Mask ROM
Mask ROM is the type of ROM that has to be custom built by the factory. Below are the key points explaining why Mask ROM is custom-built by the factory:

Factory Programming
- Mask ROM is manufactured with the data already programmed into the memory during the fabrication process.
- This data is permanently encoded into the ROM during the manufacturing process and cannot be altered or reprogrammed by the end-user.

Custom Design
- The data programmed into a Mask ROM is customized based on the specific requirements of the device or system it is intended for.
- This customization ensures that the ROM contains the necessary instructions or data for the intended application.

Specialized Production
- Mask ROMs are produced in large quantities for mass-produced devices or systems.
- The custom-built nature of Mask ROMs allows for efficient and cost-effective production of ROMs tailored to the specific needs of the end product.

One-Time Programming
- Once the data is programmed into a Mask ROM, it cannot be changed or erased.
- This makes Mask ROMs ideal for applications where the data does not need to be updated or modified after manufacturing.
In conclusion, Mask ROM is custom built by the factory to meet the specific requirements of the intended application, with the data programmed during the manufacturing process and permanently encoded into the ROM.