This memory uses Metal Oxide Semiconductor Technology. RAM is read-write memory whereas ROM is Read-Only-Memory. This memory is used to store frequently used programs in the system.
ROM can be further divided into PROM, EPROM and EEPROM.
In Case of RAM it is possible to randomly select and use any Location of this memory. Each memory Location can be as easily and speedily accessed as any other Location in memory, that is, it takes the same access time wherever the Location in the memory. It is possible to read from and write into this memory and hence this memory is also called read/ write memory.
A Read-Only-Memory is one in which information is permanently stored. This memory cannot be written into. It can only be read. When power is switched off, the contents of ROM remain unchanged. The manufacturers supply ROM chips along with the programs therein. A variation of ROM chip is PROM.
The PROM can be programmed to record information using a special facility known as PROM programmer. But, once it is programmed using this facility, it becom,2s a ROM. PROM is also
non-volatile. However, there is another type of memory chip called EPROM which can be reprogrammed several times. It is possible to erase information stored in E:'ROM chip by exposing
it to ultraviolet light and reprogram it using a special PROM programmer facility.
1.      PROM
Programmable read-only memory can be programmed once. Special circuitry is used for programming it. Once programmed, this becomes read-only memory (ROM).
2.      EPROM
Erasable programmable read-only memory can· be erased repeatedly and reprogrammed by the us·=r. Erasing is done by exposing EPROM to ultraviolet light of a specific frequency. Reprogramming requires special EPROM Programmer kit.
3.      EEPROM
Electrically erasable programmable read-only-memory can be erased electrically and       reprogrammed repeatedly.