Read-only memory

Read-only memory (ROM) is used as a storage medium in computers. It is so named because it cannot (easily) be written to, and not likely to need frequent upgrading, nor can the information stored on be lost, as with the case of RAM.

Applications
Most video game consoles from the 1970s to the mid-1990s used ROMs as their primary software distribution medium. Such ROMs are sealed into plastic cases suitable for handling and repeated insertion, known as cartridges or "carts" (called "Game Paks" by Nintendo). Some home computers also used ROM cartridges for distributing games and other types of software. The Nintendo 64 was the last major console to use ROM cartridges for software distribution. As of the 2000s, only handheld consoles like the Game Boy Advance and Nintendo DS continue using ROM cartridges.

Types of ROMs
Classic mask-programmed ROM chips are written to during production and cannot change content afterwards. But there are other types of non-volatile solid-state memory:
 * PROMs (Programmable Read-Only Memory) can be written to (programmed) via a special device, a PROM programmer. The writing often takes the form of destroying internal links with the result that a PROM can only be programmed once.
 * EPROMs (Erasable Programmable Read-Only Memory) can be erased by exposure to ultraviolet light then rewritten via an EPROM programmer. Repeated exposure to ultraviolet light will eventually destroy the EPROM but it generally takes many exposures before the EPROM becomes unusable.
 * EAROMs (Electrically Alterable Read-Only Memory) can be modified a bit at a time, but writing is intended to be an infrequent operation; most of the time the memory is used as a ROM. EAROM may be used to store critical system setup information in a non-volatile way. For many applications, EAROM has been supplanted by CMOS RAM backed-up by a lithium battery.
 * EEPROM such as Flash memory (Electrically Erasable Read-Only Memory) allow the entire ROM (or selected banks of the ROM) to be electrically erased (flashed back to zero) then written to without taking them out of the computer (camera, MP3 player, etc.). Flashing is much slower than writing to RAM (Random Access Memory) (or reading from any ROM).
 * By applying write protection, read/write memory may be turned (temporarily) into read-only memory.
 * A CD-ROM isn't a form of electrical ROM in the traditional sense, but rather a kind of compact disc. A CD-ROM is Read Only, hence the name, while CD-R is Write-Once-Read-Many (analogous to a PROM), and CD-RW is Read/Write (analogous to an EEPROM).

Reading speed
Although this has varied over time, today, large RAMs can be read faster than most large ROMs. Therefore ROM content that is used often is sometimes copied to RAM and subsequently read from there (shadowed).

Writing speed
For those types of ROMs that can be written to or electrically altered, the writing speed is always much slower than the reading speed and it may require unusual voltages or the movement of jumper plugs to apply write-enable signals to the EAROM/Flash ROM.