ROM Types

ROM, PROM, EPROM, EEPROM—non-volatile memory technologies and their evolution.

Read-Only Memory (ROM) retains data without power, making it essential for storing firmware, boot code, and configuration data. ROM technology evolved from mask-programmed chips to electrically erasable devices, each generation offering more flexibility. Understanding ROM types provides context for modern flash memory and firmware storage.

Objectives

  • Explain the basic ROM structure (address decoder + data matrix)
  • Compare mask ROM, PROM, EPROM, and EEPROM
  • Understand how each type is programmed and erased
  • Identify applications for each ROM type
  • Describe the evolution from ROM to flash memory

Key Takeaways

  • ROM is non-volatile: stores data without power
  • Mask ROM: factory-programmed, cheapest at volume
  • PROM: one-time programmable (fuse-based)
  • EPROM: UV-erasable, reprogrammable (floating gate)
  • EEPROM: electrically erasable, byte-level, ~100K cycles

Applications

  • BIOS/UEFI: Computer boot firmware stored in flash/EEPROM.
  • Microcontroller Firmware: Program code stored in on-chip flash (evolved from ROM).
  • Configuration Storage: EEPROM stores calibration data and user settings.
  • Lookup Tables: Character generators, math tables, and decoder logic.

Practice Problems

Problem 1: A ROM has 10 address lines and 8 data lines. How many bits does it store?

Problem 2: Why does an EPROM chip have a small quartz window on top?

Problem 3: A microcontroller needs firmware storage that can be updated in the field. Which ROM type is best?

Problem 4: Why can't PROM be reprogrammed?