Computer memory often called as storage or computer data storage; refer to the components of a computer that are used to store data in the digital form so as to compute at some interval of time. Computer stores information in binary system such as in 1s or 0s. Such a system called as bi-stable.

Memory provides the core function of the computer which is data retention or information retention. It is a fundamental part of the computer and incorporated in CPU (central processing unit) which implements the basic computer model since the 1940s.

This could be a surface with pit or not, a capacitor that can store or lose charge or a magnet with its polarity up or down. Now-a-days capacitors and transistors are used as the electrical switches and either hard disk or tape with a magnetic coating.

Usually memory refers to a form of semiconductor storage also known as RAM (random access memory) or in other forms fast but temporary storage whereas storage commonly refers to the optical drives or hard disk drives, the other types slower than RAM but store memory on permanent nature. Previously memory and storage were called primary and secondary memory, respectively.

Characteristics of memory:

1.    Volatility
2.    Mutability
3.    Accessibility
4.    Addressability


Volatility

Non volatile memory- it stores the information even if not supplied with continuous power supply. It is used for long term data storage. In 1960s, it is used for primary storage in the form of magnetic core memory whereas today it is mostly used in secondary, tertiary and for off-line storage.

Volatile memory- it uses constant power supply to store data. It is used for primary storage as the primary storage should be very fast. Volatile memory is the fastest memory storage technology now-a-days. It is further classified into two:

1.    Dynamic memory- it requires the stored information to be periodically re-read or re-written otherwise it will vanish.

2.    Static memory- it is similar to the dynamic memory but does not need to be refreshed.

Mutability

Mutable storage or read/write storage- it allows the user to overwrite the information any time. The computer is useless with some amount of read/write storage for primary storage purposes. But modern computer uses the read/write storage for some secondary storage also.

Read only storage- this retains the information that is stored at the time of manufacture and allows write once storage to be written at some point after manufacture. It is also known as immutable storage and used for tertiary and off-line storage. Examples are CD-ROM and CD-R.

Slow write fast read storage- it is a read/write storage that allows overwriting many times but the writing process is very much slower than the reading process. Examples are CD-RW and flash memory.

Accessibility

Random access- any location of the storage can be accessed at equal interval of time at any moment. This feature well suited for primary and secondary storage.

Sequential access- the information is accessed on the basis of the piece of information wants to access is at what location and it depends on the last piece of information accessed. The accessing will be done in serial order. This characteristic well suited for off-line storage.

Addressability

Location addressable- each accessible piece of information is selected with its numerical memory address. Location addressable storage is restricted to primary storage as it is very efficient but very burdensome for humans.

File addressable- the whole information is divided into files of various lengths and each file is selected with its file name or human-readable directory. Secondary, tertiary and off-line storage uses the file addressable in modern computers.

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