A read-only memory (ROM) is a memory storage medium in computers and other electronic devices. The ROM of PCs usually stores the firmware. The name “ROM memory” is also often used, but it is actually a pleonasm, since the M already stands for memory.
A read-only memory is used when a device, even when it is turned off, needs to keep a program (normal memory RAM loses its content when there is no power left), or when it is never or rarely necessary to change the program. ROM, contrary to what some people think, is certainly not faster than RAM; therefore on old pc’s the ROM was quickly copied into RAM by the computer before using it. This process was called ROM shadowing.
The ROM is relatively slow, and could hardly get smaller, cheaper and faster after 2003 (it would slowly lose its content if it were made even smaller).
The name ROM is often used exclusively for the mask ROM. In this archetypal form, the data is inserted into the chip during the manufacturing process. This is done by selectively omitting parts of the chip.
We know the following ROM type:
ROM
The oldest kind of ROM is ‘mask programmed’. This is provided with data in the chip factory. Such a ROM is therefore only available to order, and moreover only in large numbers. Nevertheless, such a ROM was very useful for manufacturers of electronic equipment in which a fixed computer program had to be built in.
PROM
Then came the PROM, Programmable ROM. This is sold ’empty’. With a special device, a PROM programmer or (e) prommer, data can be written into a PROM. It is no longer possible to erase afterwards.
EPROM
The next step is the remarkable EPROM, Erasable PROM. This has a quartz glass window in the housing. Exposing the EPROM to ultraviolet light for a few minutes will erase the contents. Then the memory can be reprogrammed with an EPROM programmer. This makes an EPROM particularly suitable for testing software that will later be put into a ROM or PROM.
If an EPROM is definitively programmed, that is to say without the intention to delete the contents again, a label with the name and version number of the software is placed over the small window.
Some PROMs are actually EPROMs, but with a closed housing so that erasure is practically impossible.
EEPROM
Then the EEPROM appeared. This is erased and described by certain electrical voltages. This makes it possible to change the content while the EEPROM remains in the computer. One form of EEPROM memory is flash memory.