Read Only Memory

Read only memory, or ROM, is a type of storage media that is used by computers and other electronic devices. The data stored in ROM cannot be modified, therefore it is largely used to distribute firmware.
The most straightforward type of ROM is about as old as semiconductor technology. It is called mask ROM, and it was introduced along with the integrated circuit. Mask ROM is made up of a grid of word lines, called the address input, and bit lines, called the data output, that are selectively joined together with transistor switches.

Collectively, they can represent a subjective look-up table with a regular physical layout and predictable propagation delay. The data is generally physically encoded in the circuit, so it can only be programmed during fabrication. However, this can often times cause disadvantages. Due to these problems, further developments have been made. In 1956, PROM was invented to allow users to program its contents by applying high-voltage pulses to physically alter its structure. The 1971 invention of EPROM also helped to solve more problems. It can be repeatedly reset to its original state by exposing it to strong ultraviolet light. All of the new technologies improved the flexibility of ROM, but at the same time increased its cost.

Every computer that supplies programs and other data must have some form of stable or erasable storage to store the initial program that runs when the computer is powered on or booted up. Forms of read-only memory began being used as non-volatile storage for programs in many early stored-program computers. ROM was much easier to put into practice because it only needed a device that was able to read stored values rather than to change their order. ROM and other later technologies, such as Flash, are common in embedded systems. This controls everything from industrial robots to home appliances and consumer electronics, all of which are designed for specific functions but based on general-purpose microprocessors. As of recently, most products use Flash instead of mask ROM, and many provide ways to connect to PCs for firmware updates.

Due to the fact that ROM cannot be modified, it is only appropriate for storing data that is not expected to need modification for the life of the device. ROM has been used in many computers to store look-up tables for mathematical and logical functions. Along the same lines, the display adapters of early computers stored tables of font characters in ROM. Today, the use of ROM to store such small amounts of data has almost completely disappeared in general-purpose computers. Instead, Flash ROM has taken over as a channel for mass storage or secondary storage of files.

There are many different types of ROM. Programmable read-only memory (PROM), is a type of ROM that can be programmed using a special device called a PROM programmer. This device usually uses high voltages to permanently create or destroy internal links within the chip. As a result, a PROM can only be programmed once. Erasable programmable read-only memory (EPROM) is a type of ROM that can be erased by exposure to strong ultraviolet light and then rewritten with a process that uses higher voltage amounts than usual.

Continuous exposure to ultraviolet light will eventually wear out an EPROM. Electrically erasable programmable read-only memory (EEPROM) is a type of ROM that is based on a similar semiconductor structure to EPROM, but the difference is that it allows its entire contents to be electrically erased and rewritten so that they do not need to be removed from the computer. All these different types of ROMs are used in different situations and can all be found useful. The relative speed of all ROMs has varied over time. However, writing speed is always slower than reading speed, and it requires much higher voltages. The endurance and data retention of ROMs has also varied over time. ROMs can only withstand a limited number of write and erase cycles because they are written by forcing electrons through a layer of electrical insulation onto a floating transistor gate. Earlier ROMs could only withstand 1,000 cycles, but today they have evolved to withstand up to 1,000,000 cycles.