If it’s physically impossible to write to a ROM (read-only memory) cartridge, how do manufacturers do it?

Question

Is somehow locked physically during production, or through software?

In: 210

Answer 1

Integrated circuits are made by projecting light onto the chip with a mask.

A photoresist and then etching or depositing of material can create the projected pattern on the chip it can for example be a layer of a metallic conductor.

Chips are made with multiple layers that build up the structure.

With ROM chips you make a custom mask for one conductive layer that interconnect part so the chip output the information you like to store.

So the information is part of the physical structure of the chip. The drawback it there is a high startup cost to make the mask but each chip you make then has a low cost. This is how old game cartridges were made

That is for “real” ROM. There is field programmable ROM too.

PROM just means you can write once and it can be a high voltage that destroys or create internal links with fuses or anti-fuses. So if an empty chip is all zero you can with a special machine wire 1 but never change it back to a zero.

EPROM (Erasable programmable read-only memory) can be written to and also erased by exposing the chip to ultraviolet light. The chips are behind a window so you can do that. They are not common today.

EEPROM (Electrically erasable programmable read-only memory) can be written to but also be erased. You can change the bit in one direction but the other direction requires a large block to be clear and it longer time then to read or write to it.

FLASH memory is really EEPROM. You can build chips that store data in FLASH so you can inlay change it like you what to but there is also a special command you can use that locks the memory. It is a one-way operation that makes writing or clearing the memory impossible.

There is not a large initial startup cost but each chip cost more. That is a huge advantage if you have one of just thousands of products, It might not be the best if you make millions.

So you can store the data with the physical structure of the chip or it can chip you could write data to but they set the chip in a stat where not more writing is possible.