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. 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.
This development has started to blur the definition between traditional random-access memory and "disks", dramatically reducing the difference in performance.
The value in the memory cell can be accessed by reading it. 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.
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. 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.
Free memory is reduced by the size of the shadowed ROMs. Virtual memory Main article: These persistent forms of semiconductor ROM include USB flash drives, memory cards for cameras and portable devices, and solid-state drives.
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. The ROM chip is then disabled while the initialized memory locations are switched in 4 ram slots or 2 the same block of addresses often write-protected. A second type, DRAM, is based around a capacitor.
For example, a 4 bit 'wide' RAM chip has 4 memory cells for each address. 4 ram slots or 2, 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.
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 Virtual memory Most modern operating systems employ a method of extending RAM capacity, known as "virtual memory".
Latches built out of vacuum tube triodesand later, out of discrete transistorswere used for smaller and faster memories such as registers. In SRAM, a bit of data is stored using the state of rivers casino wing night six transistor memory cell.
Ultrasonic delay lines could only reproduce data in the order it was written. For example, some hardware may be inaccessible to the operating system if shadow RAM is used. Due to this addressing, RAM devices virtually always have a memory capacity that is a power of two. The End of the Road for Conventional Microarchitectures"  which projected a maximum of In that case, external multiplexors to the device are used to activate the correct device that is being accessed.
It became a widespread form of random-access memory, relying on an array of magnetized rings. A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source.
It stored data as electrically charged spots on the face of a cathode ray tube. Within the RAM device, multiplexing and demultiplexing circuitry is used to select memory cells.
The first practical form of random-access memory was the Williams tube starting in 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. Usually several memory cells share the same address.
Magnetic core memory was the standard form of 4 ram slots or 2 system until displaced by solid-state memory in integrated circuits, starting in the early s.
On some systems the benefit may be hypothetical because the BIOS is not used after booting in favor of direct hardware access. Memory cell Main article: By changing the sense of each ring's magnetization, data could be stored with one bit stored per ring.
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.
Often more addresses are needed than can be provided by a device. RAM disk Main article: Given these trends, it was expected that memory latency would become an overwhelming bottleneck in computer performance.
Some kinds of random-access memory, such as "EcoRAM", are specifically designed for server farmswhere low power consumption is more important than speed. This means that SRAM requires very low power when not being accessed, but it is expensive and has low storage density.
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. Depending on the system, this may not result in increased performance, and may cause incompatibilities.
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. There are two 2nd generation techniques currently in development: By contrast, read-only memory ROM stores data by permanently enabling or disabling selected transistors, such that the memory cannot be altered.
Memory hierarchy One can read and over-write data in RAM.
Since the electron beam of the CRT could read and write the spots on the tube in any order, memory was random access. This process, sometimes called shadowing, is fairly common in both computers and embedded systems.
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. However, the charge in this capacitor slowly leaks away, and must be refreshed periodically.