Prom
PROM, or programmable read-only memory, is a read-only memory that is manufactured blank and programmed once by the user after fabrication, rather than having its data fixed at the factory by a mask. It occupies a middle ground between mask ROM and the later erasable memories: unlike mask ROM, it can be written by whoever buys it, giving flexibility and fast turnaround, but unlike EPROM or EEPROM, it can be written only a single time, after which its contents are permanent. PROM was an important step because it decoupled the act of programming from the act of manufacturing, letting data be set in small quantities and late in the process.
The classic PROM works by having each bit's value determined by a tiny fusible link that the user can selectively destroy during programming. A PROM chip comes from the factory with all its bits in one default state, each held by a small fuse-like connection. Programming the chip means applying higher-than-normal electrical pulses to specific locations to permanently blow those links, changing the bits that should be in the other state. Because blowing a fuse is irreversible — the physical connection is destroyed — the programming is one-time and permanent, which is exactly why PROM cannot be erased and rewritten the way later technologies can.
PROM's value is that it moves the programming step out of the factory and into the hands of the user, which changes the economics for small and medium volumes. With mask ROM, the data must be committed before any chips exist, at the cost of a custom mask, so it only makes sense at high volume with stable data. PROM lets a user buy standard blank parts and program them in whatever small quantities are needed, whenever the data is ready, with inexpensive programming equipment. This is a decisive advantage for prototyping, low-volume products, or any situation where committing to a mask would be premature or uneconomical.
The one-time nature of PROM is both its limitation and the reason it sits where it does in the technology family. A programming mistake or a later change to the data wastes the part, because a blown fuse cannot be restored, so PROM suits data that is final when written but produced in quantities or timeframes that do not justify a mask. This write-once limitation is precisely what the erasable technologies set out to remove: EPROM added the ability to erase the whole chip with ultraviolet light and reprogram it, and EEPROM added electrical erasure, each trading cost or complexity for the ability to reuse the part.
PROM is best understood as the first loosening of ROM's rigidity, a stepping stone in a clear progression toward ever more flexible non-volatile memory. The line runs from mask ROM, where data is fixed at manufacture, to PROM, where the user can write it once, to EPROM and EEPROM, where it can be erased and rewritten repeatedly. Each step added flexibility at some cost, and PROM marks the specific moment where the user first gained control over the contents of a read-only memory, even if only for a single irreversible write.
Read PROM as write-once memory handed to the user: shipped blank and permanently programmed in the field by blowing fusible links, it freed data from the factory mask for small-volume and late-decided needs, while its single irreversible write is exactly the constraint the erasable EPROM and EEPROM were built to overcome.