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Encyclopedia > Read only memory

Rom is also the name of a toy and comic book character Rom (Spaceknight).

ROM is also an abbreviation referring to the Royal Ontario Museum.

Read-Only Memory (ROM) is used as a storage medium in computers. Because it cannot (easily) be written to, its main uses lie in the distribution of firmware (software that is very closely related to hardware, and not likely to need frequent upgrading).

One common usage of ROMs are in data files for video game cartridges emulation (often a violation of copyright or sui generis mask work rights unless your jurisdiction has a fair use protection).

Modern semiconductor ROMs typically take the shape of IC packages, i.e. "computer chips", not immediately distinguishable from other chips like RAMs but for the text printed on the chips. Many microcontrollers consist of a ROM part in addition to a CPU core, some on-board peripherals, and RAM. Home computers of the early 1980s came with their complete operating system in ROM, often including a BASIC programming language interpreter. There was no reasonable alternative because floppy disk drives were generally optional. Upgrading to a newer version meant using either a soldering iron or a set of DIP sockets and replacing the old ROM chip with a new one.

Video game consoles that use ROM based software include the Super Famicom, Super Nintendo Entertainment System (North American and European versions of the Super Famicom), the Magnavox Odyssey˛, and the Game Gear. Such ROMs are sealed into plastic cases suitable for handling and repeated insertion, known as cartridges or "carts" (called "Game Paks" by Nintendo). Many home computers also used ROM cartridges for distributing games and other types of software.

The contents of these ROM cartridges could be extracted with special software, creating files known as "ROM images". Illegal distribution and trade of these "ROM images" is hugely popular in software piracy circles, where the files are commonly referred to as "roms" (or even "romz", which has a connotation with "warez"). The term is so widespread that it has even started to refer to computer games that do not come on ROM cartridges, but instead on floppy disks or CD-ROMs.

There is a trend to put less and less software into static ROMs, and more on disk storage, making changes easier. By the 2000s, operating systems for desktop computers are not generally on ROM anymore. Computers may still rely on some software in ROMs, like their BIOS, but even that is more likely to reside on a Flash-ROM (see below). Mobile phones and personal digital assistants are likely to have software in ROM (or at least flash memory).

One reason why some data still sits in ROMs is speed – disks are an order of magnitude slower. Even more important, though, is that you cannot read software that is needed to drive a disk from the disk itself – see bootstrap. Hence the BIOS or a bootloader for a computer is often stored in ROM. Also, a networking or graphics card may implement some basic functionality through software contained on a ROM chip. Another application for ROMs is in storing software for embedded systems operating in physically demanding environments (exposed to, say, vibration, or high acceleration), where rotating media like disks are more vulnerable to damage.


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.
  • Flash memory or EEPROMs (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 disk. A CD-ROM is Read Only, hence the name, while CD-R is Write-Once-Read-Many (analogous to an EPROM), and CD-RW is Read/Write (analogous to a Flash EEPROM).
  • A form of pre-semiconductor ROM technology is found in the core rope, which was used in the Apollo spacecraft computers. This ROM was programmed by hand by weaving word line wires inside or outside of ferrite transformer cores.

Speed of ROMs

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 (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.


Because they are written by forcing electrons through an layer of electrical insulation onto a floating transistor gate, re-writeable ROMs will only withstand a limited number of write cycles before the insulation is permanently damaged. In very old EAROMs, this damage could occur in as few as 1,000 write cycles. In modern Flash EEPROMs, the life may be in excess of 10,000 or even 100,000 cycles, but it is by no means infinite. It is partially for this reason (as well as their limited, more-expensive capacity) that Flash ROMs are unlikely to completely supplant magnetic disk drives in the near future.

The reading life of a ROM is not limited by the number of cycles performed. The reading life of EAROMs and EEPROMs may be limited by charge leakage from the floating gates of the storage transistors, especially at high temperatures or in high radiation environments. Masked ROMs or fused EPROMs do not suffer from this effect; once a 1 or 0, always a 1 or 0 (although fuse re-growth was once a problem in early fused PROMs).



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