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Read-only memory (ROM) is a type of memory that can permanently or semi-permanently store data. It is called read-only because it is either impossible or difficult to write to. ROM also is often referred to as nonvolatile memory because any data stored in ROM remains there, even if the power is turned off. As such, ROM is an ideal place to put the PC’s startup instructions—that is, the software that boots the system.
Note that ROM and RAM are not opposites, as some people seem to believe. Both are simply types of memory. In fact, ROM technically could be classified as a subset of the system’s RAM. In other words, a portion of the system’s random access memory address space is mapped into one or more ROM chips. This is necessary to contain the software that enables the PC to boot; otherwise, the processor would have no program in memory to execute when it was powered on.
The main ROM BIOS is contained in a ROM chip on the motherboard, but there are also adapter cards with ROMs on them. ROMs on adapter cards contain auxiliary BIOS routines and drivers needed by the particular card, especially for those cards that must be active early in the boot process, such as video cards. Cards that don’t need drivers active at boot time typically don’t have a ROM because those drivers can be loaded from the hard disk later in the boot process.
Most systems today use a type of ROM called electrically erasable programmable ROM (EEPROM), which is a form of flash memory. Flash is a truly nonvolatile memory that is rewritable, enabling users to easily update the ROM or firmware in their motherboards or any other components (video cards, SCSI cards, and so on).