ROM (Read-Only Memory) is a type of non-volatile memory that retains its contents even when power is turned off. Unlike RAM, which loses all data when the computer shuts down, ROM keeps its data permanently. Traditionally, ROM was used to store firmware and BIOS instructions that a computer needs to start up before it can load the operating system.
Why it matters
ROM plays a critical role in the boot process described in What Happens During Startup?. When you press the power button, the CPU has no instructions in RAM yet—everything was wiped when power was off. ROM provides the essential first instructions that tell the computer how to initialize hardware and begin loading the operating system from storage.
Types of ROM
The term "ROM" originally meant memory that could never be changed after manufacturing. Modern variants allow limited rewriting:
- PROM (Programmable ROM): Can be written once after manufacturing
- EPROM (Erasable PROM): Can be erased with UV light and rewritten
- EEPROM (Electrically Erasable PROM): Can be erased and rewritten electrically
- Flash memory: A type of EEPROM that's fast and cheap, used in SSDs and USB drives
Modern computers typically use flash-based storage for BIOS/UEFI, allowing firmware updates while still retaining data without power.
ROM vs RAM
The key distinction is volatility: RAM is fast but loses data without power, while ROM is slower but persistent. Your computer uses both—ROM to bootstrap the system, then RAM for active work once the operating system loads.