ROM

In computing, a memory device in the form of a collection of integrated circuits (chips), frequently used in microcomputers. ROM chips are loaded with data and programs during manufacture and, unlike RAM (random-access memory) chips, can subsequently only be read, not written to, by a computer. However, the contents of the chips are not lost when the power is switched off, as happens in RAM.

ROM is used to form a computer's permanent store of vital information, or of programs that must be readily available but protected from accidental or deliberate change by a user. ROM chips are used in electronic devices, such as calculators and video games. Most personal computers use ROM to store the code that boots (starts up) the computer. The problem with conventional ROM is that small changes are difficult to achieve because they are produced in an assembly line. PROM (programmable read-only memory) allows a high-voltage electronic pulse to ‘burn’ new sets of instructions, or update old ones. EPROM (erasable programmable read-only memory) is reprogrammed using ultraviolet light, shone through a window on top. EEPROM (electronically erasable programmable read-only memory) can be reprogrammed in the machine, a byte at a time. Flash ROM (flash read-only memory) can be erased and rewritten in whole blocks.