Volatile and non-volatile memory

Volatile memory

Volatile memory is memory which retains data whilst power is applied - when power is lost, the information is lost.

It is usually a collection of semiconductor cells, eg, a pair of transistors in a bistasble latch configuration, where for one logic state, one of the transistors is switched on, and the other is switched off; for the other logic state the roles reverse.

When power is lost, the cells have no idea which one was on, and which one was off.

Volatile memory is typically the memory referred to as RAM in a computer system. It is very fast, with access times around 60 - 70 nsec. Current PC`s are supplied with 8 to 64 Mb of RAM.

Non-volatile memory

Non-volatile memory is memory where the data is recorded by :-

• Altering a transistor cell during fabrication by masking - as in a mask ROM

• Blowing links within a cell by a particular type of applied current - as in a PROM, EPROM, or EEPROM

• Altering the magnetic state of a microscopic portion of a magnetic layer on a plastic backing - as in floppy discs, hard drives, magnetic tapes

• Creating surface indentations which can be read by laser - as in a CD ROM

In all these cases, retention of data is a physical state which does not rely on power to preserve it.

Data stored by these methods is therefore preserved when the computer is switched off, or when the storage medium is not in a computer.

© 1998 Ron Turner

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