random-access memory

storage (RAM) (Previously "direct-access memory"). A data

storage device for which the order of access to different

locations does not affect the speed of access. This is in

contrast to, say, a magnetic disk, magnetic tape or a

mercury delay line where it is very much quicker to access

data sequentially because accessing a non-sequential location

requires physical movement of the storage medium rather than

just electronic switching.

The most common form of RAM in use today is built from

semiconductor integrated circuits, which can be either

static (SRAM) or dynamic (DRAM). In the 1970s magnetic

core memory was used. RAM is still referred to as core by

some old-timers.

The term "RAM" has gained the additional meaning of

read-write. Most kinds of semiconductor read-only memory

(ROM) are actually "random access" in the above sense but are

never referred to as RAM. Furthermore, memory referred to as

RAM can usually be read and written equally quickly

(approximately), in contrast to the various kinds of

programmable read-only memory. Finally, RAM is usually

volatile though non-volatile random-access memory is also

used.

Interestingly, some DRAM devices are not truly random access

because various kinds of "page mode" or "column mode" mean

that sequential access is faster than random access.

(1995-12-05)