Programmable Read-Only Memory (PROM)

Overview of PROM: what it is, how it differs from other ROM types, main variants (PROM, EPROM, EEPROM), programming methods, typical uses, and notable distinctions.

Programmable read-only memory, commonly abbreviated PROM, is a class of nonvolatile memory whose contents are written after manufacture and are retained without power. PROM devices are a form of read-only memory (ROM) because they provide persistent storage of code or data, but unlike mask ROM they are supplied blank and are set by the user or manufacturer.

Overview

In a typical PROM the user sets bits once by a programming operation; this is often called 'burning' the PROM. Once programmed, the pattern remains until the device is either discarded or, for erasable variants, explicitly erased. The programming step must be done with a dedicated hardware tool known as a PROM programmer, and the data are usually fixed by permanently altering internal elements such as fuses or antifuses.

Characteristics and variants

One-time programmable (OTP) PROM: written once; internal fuses are irreversibly changed.
EPROM: an erasable PROM that can be cleared and rewritten; commonly erased by ultraviolet light and reprogrammed. See EPROM.
EEPROM: electrically erasable PROM that can be erased and rewritten in-circuit using electrical signals. See EEPROM.
Flash: a later development related to EEPROM, allowing block-level electrical erasure and reprogramming.

How programming works

Programming a PROM typically involves forcing higher than normal voltages or currents into selected memory cells to change their state. This is why the operation is called "burning"—it can be destructive or permanent for OTP devices. Modern erasable devices use controlled charge injection or tunneling to set and clear cell states without mechanical or optical procedures. The user-facing action is often described simply as programmed, but underlying mechanisms vary by type.

Uses and importance

PROM and its variants have been widely used to store firmware, microcode, bootloaders, and configuration data where nonvolatile, tamper-resistant storage is required. OTP PROM is economical for stable production code; EPROM and EEPROM are preferred during development and for devices that require field updates. They remain important in embedded systems, legacy equipment, and applications where permanence or predictable behavior matters.

Notable distinctions

Compared with volatile memory (RAM), PROM retains data without power. Compared with mask ROM, PROM offers post-manufacture customization. Compared with modern flash memory, classic PROMs are simpler and sometimes more robust but less flexible for frequent updates. Device selection balances cost, reprogrammability, endurance, and ease of update.

For further technical details or programming procedures, consult device datasheets or programming-tool documentation, which provide the exact voltages, timing, and recommended workflows for each PROM family.