- ROM: Read-Only Memory
- Chip functionality had to be designed prior to manufacture
- Chips were 'built' with the program hard-wired inside them
- Each run of chips was a custom manufactured batch
- Chips are application-specific
- Cannot be changed once manufactured
- PROM: Programmable Read-Only Memory
- Single design for the memory chip
- Contents of the chip can be 'flashed in' later.
- Flashing is permanent (Fuse or antifuse)
- EPROM: Eraseable Programmable Read-Only Memory
- Single design for the memory chip
- Flashable like PROM
- Erased via UV light (unlike PROM)
- Becomes unusable after too many flashes
- EEPROM: Electrically Eraseable Programmable Read-Only memory
- Single design for the memory chip
- Contents can be flashed in later
- Contents can be erased and reflashed
- Many more 'flashings' than EPROM
- Doesn't require a UV Erasing device
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