Mask ROM? Mask ROM is the type of ROM that requires sending a custom mask to the manufacturer to make. It is the most expensive type, but also has the highest storage density and is highly durable, aspired for the possibility of high-density durable archiving but out of reach simply due to the costs.
20191122/https://en.wikipedia.org/wiki/Mask_ROM
PROM is the much more practical alternative for conventional use, if you need true read-only memory. Writing out a PROM is quite easy, use a 6 V pulse to burn away a gate to convert the default “1” to a “0”. Not much different than programming EPROM at 12 V.
20191122/https://en.wikipedia.org/wiki/Programmable_read-only_memory
20191122/https://en.wikipedia.org/wiki/EPROM
Note that some EPROM chips can run off of a lower voltage when programming. See my old blog post for an example of such a chip.