--- title: "ROM types and 4x3 ROM design" subject: "Embedded System" chapter: 4 key: "semester_6/embedded_system:rom-types-4x3-design" source: https://www.bctengineer.com/q/semester_6/embedded_system/rom-types-4x3-design site: BCT Engineering --- # ROM types and 4x3 ROM design Embedded System · Chapter 4: Memory - Asked in: 2070 Bhadra, 6+2 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_6/embedded_system/rom-types-4x3-design - Whole chapter: https://www.bctengineer.com/md/semester_6/embedded_system/chapter-4 ## Question Describe ROM and introduce its types in detail. Sketch the internal design of a 4 × 3 ROM. ## Short answer ROM is non-volatile memory that retains data without power. Types include mask ROM (factory-programmed, permanent), PROM (fuse-based, one-time programmable), EPROM (UV-erasable), EEPROM (electrically byte-erasable), and Flash (block-erasable). A 4×3 ROM uses a 2-to-4 decoder to select a word line; three OR gates form the output bits, with connections at crosspoints defining stored words. ## Answer **ROM (Read-Only Memory)** is a non-volatile memory that keeps its stored data without power. In the system it is normally only read, and written rarely or never; it holds fixed programs such as firmware and boot code. #### Types of ROM | Type | Programming | Erasable | Notes | |---|---|---|---| | **Mask-programmed ROM** | At the factory using a mask | No | Cheapest for large volumes; high NRE cost, contents permanent | | **PROM / OTP ROM** | By the user, once, with a programmer (blowing fuses) | No | Good for small volumes and prototypes | | **EPROM** | By the user with a programmer | Yes, by UV light (whole chip, ~20 min) | Floating-gate transistor, quartz window | | **EEPROM** | In-system, electrically | Yes, per byte, electrically | Slower writes, limited cycles; used for settings | | **Flash** | In-system, electrically | Yes, block-wise | High density, cheap, fast reads; used for firmware, USB drives | #### Internal design of a 4 x 3 ROM A 4 x 3 ROM stores 4 words of 3 bits. It needs 2 address lines (A1, A0), a 2-to-4 decoder, 4 word lines and one OR gate per output bit (3 OR gates). A 1 is stored wherever a word line is connected to an OR gate input. Example contents: w0 = 101, w1 = 011, w2 = 110, w3 = 001. ```text D2 col D1 col D0 col +---------+ | | | A1 ----->| |--w0--X-------+-------X---- w0 = 101 | 2-to-4 |--w1--+-------X-------X---- w1 = 011 A0 ----->| decoder |--w2--X-------X-------+---- w2 = 110 | |--w3--+-------+-------X---- w3 = 001 +---------+ | | | [OR] [OR] [OR] | | | D2 D1 D0 X = connection (bit 1); + = crossing with no connection (bit 0) ``` So D2 = w0 + w2, D1 = w1 + w2 and D0 = w0 + w1 + w3. Each connection is made by a transistor or fuse at the crosspoint. When address A1A0 is applied, the decoder activates one word line and the three OR outputs give the stored word. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Mask-programmed ROM is programmed: A. At the factory using a mask B. By the user with a UV lamp C. In-system electrically D. By blowing fuses once Answer: A. Mask ROM contents are set at manufacture; it is cheapest for large volumes but not erasable. (Key point: Mask ROM) 2. Which ROM type is erased block-wise electrically and used for firmware and USB drives? A. PROM B. EPROM C. Flash D. Mask ROM Answer: C. Flash is in-system, electrically erasable block-wise, dense and cheap. (Key point: Flash vs EEPROM) 3. What does a 4 x 3 ROM need for its internal design? A. 2 address lines, a 2-to-4 decoder and 3 OR gates B. 3 address lines, a 3-to-8 decoder and 4 OR gates C. 4 address lines and 3 flip-flops D. 2 address lines and 4 OR gates Answer: A. 4 words need 2 address bits and a 2-to-4 decoder; 3-bit words need 3 OR gates. (Key point: 4x3 ROM structure) 4. With w0=101, w1=011, w2=110, w3=001, which expression gives D0? A. w0 + w1 + w3 B. w0 + w2 C. w1 + w2 D. w0 + w1 + w2 Answer: A. D0 is 1 in w0 (101), w1 (011) and w3 (001), so D0 = w0 + w1 + w3. (Key point: OR gate connections) ## Related in Embedded System - [Design ROM for A,B,C to F1-F4 table](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Arom-design-abc-f1-f4) — Embedded System, chapter 4, asked 2× - [Design ROM for X,Y,Z to F1-F4 table](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Arom-design-xyz-f1-f4) — Embedded System, chapter 4, asked 2× - [Design ROM for X,Y,Z to F1,F2 table](https://www.bctengineer.com/question.md?key=semester_6%2Fembedded_system%3Arom-design-xyz-f1-f2) — Embedded System, chapter 4 ## Related in other subjects - [Interface two 8K x 8 ROM at 4000H](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Adecoder-two-8k-rom-4000h) — Microprocessor, chapter 4 - [Risk planning; internal, external; risk reduction](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Arisk-reduction-internal-external-planning) — Project Engineering, chapter 5 - [Internal (hot spot) temperature of core](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ahot-spot-core-temperature) — Electric Machine Design, chapter 2, asked 6× - [Internal temperature of a homogeneous material](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainternal-temperature-homogeneous-material) — Electric Machine Design, chapter 2