Types of Computer Memory

 Computer memory, a device for temporary or permanent storage of data or programs (sequences of instructions) for use in an electronic digital computer. Computers represent information in binary code, written as sequences of 0s and 1s. Any binary digit (or "bit") can be stored by any physical system, which can be in one of two stable states to represent 0s and 1s called bistable . This can be an on/off switch, an electrical capacitor that can store or release a charge, a magnet with its polarity pointing up or down, or a surface that may or may not have a hole. Today, capacitors and transistors, which work like small electrical switches, are used for temporary storage, and magnetically coated disks or tapes or pit-patterned plastic disks are used for long-term storage.

Computer memory is divided into main (or primary) storage and auxiliary (or secondary) storage. Main memory holds instructions and data when a program is running, while auxiliary memory holds data and programs that are not currently in use and provides long-term storage.

main memory

The earliest storage devices were electromechanical switches or relays (see Computers: the first computers) and electron tubes (see Computers: the first programmable logic machines). In the late 1940s, the first programmable logic controllers used ultrasonic waves in mercury tubes or charges in special electron tubes for main storage. The latter was the first random access memory (RAM). RAM contains memory cells that can be accessed directly for read and write operations, unlike serial-access memory like magnetic tape, where each cell in the sequence must be accessed until it is found. An example of this type of memory is Dell M1G12

magnetic drum storage

In the 1950s, both main and auxiliary storage used magnetic drums, which had fixed read/write heads for each of the many tracks on the outer surface of a rotating cylinder coated with a ferromagnetic material, although their Access to the data was serial.