In
computing,
memory refers to the physical devices used to store programs (sequences of instructions) or data (e.g. program
state information) on a temporary or permanent basis for use in a
computer or other
digital electronic device. The term
primary memory is used for the information in physical systems which function at high-speed (i.e.
RAM), as a distinction from
secondary memory, which are physical devices for
program and data storage which are slow to access but offer higher memory capacity. Primary memory stored on secondary memory is called "
virtual memory". An archaic synonym for memory is
store.
The term "memory", meaning
primary memory is often (but not always) associated with addressable
semiconductor memory, i.e.
integrated circuits consisting of
silicon-based
transistors, used for example as
primary memory but also other purposes in computers and other
digital electronic devices.
There are two main types of semiconductor memory:
volatile and
non-volatile. Examples of
non-volatile memory are
flash memory (sometimes used as secondary, sometimes primary computer memory) and
ROM/
PROM/
EPROM/
EEPROM memory (used for
firmware such as boot programs). Examples of
volatile memory are
primary memory (typically dynamic RAM,
DRAM), and fast
CPU cache memory (typically static RAM,
SRAM, which is fast but energy-consuming and offer lower memory capacity per area unit than DRAM).
Most semiconductor memory is organized into
memory cells or bistable flip-flops, each storing one
bit (0 or 1).
Flash memory organization includes both one bit per memory cell and multiple bits per cell (called MLC, Multiple Level Cell). The memory cells are grouped into words of fixed
word length, for example 1, 2, 4, 8, 16, 32, 64 or 128 bit. Each word can be accessed by a binary address of
N bit, making it possible to store 2 raised by
N words in the memory. This implies that
processor registers normally are not considered as memory, since they only store one word and do not include an addressing mechanism.
The term
storage is often (but not always) used to describe
secondary memory such as tape,
magnetic disks and optical discs (
CD-ROM and
DVD-ROM).