Mask ROM

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Mask ROM (MROM) refers to a kind of ROM (read-only memory) whose contents are programmed by the Integrated Circuit manufacturer (rather than by the user). The terminology "mask" comes from Integrated Circuit fabrication, where regions of the chip are masked off before during the process of lithography.

It is common practice to use rewritable UV-EPROM or EEPROM for the development phase of a project, and to switch to mask ROM when the code has been finalized. For example, Atmel microcontrollers come in both EEPROM and Mask ROM formats.

The main advantage of mask ROM is its cost, since it is significantly cheaper than other kinds of ROM. It is also very compact. However, the one-time masking cost is high and there is a long turn-around time from design to product phase. Design errors are costly: if an error in the data or code is found, the mask ROM is useless and must be replaced in order to change the code or data.