ROM (read-only memory)

In short: ROM (read-only memory) is memory whose content is kept permanently, even without power, and is only read in normal operation. It contains basic startup software (firmware).

In more detail: At power-on main memory is empty. Something must tell the computer how to get going. Code in ROM does that. The name comes from the time when this memory was hardwired during manufacturing and could not be changed at all.

In Depth

Kinds:

Kind	Special
Mask ROM	Content burned in during manufacturing, never changeable. Cheap in large quantities
PROM	Programmable once by the user
EPROM	Erasable with UV light (window in the package) and rewritable
EEPROM	Electrically erasable, single bytes. For small settings
Flash	Electrically erasable block by block. Today for BIOS/UEFI, firmware, SSDs

Use:

• BIOS/UEFI firmware on the motherboard.
• Firmware of routers, printers, remote controls, washing machines.
• Game modules and old console cartridges.
• “CD-ROM/DVD-ROM”: optical media that are read-only (read-only memory in the wider sense).

Difference from RAM: ROM is non-volatile and meant for reading. RAM is volatile, fast and freely writable. ROM is slower.

Use today: True “read only” is rare. It is mostly flash that you rewrite by firmware update. The term ROM remains for “programs that sit permanently in the device” (also boot ROM in the CPU: hardwired first startup code, for example for Secure Boot).

Security: True ROM cannot be infected. Writable firmware memory can, which is why signed updates matter. A small boot ROM as an unchangeable “root of trust” is therefore the basis of many security architectures.

See also: RAM, [[rom-(festwertspeicher)|ROM and non-volatile storage]], Firmware, NAND flash