What Is ROM?
ROM (Read Only Memory) is a memory type whose contents are persistent and not erased even when power is cut. Unlike RAM, it keeps data for a long time.
How Does It Work?
The data written to ROM holds the system's basic program (firmware) and unchanging data. Classic ROM is programmed once; it can't be easily changed afterward and is mostly read-only.
It's Non-Volatile Memory
Its most important property is being non-volatile: the content is preserved even when the device is off. So the boot code and persistent settings are stored in ROM/its derivatives.
| Property | ROM | RAM |
|---|---|---|
| Persistence | Persistent | Volatile |
| Writing | Limited/hard | Free |
| Role | Firmware/fixed data | Working area |
Types
Over time writable types were developed: PROM (write once), EEPROM (electrically erasable and writable) and Flash (fast, erasable block by block). All of these are in the "non-volatile memory" family.
Where Is It Used?
Device boot code (BIOS/bootloader), microcontroller firmware, game cartridges and unchanging tables.
Mask ROM and Programmable Types
Classic "mask ROM" content is written permanently to the chip during production; it's very cheap but can't be changed at all and only makes sense in large productions. Programmable types like PROM, EEPROM and Flash, being writable afterward, are preferred in development and updatable products.
Common Mistakes
- Assuming it can never be written: modern ROM derivatives (EEPROM/Flash) are writable.
- Using it like working memory: ROM isn't suited to fast and free writing.
- Write endurance: writable types have a limited erase/write cycle.
Frequently Asked Questions
Is ROM really read-only?+
Is firmware stored in ROM?+
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