Read-only memory

Read-only memory (ROM) is nonvolatile computer memory designed to be read rather than routinely modified. It retains stored data when power is removed.

Mask ROM: Read-only memory whose contents are set during chip fabrication. Its data is built into the chip’s physical layout and cannot be rewritten.
BIOS: Firmware that initializes a computer’s hardware and starts the boot process. Traditional BIOS code is stored in nonvolatile memory so it is available at startup.
Random-access memory: Volatile memory that allows data to be read and written directly at arbitrary addresses. Unlike ROM, ordinary RAM is routinely writable and loses its contents without power.
Plugboard: A panel of sockets and wires used to configure or program an electromechanical machine. Plugboards were an earlier way to encode fixed control instructions before semiconductor ROM.
Programmable read-only memory: Nonvolatile memory that can be programmed once after manufacture. It allows data to be added after fabrication, but programming is irreversible.
UEFI: A firmware interface that initializes hardware and launches an operating system. UEFI firmware resides in rewritable nonvolatile storage on modern computers.
Dynamic random-access memory: Volatile memory that stores each bit as charge in a capacitor and requires refreshing. DRAM is writable working memory, whereas ROM preserves fixed contents without power.
Read-only memory (IBM 360): The fixed microcode storage used in IBM System/360 computers. IBM System/360 helped establish ROM as a practical store for processor control instructions.
EPROM: Erasable programmable read-only memory cleared by ultraviolet light before reprogramming. Its erasable storage made ROM contents updateable through a special physical process.
Bootloader: Software that prepares a system and loads its operating system. Boot code must remain available before the operating system can access storage.

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