Mask ROM is a type of read-only memory, or ROM, that is masked off during production. "Mask" refers to parts of an integrated circuit, a thin electronic circuit for processing data, that is covered with opaque plates called photomasks. These plates contain transparencies, or holes, to allow light in certain areas while blocking light in others to create distinctive patterns. Mask work, comprising a process known as photolithography, primarily serves as a sort of intellectual property protection, reserving reproduction of the product to the integrated circuit manufacturer.
Like other types of ROM, mask ROM cannot enable the user to change the data stored in it. If it can, the process would be difficult or slow. Thus, ROM is primarily distributed as firmwire—software, or coded instructions—permanently stored in ROM. Firmwire is particularly suitable for field programmability, which involves programming into the electronic device without taking it apart. More specifically, this type of ROM is subcategorized as semiconductor memory. This term is used for data storage that is placed on a semiconductor-based integrated circuit, or a chip built on a piece of material that possesses electrical conductivity.
Some users rely on mask ROM for storing the finalized code of their projects. When projects are being developed, data can be stored using non-volatile memory. This is a type of computer memory that can store data even with the lack of power or a power source. Examples of non-volatile memory include erasable programmable read-only memory (EPROM) and electrically erasable programmable read-only memory (EEPROM). Some microcontrollers, or the tiny computer on the integrated circuit responsible for controlling the electronic device on which it is installed, have non-volatile memory for development purposes and mask ROM for finalized code.
There are some semiconductor or electronics manufacturers that make chips containing only this type of ROM. A prime example is Samsung Electronics, the premier subsidiary of South Korean conglomerate the Samsung Group. Other such manufacturers include Japan-based technology companies NEC Corporation and Oki Electric Industry Co., Ltd.
The main advantage of mask ROM is that it is inexpensive compared with other types of semiconductor memory. This is because integrated circuit costs are tied to how big or small the component is. This type of ROM is generally smaller than other types of semiconductor memory, which consequently makes it cheaper.
The ironic downside of mask ROM concerns the mask work involved in manufacturing it. Not only does it take a considerable amount of time to complete this phase, but there is always the possibility of a data or code error. If that occurs, the component becomes completely useless.