Read-only memory spent fifty years learning how to be rewritten
The Apollo Guidance Computer stored its programs by threading wires through tiny magnetic rings, a memory nobody could change once woven. Early chip-based ROM was just as permanent, etched into silicon at the factory. Step by step, engineers made it erasable, until its descendant, flash memory, began replacing hard disks in the 2000s and 2010s.
Classic mask ROM stores bits as physical features laid down by photolithography during chip manufacture: a transistor present means one value, absent means the other. Because the data is literally part of the circuit, no electrical trick can alter it. That made it cheap per bit, since a ROM cell can be a single transistor while a static RAM cell needs a latch of five to twenty, but inflexible. Every software revision required a new mask, orders only made sense in bulk, turnaround was slow, and a bug in shipped hardware meant recalling every unit.
Each shortcoming found a fix. Wen Tsing Chow invented programmable ROM in 1956, letting customers burn their own contents once with high-voltage pulses. In 1971 Dov Frohman at Intel created EPROM, which could be wiped clean under strong ultraviolet light and reused. The following year, researchers at Japan's Electrotechnical Laboratory developed EEPROM, which could be rewritten electrically while still inside a device. Fujio Masuoka at Toshiba then invented flash in the early 1980s, able to erase selected blocks quickly and survive thousands of rewrites. By 2000 falling prices had nearly killed the market for mask ROM.
For decades ROM held the software a machine needed at switch-on. The Commodore 64 paired 64 kilobytes of RAM with 20 kilobytes of ROM containing its BASIC interpreter and operating system. Later PCs loaded their systems from disk and kept only a small startup routine, the BIOS, in ROM. ROM also stored lookup tables so slow processors could fetch answers such as sine values instead of calculating them.
Toshiba's NAND flash designers stated openly that their goal was to replace hard disks, and by 2021 they had largely succeeded, offering speed, shock resistance and tiny formats such as microSD cards. The old name lingers: Android enthusiasts still call a replacement operating system a custom ROM, and hobbyists have reflashed music players and routers to run Linux.
Source: Read-only memory
