Read-only memory
Non-volatile memory that cannot be electronically modified after manufacture.
Read-only memory (ROM) is a form of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. ROM is useful for storing software that is rarely changed during the life of the system, also known as firmware, and has been essential for bootstrapping computers and storing programs in early stored-program computers.
- field
- Computer memory technology
- known_for
- Non-volatile storage of firmware and boot programs
- types
- Mask ROM, PROM, EPROM, EEPROM, flash memory
Lore & Background
The simplest solid-state ROM is as old as semiconductor technology itself, using combinational logic gates to map address input to data output. With the integrated circuit came mask ROM, programmed with photomasks during semiconductor manufacturing, where data is physically encoded in the circuit. Mask ROM is economical only in large quantities and has long turnaround times, making it impractical for R&D. The invention of the MOSFET enabled MOS transistors as memory cell storage elements. Flash memory, invented in the early 1980s and commercialized in the late 1980s, is a form of EEPROM that permits high-speed erasure and programming of specific parts of the device.
Reader's Guide
Read-only memory has been fundamental to computing, providing non-volatile storage for firmware and boot programs. Early stored-program computers used forms of ROM, such as IBM's capacitor read-only storage (CROS) and transformer read-only storage (TROS), and the Apollo Guidance Computer's core rope memory. ROM was simpler to implement than mutable memory, needing only a mechanism to read stored values. With integrated circuits, ROM memory cells could use fewer transistors than SRAM cells, offering lower cost-per-bit for many years. The evolution from mask ROM to PROM, EPROM, EEPROM, and flash memory progressively addressed disadvantages: mask ROM required costly custom fabrication for each software revision and could not be corrected after shipment; PROM allowed one-time user programming; EPROM enabled erasure and reprogramming; EEPROM allowed in-place electrical reprogramming; and flash memory provided high-speed, partial erasure and reprogramming. By the year 2000, decreasing cost of reprogrammable devices had nearly eliminated the market for mask ROM. The most recent development, NAND flash, aimed to replace hard disks, offering higher throughput, lower latency, greater shock tolerance, miniaturization, and lower power consumption. ROM and RAM remain essential components, with ROM preserving data when power is off, while RAM is volatile.
Did You Know?
- The Apollo Guidance Computer used core rope memory, programmed by threading wires through magnetic cores.
- Flash memory was invented by Fujio Masuoka at Toshiba in the early 1980s and commercialized in the late 1980s.
Frequently Asked Questions
What is Read-only memory (ROM)?
ROM is a non-volatile memory chip whose stored data is permanently fixed at the time of manufacture and cannot be altered electronically afterward. It acts as a fixed repository for the low-level code a system needs every time it powers on.
What types of ROM exist?
The principal variants are Mask ROM (data etched during fabrication), PROM (programmable once by the user), EPROM (erasable with ultraviolet light), EEPROM (electrically erasable), and flash memory (the modern successor found in USB sticks and phones).
Why is ROM important in computing history?
ROM gave early stored-program computers a reliable, fixed location for their bootstrap code, removing the need to manually load instructions at every startup. It went on to anchor firmware storage across decades of consumer and industrial electronics.
How does ROM differ from ordinary RAM?
Unlike RAM, which is volatile and freely readable and writable during operation, ROM keeps its contents without power and resists any electronic modification once it leaves the factory. This makes it well suited for code that must survive power cycles and tampering.
Where is ROM (or its descendants) used in everyday devices?
You'll find it in the boot firmware of smartphones, the BIOS/UEFI chip on a motherboard, and the embedded controllers inside appliances and car ECUs. In each case it holds the low-level instructions that bring the device to life before any user software runs.
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