---
title: "Short note: PROM"
subject: "Digital Logic"
chapter: 4
key: "semester_3/digital_logic:short-note-prom"
source: https://www.bctengineer.com/q/semester_3/digital_logic/short-note-prom
site: BCT Engineering
---

# Short note: PROM

Digital Logic · Chapter 4: Data Processing Circuits

- Asked in: 2081 Baisakh, 2 marks
- Video: [PROM (Programmable Read Only Memory) || Implementing Boolean Functions using PROM || PROM Example](https://www.youtube.com/watch?v=5rcdQahTZh8) — Sudhakar Atchala
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/digital_logic/short-note-prom
- Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-4

## Question

Write a short note on PROM.

## Short answer

PROM is a one-time programmable ROM with a fixed AND array (decoder) and a programmable OR array of fusible links. All cells initially store 1; blowing selected fuses creates 0s. It stores fixed programs and implements combinational logic.

## Answer

**PROM (Programmable Read-Only Memory)** is a ROM that is manufactured blank and can be programmed **once** by the user with a PROM programmer. After programming its contents cannot be changed (one-time programmable, OTP).

- **Structure:** a fixed AND array (decoder producing all minterms of the address lines) and a programmable OR array. Each cross-point has a **fusible link** (nichrome or polysilicon fuse, or a diode).
- **Programming:** all fuses are intact at first (all bits 1). A high current pulse blows selected fuses, making those bits 0. This is permanent.
- **Uses:** storing fixed programs, look-up tables and code converters; it can implement any combinational function in sum-of-minterms form.

Unlike EPROM or EEPROM, a PROM cannot be erased; a mistake means a new chip.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How many times can a PROM be programmed?
   A. Many times using UV light
   B. Once, by the user
   C. Many times electrically
   D. Never, it is set at manufacture
   Answer: B. PROM is one-time programmable. (Key point: Programmability)

2. What is the structure of a PROM?
   A. Programmable AND array and fixed OR array
   B. Both arrays programmable
   C. Fixed AND array and programmable OR array with fusible links
   D. Floating-gate MOSFETs erased by UV
   Answer: C. The fixed AND array decodes addresses and the fuse pattern in the OR array stores the data. (Key point: Structure)

3. How is a 0 stored in a PROM bit?
   A. Electrons are injected into a floating gate
   B. A mask leaves out the connection at manufacture
   C. The fuse is left intact
   D. A high current pulse blows the fuse
   Answer: D. Intact fuses are 1; blowing a fuse permanently makes the bit 0. (Key point: Programming)