ROM stores a table intended to be read during normal operation. Mask ROM is fixed during manufacture; PROM is programmable once. EPROM can be erased using ultraviolet light in suitable packages, while EEPROM is electrically erasable. Flash is a related nonvolatile technology with erase organization that commonly uses blocks.
Worked example
A controller may read its startup constants from nonvolatile memory after power returns. Unlike volatile working storage, those programmed values remain without continuous power. Updating an EEPROM-based setting uses a supported write operation rather than merely changing the read address.
What to watch for
Nonvolatile memories have device-specific write timing, endurance and erase rules. The word ROM in a system description does not prove every underlying device is permanently unchangeable.
Try it yourself
Which listed type permits electrical erasure rather than ultraviolet erasure?
Answer: EEPROM. Its allowed update procedure and endurance must still be taken from the particular device's specification.
How to study this topic
Separate the ideal logical model from the physical device interface. Write units beside capacities, voltages and timing values. The architecture explains the idea; the chosen part's documentation supplies the operating limits.
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Digital Electronics