Pronounced rahm, a read-only memory (ROM) is a computer memory on which data has been pre-recorded. Once data has been written onto a ROM chip, it cannot be removed and can only be read. Unlike main memory (RAM), ROM retains its contents even when the computer is turned off. ROM is referred to as being non-volatile, whereas RAM is considered volatile. Read more about the difference between RAM and ROM here.
Most personal computers contain a small amount of ROM for critical programs that will not need to change over a device s lifetime, known as firmware. In addition, ROMs are used extensively in calculators and peripheral devices such as laser printers, whose fonts are sometimes stored in ROMs.
Three Types of ROM
There are three types of ROM that have historically been used in PCs, with each subsequent version replacing its predecessor:
- Programmable ROM (PROM) is manufactured as blank memory that can then be written by a programmer device, a change from earlier versions of ROM that were programmed during the manufacturing process.
- Erasable and Programmable ROM (EPROM) is a type of PROM that can be erased using ultraviolet light and then reprogrammed.
- Electrically Erasable and Programmable ROM (EEPROM) is a type of PROM that can be erased using an electrical charge and then reprogrammed. EEPROM is similar to its derivative flash memory; however, flash memory can be programmed and erased in large blocks instead of one byte at a time (like EEPROM), so it can also be used as a stand-alone memory for removable storage, such as a USB flash drive.
