Random-access
memory
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"RAM"
redirects here. For other uses of the word, see Ram (disambiguation).
Example
of writable volatile random-access memory: Synchronous Dynamic RAM modules, primarily used as
main memory in personal computers,workstations,
and servers.
Random access memory (RAM)
is a form of computer data storage. Today, it takes the
form of integrated circuits that allow stored data to be accessed in any order with a
worst case performance of constant time.
Strictly speaking, modern types of DRAM are therefore 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.
Other memory devices
(magnetic tapes, disks) can access the storage data only in a predetermined
order, because of mechanical design limitations.
History
1
Megabit chip - one of the last models developed by VEB Carl
Zeiss Jena in 1989
An early type of
widespread writable random-access memory was magnetic core memory, developed from 1955
to 1975, and subsequently used in most computers up until the development and
adoption of the static and dynamic integrated RAM circuits in the late 1960s
and early 1970s. Before this, computers used relays, delay line/delay
memory, or various kinds of vacuum tube arrangements to implement "main"
memory functions (i.e., hundreds or thousands of bits), some of which were random access, some not. 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. 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, or specially wound core memory planes.Robert H.
Dennard invented Dynamic random access memory in 1968.
Types of RAM
Top
L-R, DDR2 with heat-spreader, DDR2 without
heat-spreader, Laptop DDR2, DDR, Laptop DDR
The two main forms of
modern RAM are static RAM (SRAM) and dynamic RAM (DRAM). In static RAM, 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-RAM is somewhat of a misnomer since, like CD-RW, a rewriteable DVD
must be erased before it can be rewritten.
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, 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. 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 its
applications, RAM is used in numerous other ways.
Virtual
memory
Main article: 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
Main article: 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 source.
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.
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 the magnetic 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) which is being developed by Crocus
Technology, and Spin Torque Transfer (STT) on which Crocus, Hynix, IBM, and several other
companies are working. Nantero built 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.
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.
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 Duoprocessors
(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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