A ROM (read only memory) is anther non-volatile memory. It is used to store the information permanently. It is used by the manufacturers to store programs or OEM software known as firmware. It is not accessible to the user. It is used to initialize the programs or OS (operating system) of computer, microcodes of a CISC processor etc. The content is written into the ROM during its IC fabrication. It uses one-transistor memory cell.
Types of ROM:
- PROM (Programmable ROM)
- EEPROM (Electrically erasable ROM)
- EPROM (Erasable programmable read-only-memory)
PROM: In Programmable ROM (P-ROM) user can write a program or data permanently. It is programmable only once, means user will not be able to write data twice. The program is written into it using a special equipment known as PROM programmer.
EEPROM: An Electrically Erasable PROM is also called as Electrically Alterable ROM. In EE-PROM we can only erase the selective part of a program. It requires different voltages for reading, writing and erasing the data. The content can be erased very easily and programmed on the system board via byte by byte basis.
EPROM: An Erasable Programmable ROM can be erased and programmed more than once. In order to erase the content of EP-ROM, it is exposed to high-intensity short wave ultraviolet light for about 20 minutes. Unlike the EEP-ROM, the entire contents are erased in EPROM.