Glossary · Hardware

ROM (Read-Only Memory)

ROM is non-volatile memory whose contents persist without power and are normally written once, used to store firmware and boot code.

Updated October 6, 2026

ROM (Read-Only Memory) is non-volatile memory that keeps its contents when the power is off. It is traditionally written at manufacture or very rarely changed, and holds the code a device needs to start up.

Why it matters

Every device needs some permanent instructions to begin booting before storage is available. Those live in ROM or in flash memory that behaves like it. Modern devices usually use rewritable flash, which allows firmware updates.

The name is mostly historical. Types include mask ROM, PROM, EPROM and EEPROM.

Example

When you press the power button, the processor starts by running code from a ROM or flash chip:

power on -> boot code in ROM/flash -> firmware init -> bootloader -> operating system

If that chip is corrupted, the device may not start at all, often called being bricked.

Common misconceptions

  • ROM can never be changed. Flash and EEPROM variants are updated through firmware flashing.
  • ROM is the same as storage. It is a tiny amount of boot code, not your files.
  • Phone ROM means the whole system image. In custom firmware communities the term is used loosely.

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