ROM

• Read only memory
• Non-volatile memory
• In computer engineering, “Read-Only” does not mean a chip can never be altered
• It means it cannot be modified during normal, high-speed CPU operation cycles
• Writes require special program/erase commands via a flash-interface controller
• why CPU cycles cannot write flash?
  • flash write needs special erase/program command sequences and timing, not a simple direct CPU store

MROM

• Mask ROM
• Data hardwired during manufacturing
• Not relevant today
  • reason: fixed at fabrication, no field updates
• used in NES cartridges

PROM

• Programmable ROM
• Can only be programmed once
• Not relevant today
  • reason: one-time programmable, cannot patch after deployment
• was used in RFID tags, electronic car keys
• replaced by flash memory

EPROM

• Erasable Programmable ROM
• Data can be erased by exposing chip to UV rays
• Not relevant today
  • reason: erase process is slow and inconvenient, often needs chip removal
• older BIOS chips in 1900s

EEPROM

• Electrically Erasable Programmable ROM
• used in Real-time clocks, microcontrollers
• Highly relevant

Flash memory

• aka Advanced EEPROM
• a block-erasable form of EEPROM
• architecture evolved from EEPROM, but with erase/write operations optimized in blocks
• motherboard BIOS/UEFI firmware is usually stored on flash (often NOR flash)
  • commonly called the “ROM chip”
• Storage
  • SSDs
  • USB Drives
  • Smartphone internal storage
• Highly relevant

Feature	RAM (Volatile Memory)	EEPROM / Flash (Non-Volatile)
CPU Access	Reads/writes via CPU DRAM memory controller at bus speed	Reads directly; writes use flash-controller command sequences (example: SPI/QSPI), not DRAM load/store cycles
Write Method	Single-step electrical pulse	Complex multi-step process (Erase entire block —> Write new data)
Frequency	Millions of writes per second	Limited writes over its entire lifespan (wear-and-tear)