memory

The position of a motherboard within a computer's system unit. The motherboard contains the central processing unit, random access memory (RAM) chips, read-only memory (ROM), and a number of expansion slots.

In computing, the part of a system used to store data and programs either permanently or temporarily. There are two main types: immediate access memory and backing storage. Memory capacity is measured in bytes or, more conveniently, in kilobytes (units of 1,024 bytes) or megabytes (units of 1,024 kilobytes).

Immediate access memory, or internal memory, describes the memory locations that can be addressed directly and individually by the central processing unit. It is either read-only (stored in ROM, PROM, and EPROM chips) or read/write (stored in RAM chips). Read-only memory stores information that must be constantly available and is unlikely to be changed. It is nonvolatile - that is, it is not lost when the computer is switched off. Read/write memory is volatile - it stores programs and data only while the computer is switched on.

Backing storage, or external memory, is nonvolatile memory, located outside the central processing unit, used to store programs and data that are not in current use. Backing storage is provided by such devices as magnetic disks (floppy and hard disks), magnetic tape (tape streamers and cassettes), optical disks (such as CD-ROM), flash memory devices (USB flash or pen drives), and bubble memory. By rapidly switching blocks of information between the backing storage and the immediate-access memory, the limited size of the immediate-access memory may be increased artificially. When this technique is used to give the appearance of a larger internal memory than physically exists, the additional capacity is referred to as virtual memory.