From ECDL.web

Read-only memory (often referred to as its acronym ROM) is a class of storage media used in computers and other electronic devices. 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).

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. "ROM" in its strictest sense can only be read from, but all ROMs allow data to be written into them at least once, either during initial manufacturing or during a step called "programming". Some ROMs can be erased and re-programmed multiple times, although they are still referred to as "read only" because the reprogramming process is relatively infrequent.

Contents 1 Types of ROMs 1.1 Semiconductor based 1.2 Other technologies 2 Speed of ROMs

[edit] Types of ROMs

[edit] Semiconductor based

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 (ie. programmed) via a special device, a PROM programmer. The writing often takes the form of permanently destroying or creating internal links (fuses or antifuses) 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 (greater than 1000) exposures before the EPROM becomes unusable. EPROMs can be easily identified by the circular 'window' in the top which allows the UV light to enter. Once programmed, this window is typically covered by a label to prevent accidental erasure.
• EAROMs (Electrically Alterable Read-Only Memory) can be modified a bit at a time, but writing is a slow process and uses non-standard voltages (usually higher voltages around 12 volts). Rewriting an EAROM 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 Programmable 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 (or camera, MP3 player, etc.). Flashing is much slower (milliseconds) than writing to RAM (nanoseconds) or reading from any ROM.
• By applying write protection, read/write memory may be turned temporarily into read-only memory.

[edit] Other technologies

There are also other types of non-volatile memory that are not solid-state chips:

• A CD-ROM is 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).
• Diode matrix ROM, which was used in small amounts in many computers in the 1960s. This ROM was programmed by installing discrete semiconductor diodes at selected locations between a matrix of word line traces and bit line traces on a printed circuit board.
• Resistor, capacitor, or transformer matrix ROM was used in many computers until the 1970s. Like the diode matrix ROM, it was programmed by placing components at selected locations between a matrix of word lines and bit lines.
• Core rope memory was a form of transformer matrix ROM technology used where size and/or weight were critical. This was used in NASA/MIT's Apollo Spacecraft Computers, DEC's PDP-8 computers, and other places. This type of ROM was programmed by hand by weaving "word line wires" inside or outside of ferrite transformer cores.
• Various other mechanical devices have been used as ROM in early computing equipment.

[edit] 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 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.