What is ROM ? List out the different types of ROM. [2+3]

ROM (Read-Only Memory) is a type of non-volatile memory used in computers and other electronic devices. Non-volatile memory retains its stored data even when the power is turned off. ROM is primarily used to store firmware or software that is rarely changed, such as the BIOS in a computer. The data in ROM is pre-written during manufacturing and cannot be modified by normal programming operations. It is designed to be read-only, ensuring data integrity and protection.

Types of ROM

ROM can be classified into different types based on their programmability and erasability:

1. Mask ROM (MROM)

   • Data is written during the manufacturing process using a photomask.
   • Cannot be altered or reprogrammed after manufacturing.
   • Used in applications where the program is fixed and does not require updates (e.g., BIOS in older computers).

2. Programmable ROM (PROM)

   • Blank ROM chips that can be programmed once by the user using a special device called a PROM programmer.
   • Once programmed, the data cannot be erased or rewritten.
   • Used in applications where a one-time programming is sufficient.

3. Erasable Programmable ROM (EPROM)

   • Can be programmed multiple times.
   • Data can be erased by exposing the chip to ultraviolet (UV) light for a specific duration (typically 15-20 minutes).
   • After erasure, it can be reprogrammed using a PROM programmer.
   • Identified by a transparent quartz window on the chip that allows UV light to reach the internal circuitry.
   • Used in development and prototyping stages.

4. Electrically Erasable Programmable ROM (EEPROM)

   • Can be programmed and erased electrically, byte-by-byte or page-by-page.
   • No need for UV light; erasure and reprogramming can be done in-circuit.
   • Slower write/erase times compared to Flash memory but more flexible for small-scale updates.
   • Used in applications requiring infrequent updates, such as storing configuration data.

5. Flash Memory

   • A type of EEPROM that allows data to be erased and rewritten in blocks rather than byte-by-byte.
   • Faster erase/write times and higher density compared to EEPROM.
   • Used in USB drives, SSDs, and modern BIOS systems.
   • Can be further divided into:
     • NOR Flash: Used for executing code (e.g., firmware in embedded systems).
     • NAND Flash: Used for mass storage (e.g., SSDs, memory cards).

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