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Understanding random access memory ram in a computer

These IBM tabulating machines from the 1930s used mechanical counters to store information A portion of a core memory with a modern flash SD card on top 1 Megabit chip — one of the last models developed by VEB Carl Zeiss Jena in 1989 Early computers used relaysmechanical counters [3] 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 triodesand later, out of discrete transistorswere used for smaller and faster memories such as registers. Such registers were relatively large and too costly to use for large understanding random access memory ram in a computer 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 program was implemented in the Manchester Baby computer, which first successfully ran a program on 21 June 1948.

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. Data was stored in the tiny capacitance of each transistor, and had to be periodically refreshed every 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 decodersor specially wound core rope memory planes.

  • So, for example, if a document has been saved to a hard drive prior to a power outage or system crash, the user will still be able to retrieve it when the system is back up and running;
  • In addition to serving as temporary storage and working space for the operating system and applications, RAM is used in numerous other ways;
  • Multiple levels of caching have been developed to deal with the widening gap, and the performance of high-speed modern computers relies on evolving caching techniques.

In SRAM, a bit of data is stored using the state of a six transistor memory cell. The capacitor holds a high or low charge 1 or 0, respectivelyand 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. These persistent forms of semiconductor ROM include USB flash drives, memory understanding random access memory ram in a computer for cameras and portable devices, and solid-state drives.

Memory cell Main article: Memory cell computing 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.

The value in the memory cell can be accessed by reading it. 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, the charge in this capacitor slowly leaks away, and must be refreshed periodically. Within the RAM device, multiplexing and demultiplexing circuitry is used to select memory cells.

  • Developed at the University of Manchester in England, the Williams tube provided the medium on which the first electronically stored program was implemented in the Manchester Baby computer, which first successfully ran a program on 21 June 1948;
  • DRAM is typically used for the main memory in computing devices;
  • Given these trends, it was expected that memory latency would become an overwhelming bottleneck in computer performance;
  • Depending on the system, this may not result in increased performance, and may cause incompatibilities;
  • This refresh operation happens automatically thousands of times per second.

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.

RAM - random access memory

Often more addresses are needed than can be provided by a device. In that case, external multiplexors to the device are understanding random access memory ram in a computer to activate the correct device that is being accessed.

Memory hierarchy Main article: Memory hierarchy One can read and over-write data in RAM. Many computer systems have a memory hierarchy consisting of processor registerson-die SRAM caches, external cachesDRAMpaging 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 timesviolating the original concept behind the random access term in RAM.

Even within a hierarchy level such as DRAM, the specific row, column, bank, rankchannel, or interleave organization of the components make the access time variable, although not to the extent that access time to rotating storage media or a tape is variable. The overall goal of using a memory hierarchy is to obtain the highest 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. 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 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.

RAM (Random Access Memory)

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: A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source.

The ROM chip is then disabled while the initialized memory locations understanding random access memory ram in a computer 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.

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.

The technologies used include carbon nanotubes and approaches utilizing Tunnel magnetoresistance. There are two 2nd generation techniques currently in development: Whether some of these technologies can eventually take 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.

  • By synchronizing a computer's memory with the inputs from the processor, computers were able to execute tasks faster;
  • Flash memory is used for storage memory, while RAM is used as active memory that performs calculations on the data retrieved from storage;
  • And, alternatively, an SSD stores data in memory chips that, unlike RAM, are nonvolatile, don't depend on having constant power and won't lose data once the power is turned off.

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 farmswhere low power consumption is more important than speed.

How RAM Works

An important reason for this disparity is the limited communication bandwidth beyond chip boundaries, which is also referred to as bandwidth wall. Given these trends, it was expected that memory latency would become an overwhelming bottleneck in computer performance.

Intel summarized these causes in a 2005 document. 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.

Random-access memory

Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster due to the so-called Von Neumann bottleneckfurther 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 End of the Road for Conventional Microarchitectures" [15] which projected a maximum of 12. 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. Multiple levels of caching have been developed to deal with the widening gap, and the performance of high-speed modern computers relies on evolving caching techniques.