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Before moving into RAM types lets take a look at ROM types.
The classification reflects the evolution of ROM devices from hardwired to programmable to erasable-and-programmable devices. A common feature of all these devices is their ability to retain data and programs forever, even during a power failure.
Masked ROMs: The very first ROMs were hardwired devices the contents of which had to be specified before chip production. They are now called masked ROMs to distinguish them from other types of ROM.
PROM: One step up from the masked ROM is the PROM (programmable ROM. A Device programmer is used for writing the data to the PROM; it writes data one word at a time by applying an electrical charge to the input pins of the chip. Once programmed the PROM's contents can never be changed.
EPROM (erasable-and-programmable ROM): is programmed in the same manner as a PROM. The advantage EPROM has over PROM is that it can be erased and reprogrammed repeatedly. Data on an EPROM is erased by exposing it to a strong source of ultraviolet light which resets EPROM to its initial un-programmed state.
