Field Guide · hardware

Also known as: ROM

Read-only memory (ROM) is non-volatile memory whose contents are written once or rarely and retained without power, traditionally used to hold code a device needs the moment it switches on.1

Mask ROM: the data is the wiring pattern row decoder bit lines ● present = 1 blank = 0 the 1s and 0s are fixed by the factory mask — non-volatile and read-only
Classic mask ROM stores each bit physically: at every crossing of a word line and a bit line there either is or isn't a transistor, and that pattern is decided by the photomask at manufacture. Reading just selects a row and senses the columns. Nothing rewrites it in normal use, and it keeps its contents with the power off.

Overview

Classic mask ROM has its data baked in during manufacture and can never be changed. Later variants made ROM progressively more editable: PROM (programmable once), EPROM (erasable with ultraviolet light), and EEPROM (electrically erasable). Flash memory is the modern, block-erasable descendant of EEPROM and now fills most roles once handled by ROM chips. Despite the name, today’s “ROM” is usually rewritable — but only deliberately, which is the point: it survives power loss and is not casually overwritten like RAM.

Where it fits

ROM’s job is to store the firmware and boot code that bring hardware to life, such as a BIOS/UEFI on a PC or the bootloader on a microcontroller. It is the non-volatile counterpart to volatile RAM: RAM holds the running program and is wiped at power-off, while ROM holds the unchanging instructions that start the machine. In a GopherTrunk capture node, the SDR dongle and the host both rely on firmware held in this kind of memory.

Sources

  1. Read-only memory — Wikipedia, on ROM, its PROM/EPROM/EEPROM variants, and firmware storage. ↩

See also