Is somehow locked physically during production, or through software?
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Mask ROM as the name implies uses lithography to “print” the wire connections onto the memory region of the chip, since data’s are all 1s and zeros or connected, not connected.
Once the data has been printed on, the silicon wafer goes into the package, the black thing with metal legs, and no further modifications can be made.
Otherwise a cartridge could use EEPROM, a form of NOR flash, which can be written electronically but the “writing function” is just disabled once the chip goes in the cartridge.
Mask ROM as the name implies uses lithography to “print” the wire connections onto the memory region of the chip, since data’s are all 1s and zeros or connected, not connected.
Once the data has been printed on, the silicon wafer goes into the package, the black thing with metal legs, and no further modifications can be made.
Otherwise a cartridge could use EEPROM, a form of NOR flash, which can be written electronically but the “writing function” is just disabled once the chip goes in the cartridge.
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.
With ROM the data is basically burned into a chip. It can’t be rewritten because the process of getting the data onto it physically alters it. It would be like trying to re-carve a stone tablet.
The best explanation I have is that your question is wrong. It’s impossible to REWRITE a piece of ROM. When it gets made, the information is built into it, for instance an integrated circuit (in which they build the part and lay the circuitry in it as it’s being made, the circuit functioning as the data). It’s written in the process of manufacturing. If you wanted to rewrite it as the end user, you’d need to somehow rip the whole thing apart and put new circuits in, IE destroying the part and technically just recycling the raw materials through the same manufacturing process.
I used a lot of incorrect terminology, sorry, but if I was more technical it would stop being an ELI5. If you want more extensive information, you want to look at how ROM is manufactured because there are a lot of different storage methods, just like editable memory.
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.
The best explanation I have is that your question is wrong. It’s impossible to REWRITE a piece of ROM. When it gets made, the information is built into it, for instance an integrated circuit (in which they build the part and lay the circuitry in it as it’s being made, the circuit functioning as the data). It’s written in the process of manufacturing. If you wanted to rewrite it as the end user, you’d need to somehow rip the whole thing apart and put new circuits in, IE destroying the part and technically just recycling the raw materials through the same manufacturing process.
I used a lot of incorrect terminology, sorry, but if I was more technical it would stop being an ELI5. If you want more extensive information, you want to look at how ROM is manufactured because there are a lot of different storage methods, just like editable memory.
The idea that you can “etch” data onto a chip is fascinating!
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.
With ROM the data is basically burned into a chip. It can’t be rewritten because the process of getting the data onto it physically alters it. It would be like trying to re-carve a stone tablet.
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