A read-only memory, or read-only memory (English read-only memory, ROM) is a data store that can be accessed read-only during normal operation, do not write and is not volatile. That is: It keeps its data in a de-energized state. It is used in embedded systems with unchangeable software. On PCs, there is, for example, the " hard-wired " in the BIOS ROM.
The writing of data into a ROM is called the program the block and is not affiliated with the write access to a read-write memory ( Random Access Memory, HDD ) comparable. A distinction is made between blocks with reversible and irreversible programming. However, like RAM ROM allows random access to the data.
Originally ROMs are "hard-wired " only in production. Since this wiring with a "mask" ( a kind of film negative ) is directly exposed onto the raw chip, we speak here of a mask-programmed ROM or just mask ROM. Constantly growing - - Another family memory devices of this type developed, which can be filled with information even after the production Since this method, however, credited only in big production, was one.
There are now quite a large number of different types of ROM:
- Mask ROM - only programmable time of manufacturing, the cheapest version for mass production
- Programmable Read - Only Memory (PROM ), one-time programmable
- Erasable Programmable Read-Only Memory (EPROM), erasable with UV light
- Electrically Erasable Programmable Read - Only Memory (EEPROM ), electrically erasable
As a replacement for the ROM flash memory is used often today - available as a FLASH EEPROM with random addressing.