Mask ROM

Also called: mask-programmed ROM, MROM, factory-programmed ROM

A ROM whose contents are built into the chip during manufacture by the pattern of its wiring. It can never be changed afterward.

A mask ROM gets its contents at the factory. The 1s and 0s are set by the photographic masks used to make the chip: a stored 1 is a connection that exists, a 0 is one that doesn't. Once made, the contents are fixed forever.

It's the original, simplest member of the ROM family, and it's non-volatile in the strongest sense: there's nothing to leak or wear out.

Trade-offs:

Cheapest per chip in huge volumes, since there's no programming step.
Expensive to set up: a custom mask must be made for each set of contents.
No fixes: a bug in the stored data means making new chips.

So mask ROMs suit products made by the million whose contents never change, such as character tables or the firmware of a simple toy. Everything else moved to programmable parts: PROM, EPROM, EEPROM and flash.

Worked example

Example

Choosing a ROM type

A company will make 5 million identical greeting cards that play a tune, and the tune will never change.

1.
The contents are fixed and the volume is huge.
2.
A mask ROM avoids any per-chip programming step, so it's the cheapest choice per card.
3.
If the tune might need fixing after release, a rewritable part such as flash would be safer.

Common mistakes

Thinking a user can program a mask ROM. Its contents are fixed when it is manufactured.
Confusing mask ROM with PROM. A PROM leaves the factory blank and is programmed once by the user.

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