In computing, memory applies to the state information of a computing system because it is kept active in some physical structure. The term “memory” is used for the fast information in physical systems, as a distinction from physical systems which are slow to access. The term also refers to temporary state devices. Informally, computer memory refers to the physical devices used to store data or programs for later use in an electronic computer. Computers use information in a binary code that is written as sequences of 0s and 1s. Each binary digit can be stored by any physical system that to represent 0 and 1, called a bistable. It could be an on-off switch, an electrical capacitor, or a magnet. Today, capacitors and transistors function as tiny electrical switches for temporary storage and plastic discs for long-term storage.

In the early 1940s, memory technology frequently allowed a capacity of only a few bytes. The first programmable digital computer was the ENIAC, and it could perform simple calculations involving 20 numbers of ten decimal digits. Soon after, J. Presper Eckert developed the acoustic delay line memory. Delay lines could store information within the quartz and transfer it through sound waves through a glass tube filled with mercury and plugged at each end with a quartz crystal. In order to remain efficient, delay line memory is limited to a capacity of up to a few hundred thousand bits. The Williams tube and Selectron tube were developed in 1946 as two alternatives to the delay line, with both using electron beams in glass tubes for storage. At the same time, efforts were made to find non-volatile memory. Engineers created magnetic core memory so memory could be recalled after power loss. Magnetic core memory would become the dominant form of memory for the next couple decades.
There are two main types of memory: volatile and non-volatile. Volatile memory is computer memory that needs power to maintain the stored information. Current semiconductor volatile memory technology is usually either static RAM or dynamic RAM. Non-volatile memory is computer memory that can retain the stored information with or without power. Some examples of non-volatile memory include read-only memory, flash memory, floppy discs, optical discs, and early computer storage methods such as paper tape and punched cards.

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