RANDOM-ACCESS MEMORY (RAM /ræm/ ) is a form of computer data storage which stores frequently used program instructions to increase the general speed of a system. A random-access memory device allows data items to be read or written in almost the same amount of time irrespective of the physical location of data inside the memory. In contrast, with other direct-access data storage media such as hard disks , CD-RWs , DVD-RWs and the older magnetic tapes and drum memory , the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement.
RAM contains multiplexing and demultiplexing circuitry, to connect the data lines to the addressed storage for reading or writing the entry. Usually more than one bit of storage is accessed by the same address, and RAM devices often have multiple data lines and are said to be '8-bit' or '16-bit' etc. devices.
In today's technology, random-access memory takes the form of
integrated circuits . RAM is normally associated with volatile types
of memory (such as
* 1 History * 2 Types of random-access memory * 3 Memory cell * 4 Addressing * 5 Memory hierarchy
* 6 Other uses of RAM
* 7 Recent developments * 8 Memory wall * 9 See also * 10 References * 11 External links
Early computers used relays , mechanical counters 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 relatively low cost but efficient retrieval of 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 registers. Such registers were relatively large and too costly to use for large amounts of data; generally only a few dozen or few hundred 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. Developed at the University of Manchester in England, the Williams tube provided the medium on which the first electronically stored-memory program was implemented in the Manchester Small-Scale Experimental Machine (SSEM) computer, which first successfully ran a program on 21 June 1948. In fact, rather than the Williams tube memory being designed for the SSEM, the SSEM was a testbed to demonstrate the reliability of the memory.
Magnetic-core memory was invented in 1947 and developed up until the mid-1970s. It became a widespread form of random-access memory, relying on an array of magnetized rings. By changing the sense of each ring's magnetization, data could be stored with one bit stored per 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.
Dynamic random-access memory (DRAM) 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.
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 RANDOM-ACCESS MEMORY
The two widely used 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 six transistor memory cell . This form of RAM is more expensive
to produce, but is generally faster and requires less dynamic power
than DRAM. In modern computers, SRAM is often used as cache memory for
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 E
EPROM 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
In general, the term _RAM_ refers solely to solid-state memory
Main article: Memory cell (binary)
The memory cell is the fundamental building block of computer memory . The memory cell is an electronic circuit that stores one bit of binary information and it must be set to store a logic 1 (high voltage level) and reset to store a logic 0 (low voltage level). Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it.
In SRAM, the memory cell is a type of flip-flop circuit, usually implemented using FETs . This means that SRAM requires very low power when not being accessed, but it is expensive and has low storage density.
A second type, DRAM, is based around a capacitor. Charging and
discharging this capacitor can store a '1' or a '0' in the cell.
However, this capacitor will slowly leak away, and must be refreshed
periodically. Because of this refresh process,
To be useful, memory cells must be readable and writeable. Within the RAM device, multiplexing and demultiplexing circuitry is used to select memory cells. Typically, a RAM device has a set of address lines A0... An, and for each combination of bits that may be applied to these lines, a set of memory cells are activated. Due to this addressing, RAM devices virtually always have a memory capacity that is a power of two.
Usually several memory cells share the same address. For example, a 4 bit 'wide' RAM chip has 4 memory cells for each address. Often the width of the memory and that of the microprocessor are different, for a 32 bit microprocessor, eight 4 bit RAM chips would be needed.
Often more addresses are needed than can be provided by a device. In that case, external multiplexors to the device are used to activate the correct device that is being accessed.
Main article: Memory hierarchy
One can read and over-write data in RAM. Many computer systems have a
memory hierarchy consisting of processor registers , on-die SRAM
caches, external caches ,
In many modern personal computers, the RAM comes in an easily
upgraded form of modules called memory modules or
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.
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.
Main article: RAM drive
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.
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.
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 Tunnel
magnetoresistance . Amongst the 1st generation MRAM , a 128 KiB (128
× 210 bytes) 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-transfer torque (STT) on which Crocus ,
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.
The "memory wall" is the growing disparity of speed between
“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
A different concept is the processor-memory performance gap, which can be addressed by 3D integrated circuits that reduce the distance between the logic and memory aspects that are further apart in a 2D chip. Memory subsystem design requires a focus on the gap, which is widening over time. The main method of bridging the gap is the use of caches ; small amounts of high-speed memory that houses recent operations and instructions nearby the processor, speeding up the execution of those operations or instructions in cases where they are called upon frequently. Multiple levels of caching have been developed in order to deal with the widening of the gap, and the performance of high-speed modern computers are reliant on evolving caching techniques. These can prevent the loss of processor performance, as it takes less time to perform the computation it has been initiated to complete. There can be up to a 53% difference between the growth in speed of processor speeds and the lagging speed of main memory access.
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* CAS latency (CL) * Hybrid Memory Cube * Multi-channel memory architecture * Registered/buffered memory * RAM parity * Memory Interconnect/RAM buses * Memory geometry * Chip creep * Evolution of storage devices
* ^ Gallagher, Sean. "Memory that never forgets: non-volatile DIMMs
hit the market". Ars Technica.
* ^ Bellis, Mary. "The Invention of the Intel 1103".
* ^ "
* ^ Toscal BC-1411 calculator,
Science Museum, London
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