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ROM stands for Read-Only Memory -- the digital equivalent of look but don't touch. ROM information is etched onto tiny microchips inside the computer, and it can be read by the computer, but not changed.
ROM chips are found almost everywhere in a computer system, from the basic input/output system (BIOS) to peripheral devices like printers and scanners. Each ROM chip contains specific instructions important to the operation of that particular part of the system.
Your computer uses ROM all the time. When you turn on the power switch, your computer goes through a series of tests, checking the amount of RAM, testing disk drives and other tasks, all before loading the operating system. The checklist for performing this sequence is contained in ROM, and it never changes.
While in use, your computer constantly calls on different ROM chips to supply the necessary information to perform various tasks.
ROM technology is fairly standard among competing companies, but the actual instructions contained on chips varies depending on the manufacturer. ROM chips are also commonly referred to as "black boxes," because the cargo gets more attention than the transport device.
Despite common design elements, there are a few differences in ROM technology.
One variation of ROM is called PROM (Programmable Read-Only Memory). A PROM chip is like a blank canvas awaiting an artist, unlike a ROM chip that's already been constructed. Programmers can tell PROMs to hold specific instructions, but they can only do this once. Afterward, the information is burned into the PROM chip, and it behaves like regular ROM.
EPROM (Erasable Programmable Read-Only Memory) improves on PROM. EPROM chips can be erased using specialized equipment after they've been removed from a computer. Once the chip is blank, a programmer is able to install new instructions.
A further refinement in ROM technology is EEPROM (Electronically Erasable Programmable Read-Only Memory). EEPROMs allow programmers to erase the read-only instructions without physically removing the chip. This is accomplished by running a specialized program that replaces the read-only memory with new information.
A similar but faster chip is called "Flash ROM." It is used in most systems for basic input/output functions (it's often called "Flash BIOS"). These chips feature electronically reprogrammable read-only memory and rely on an internal battery for power.
All four types of ROM chips are in use today, often within the same computer system.
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