Read-Only Memory (ROM)

Last Updated: June 5, 2018

Definition - What does Read-Only Memory (ROM) mean?

Read-only memory (ROM) is a type of storage medium that permanently stores data on personal computers (PCs) and other electronic devices. It contains the programming needed to start a PC, which is essential for boot-up; it performs major input/output tasks and holds programs or software instructions.

Because ROM is read-only, it cannot be changed; it is permanent and non-volatile, meaning it also holds its memory even when power is removed. By contrast, random access memory (RAM) is volatile; it is lost when power is removed.

There are numerous ROM chips located on the motherboard and a few on expansion boards. The chips are essential for the basic input/output system (BIOS), boot up, reading and writing to peripheral devices, basic data management and the software for basic processes for certain utilities.

Techopedia explains Read-Only Memory (ROM)

Other types of non-volatile memory include:

  • Programmable Read-Only Memory (PROM)
  • Electrically Programmable Read-Only Memory (EPROM)
  • Electrically Erasable Programmable Read-Only Memory (EEPROM; also called Flash ROM)
  • Electrically Alterable Read-Only Memory (EAROM)

However, these types of non-volatile memory can be altered and are often referred to as programmable ROM. One of the original forms of non-volatile memory was mask-programmed ROM. It was designed for specific data such as bootstrap, which contains the startup code. Mask-programmed ROM can never be changed.

Because ROM cannot be changed and is read-only, it is mainly used for firmware. Firmware is software programs or sets of instructions that are embedded into a hardware device. It supplies the needed instructions on how a device communicates with various hardware components. Firmware is referred to as semi-permanent because it does not change unless it is updated. Firmware includes BIOS, erasable programmable ROM (EPROM) and the ROM configurations for software.

ROM may also be referred to as maskROM (MROM). MaskROM is a read-only memory that is static ROM and is programmed into an integrated circuit by the manufacturer. An example of MROM is the bootloader or solid-state ROM, the oldest type of ROM.

Some ROM is non-volatile but can be reprogrammed, this includes:

  • Erasable Programmable Read-Only Memory (EPROM): This is programmed with the use of very high voltages and exposure to approximately 20 minutes of intense ultraviolet (UV) light.
  • Electrically-Erasable Programmable Read-Only Memory (EEPROM): This is used in many older computer BIOS chips, is non-volatile storage that can be erased and programmed several times and allows only one location at a time to be written or erased. An updated version of EEPROM is flash memory; this allows numerous memory locations to be altered simultaneously.
  • Ultraviolet-Erasable Programmable Read-Only Memory (UV-EPROM): This is read-only memory that can be erased by the use of ultraviolet light and then reprogrammed.

ROM is also often used in optical storage media such as various types of compact discs, including read-only memory (CD-ROM), compact disc recordable (CD-R) and compact disc rewritable (CD-RW).

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