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

There are a few different technologies. One for example is where the data is physically etched into the IC during manufacture, computer chips are made by a process called photolithography and it is possible to just engineer the logic in the chips from the ground up to be memory with certain data. We can actually further this process and make it possible for it to have “multiple passes,” where you use one photolithography process to make the blank chip itself (perhaps using a more expensive process thats only largely possible in a high end semiconductor lab), then pass that same chip through another process of photolithography, this pass actually writing the data itself.

Later on we could get more fancy with it, we would get one time writable ROMs. Usually the way this worked was you had a chip/storage engineered with all 0s initially in memory and a mechanism to flip a 0 to a 1. You however couldn’t flip it back, at least not easily.

For example there is fuse technology, where the chips had parts where if a high enough voltage was applied to these parts, it would “burn” new paths for electricity to go through, switching a chip from a 0 to a 1. Naturally damaging the chip this way means you can’t go back to writing a 0 to it.

Another more modern example is the classic CD. You could use your CD writer to physically burn holes in the CD surface, but usually once you burned those in you couldn’t “unburn them.”

Most more modern ROMs are technically rewritable but this requires jumping through hoops, and usually the actual thing the ROM is running on isn’t able to do it. Same thing as before usually as well, you could only write 1s to the chip, however now we have techniques of wiping the whole chip down to 0s.

For example, we have the EPROM requires you to shine UV light on the chip itself to erase the chip. Naturally this can’t be done on the fly. We also have EEPROMs, which are electronically erasable, but similarly require you to wipe the whole chip before writing to it again.