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Computer memory refers to devices that are used to store data or programs (sequences of instructions) on a temporary or permanent basis for use in an electronic digital computer. Computers represent information in binary code, written as sequences of 0s and 1s. Each binary digit (or "bit") may be stored by any physical system that can be in either of two stable states, to represent 0 and 1. Such a system is called bistable. This could be an on-off switch, an electrical capacitor that can store or lose a charge, a magnet with its polarity up or down, or a surface that can have a pit or not. Today, capacitors and transistors, functioning as tiny electrical switches, are used for temporary storage, and either disks or tape with a magnetic coating, or plastic discs with patterns of pits are used for long-term storage.

Computer memory is usually meant to refer to the semiconductor technology that is used to store information in electronic devices. Current primary computer memory makes use of integrated circuits consisting of silicon-based transistors. There are two main types of memory: volatile and non-volatile.

Detail of the back of a section of ENIAC, showing vacuum tubes
In the early 1940s, memory technology mostly permitted a capacity of a few bytes. The first digital computer, the ENIACmarker, using 20,000 octal-base radio vacuum tubes allowed simple calculations involving 20 numbers of ten decimal digits which were held in the vacuum tube accumulators.

The next significant is that advance in computer memory was with acoustic delay line memory developed by J. Presper Eckert in the early 1940s. Through the construction of a glass tube filled with mercury and plugged at each end with a quartz crystal, delay lines could store bits of information within the quartz and transfer it through sound waves propagating through mercury. Delay line memory would be limited to a capacity of up to a few hundred thousand bits to remain efficient.

Two alternatives to the delay line, the Williams tube and Selectron tube, were developed in 1946, both using electron beams in glass tubes as means of storage. Using cathode ray tubes, Fred Williams would invent the Williams tube, which would be the first random access computer memory. The Williams tube would prove to be advantageous to the Selectron tube because of its greater capacity (the Selectron was limited to 256 bits, while the Williams tube could store thousands) and being less expensive. The Williams tube would nevertheless prove to be frustratingly sensitive to environmental disturbances.

Efforts began in the late 1940s to find non-volatile memory. Jay Forrester, Jan A. Rajchman and An Wang would be credited with the development of magnetic core memory, which would allow for recall of memory after power loss. Magnetic core memory would become the dominant form of memory until the development of transistor based memory in the late 1960s.

Volatile memory

Volatile memory is computer memory that requires power to maintain the stored information. Current semiconductor volatile memory technology is usually either static RAM (see SRAM) or dynamic RAM (see DRAM). Static RAM exhibits data remanence, but is still volatile, since all data is lost when memory is not powered. Whereas, dynamic RAM allows data to be leaked and disappear automatically without a refreshing. Upcoming volatile memory technologies that hope to replace or compete with SRAM and DRAM include Z-RAM, TTRAM and A-RAM.

Non-volatile memory

Non-volatile memory is computer memory that can retain the stored information even when not powered. Examples of non-volatile memory include read-only memory (see ROM), flash memory, most types of magnetic computer storage devices (e.g. hard disks, floppy discs and magnetic tape), optical discs, and early computer storage methods such as paper tape and punch cards.Upcoming non-volatile memory technologies include FeRAM, CBRAM, PRAM, SONOS, RRAM, Racetrack memory, NRAM and Millipede.

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