persistent storage, such as magnetic disks and tapes, while NOR flash is being used in place of ROM in netbooks and rugged
computers, since it is
capable of true random access, allowing direct code execution.
ECC memory (which can be either SRAM or DRAM) includes
special circuitry to detect and/or correct random faults (memory errors) in the
stored data, using parity bits or error
correction code.
In general, the term RAM refers solely to solid-state memory devices
(either DRAM or SRAM), and more specifically the main memory in most computers.
In optical storage, the term DVD-RAMis somewhat of a misnomer since, unlike CD-RW or DVD-RW it does not require to be erased before
reuse. Nevertheless a DVD-RAM behaves much like a hard disc drive if somewhat
slower.
[Memory hierarchy
One can read and over-write data in RAM. Many
computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems, andvirtual 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 interleaveorganization of the components make the
access time variable, although not to the extent that rotating storage media or a tape is variable. The overall goal of
using a memory hierarchy is to obtain the higher possible average access
performance while minimizing the total cost of the entire memory system
(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 when changing needs demand more storage capacity. 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.
Other uses of RAM
In addition to serving as temporary storage
and working space for the operating system and applications, RAM is used in
numerous other ways.
Virtual memory
Most modern operating systems employ a method
of extending RAM capacity, known as "virtual memory". A portion of
the computer's hard drive is set aside for a paging file or a scratch partition, and the combination of physical RAM and the
paging file form the system's total memory. (For example, if a computer has 2
GB of RAM and a 1 GB page file, the operating system has 3 GB total memory
available to it.) When the system runs low on physical memory, it can "swap" portions of RAM to the paging file to
make room for new data, as well as to read previously swapped information back
into RAM. Excessive use of this mechanism results in thrashing and generally hampers overall system
performance, mainly because hard drives are far slower than RAM.
RAM disk
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. A RAM disk loses the stored data when the
computer is shut down, unless memory is arranged to have a standby battery
Shadow RAM
Sometimes, the contents of a relatively slow
ROM chip are copied to read/write memory to allow for shorter access times. 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 not result in increased performance, and may cause incompatibilities.
For example, some hardware may be inaccessible to the operating
system if shadow RAM is used. On some systems the
benefit may be hypothetical because the BIOS is not used after booting in favor
of direct hardware access. Free memory is reduced by the size of the shadowed
ROMs.[1]
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 approaches utilizing themagnetic
tunnel effect. Amongst the
1st generation MRAM, a 128 KiB (128 × 210 bytes) magnetic RAM (MRAM) chip was manufactured with
0.18 µm technology in the summer of 2003. In June 2004, Infineon
Technologies unveiled a 16 MiB (16 × 220 bytes) prototype again based on 0.18 µm
technology. There are two 2nd generation techniques currently in development: Thermal
Assisted Switching (TAS)[2] which is being developed by Crocus Technology, and Spin Torque
Transfer (STT) on which Crocus, Hynix, IBM, and several other companies are working.[3] Nanterobuilt a functioning carbon nanotube memory
prototype 10 GiB (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 256 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.
Some kinds of random-access memory, such as
"EcoRAM", are specifically designed for server farms, where low power consumption is more important than speed.[4]
Memory wall
The "memory wall" is the growing
disparity of speed between CPU and 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% while
memory speed only improved at 10%. Given these trends, it was expected that
memory latency would become an overwhelming bottleneck in computer performance.[5]
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. Intelsummarized 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... 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 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 Core 2 Duo processors (codenamed Conroe) showed a
significant improvement over previous Pentium 4 processors; due to a more efficient
architecture, performance increased while clock rate actually decreased
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