Evolution from ROM to modern Flash and emerging technologies
Data permanently encoded during manufacturing via photolithographic masks. True "read-only" — cannot be modified after fabrication.
Erased by UV light through quartz window (15-20 min exposure). The classic chip with the little window on top.
Electrically erasable at byte granularity. Still used in microcontrollers for small config storage (I²C serial EEPROMs).
Random-access reads, execute-in-place (XIP) capable. Used for firmware/BIOS storage. Block-erasable.
Page-based access, optimized for density and sequential ops. Dominant in SSDs, SD cards, USB drives. 3D stacking now common.
Intel/Micron technology using phase-change materials. Byte-addressable with DRAM-like latency. (Discontinued 2022)
Stores bits via magnetic tunnel junctions. SRAM-like speed, unlimited endurance, radiation-hard. Growing in embedded use.
Resistance switching in metal-oxide films. Simple structure, good scaling potential. Used in some MCUs as embedded NVM.