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

# Short note: ROM

Digital Logic · Chapter 4: Data Processing Circuits

- Asked in: 2074 Asoj, 3 marks
- Video: [ROM - Read Only Memory: Basics, Structure, Size, and Classifications in Digital Electronics](https://www.youtube.com/watch?v=Cd19ohX0770) — Engineering Funda
- 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-rom
- Whole chapter: https://www.bctengineer.com/md/semester_3/digital_logic/chapter-4

## Question

Write a short note on ROM.

## Short answer

ROM is non-volatile memory that retains data without power, used for firmware. It contains a decoder and programmable OR array to implement any set of Boolean functions. Types include Mask ROM, PROM, EPROM, EEPROM, and Flash.

## Answer

**ROM (Read-Only Memory)** is a non-volatile semiconductor memory whose contents are written once (at manufacture or by programming) and then mainly read. It keeps data when power is removed, so it stores fixed programs such as BIOS/boot code, look-up tables and microcode.

**Structure:** a $2^n \times m$ ROM has $n$ address lines and $m$ data outputs. An $n$-to-$2^n$ decoder selects one word (row); a programmable OR array (links at row–column crossings) gives the $m$ output bits. So a ROM can implement any set of $m$ Boolean functions of $n$ variables in sum-of-minterms form.

```text
 A(n-1)..A0 -->[ n:2^n decoder ]--> 2^n word lines
                     |
               [ OR array (links) ]
                     |
               D(m-1) ... D0  (outputs)
```

**Types:**

| Type | How programmed / erased |
|---|---|
| Mask ROM | Fixed by the maker using a photo-mask |
| PROM | Fuses blown once by the user |
| EPROM | Electrically written, erased by UV light |
| EEPROM | Electrically written and erased, byte-wise |
| Flash | Electrically erased in blocks |

Example: a $32 \times 8$ ROM has 5 address lines and 8 outputs, storing 256 bits.

## Check yourself

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

1. Which describes ROM?
   A. Volatile memory, freely rewritable
   B. Non-volatile memory that keeps data without power and is mainly read
   C. Memory used only for temporary data
   D. Memory that needs refresh
   Answer: B. ROM is non-volatile and used for fixed programs such as boot code or look-up tables. (Key point: Nature of ROM)

2. What are the main parts of a 2^n x m ROM?
   A. An n-to-2^n decoder and an OR array giving m outputs
   B. An encoder and a flip-flop array
   C. Counter and adder
   D. A MUX and a latch
   Answer: A. The decoder selects a word and the programmable OR array gives the m data bits, so ROM can implement functions in sum-of-minterms form. (Key point: ROM structure)

3. A 32 x 8 ROM has how many address lines and bits stored?
   A. 8 address lines, 32 bits
   B. 5 address lines, 256 bits
   C. 32 address lines, 8 bits
   D. 5 address lines, 40 bits
   Answer: B. 2^5 = 32 words, so 5 address lines, and 32 x 8 = 256 bits. (Key point: ROM sizing)

4. Which ROM type is erased by UV light?
   A. Mask ROM
   B. PROM
   C. EPROM
   D. EEPROM
   Answer: C. EPROM is electrically written and erased with ultraviolet light; EEPROM is erased electrically. (Key point: ROM types)