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.
[edit] 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 1970's 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 1970's.
Robert H. Dennard invented
Dynamic random access memory
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 voltatility. 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
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
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.
[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 the
magnetic tunnel effect. Amongst the 1st generation MRAM, a 128
KiB (128 × 2
10 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 × 2
20 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] Nantero built a functioning carbon nanotube memory prototype 10
GiB (10 × 2
30
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.
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... 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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