Is somehow locked physically during production, or through software?
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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.
Consider a regular book, that book is read-only. This doesn’t mean that the paper can’t be printed on, it just means that once it’s printed, you can’t change it.
The idea that you can “etch” data onto a chip is fascinating!
Consider a regular book, that book is read-only. This doesn’t mean that the paper can’t be printed on, it just means that once it’s printed, you can’t change it.
ROM can be hardwired when it’s made, or it can be programmable (PROM).
The simplest form of PROM to think about is a bunch of tiny fuses that can be accessed by an addressing system, if you put a normal current though, they pass it normally, but if you push too much through, the fuses blow and now when you try to put the normal current though, it doesn’t work. This lets you program the ROM after it has been made.
The circuit the PROM is connected to needs to be able to generate that higher current to blow the fuses. You could use such a circuit to blow more of the fuses if you wanted but in this basic kind of PROM you can’t unblow the fuses though.
EPROMs are erasable proms that have something more like a circuit breaker instead of a fuse. Different EPROMs have different ways of resetting the breakers. Some have a little window you can shine light into, others accept an electrical signal to reset their breakers and these are called “electronically erasable programmable read only memory” or “EEPROM”
Some EEPROM is arranged in blocks that can be erased separately from one another. This is what ‘Flash Memory’ is. With some additional cleverness wrapped around it to read the block before erasing then writing it back with modifications, you can treat it as writeable storage like a hard drive.
In the case of cartridges, the device reading the cartridge probably can’t generate the current needed to blow more cartridges, or direct specific patterns of into it. The ROM inside it was programmed with special hardware either before being soldered into the cartridge or after.
Same way if you get a piece of burnt toast, you can’t re-toast it. The storage medium is altered in the original write, and remains in its final state.
Same way if you get a piece of burnt toast, you can’t re-toast it. The storage medium is altered in the original write, and remains in its final state.
ROM can be hardwired when it’s made, or it can be programmable (PROM).
The simplest form of PROM to think about is a bunch of tiny fuses that can be accessed by an addressing system, if you put a normal current though, they pass it normally, but if you push too much through, the fuses blow and now when you try to put the normal current though, it doesn’t work. This lets you program the ROM after it has been made.
The circuit the PROM is connected to needs to be able to generate that higher current to blow the fuses. You could use such a circuit to blow more of the fuses if you wanted but in this basic kind of PROM you can’t unblow the fuses though.
EPROMs are erasable proms that have something more like a circuit breaker instead of a fuse. Different EPROMs have different ways of resetting the breakers. Some have a little window you can shine light into, others accept an electrical signal to reset their breakers and these are called “electronically erasable programmable read only memory” or “EEPROM”
Some EEPROM is arranged in blocks that can be erased separately from one another. This is what ‘Flash Memory’ is. With some additional cleverness wrapped around it to read the block before erasing then writing it back with modifications, you can treat it as writeable storage like a hard drive.
In the case of cartridges, the device reading the cartridge probably can’t generate the current needed to blow more cartridges, or direct specific patterns of into it. The ROM inside it was programmed with special hardware either before being soldered into the cartridge or after.
Imagine a memory chip is like a chalkboard.
A read-and-write chip is a blank board. You can write on it with chalk as you want and erase and add as you please.
A read-only chip is akin to the board manufacturer carving words into the board when its made. These are now permanent and cant be altered by you. Theres no space for you to write with chalk, all of it is made of pre-carved information.
This was typically done in the factory by etching this data with a process called lithography onto the chip instead of having writable memory cells. Over time many different ways of making irreversible (or nearly so) data sets have been developed.
Imagine a memory chip is like a chalkboard.
A read-and-write chip is a blank board. You can write on it with chalk as you want and erase and add as you please.
A read-only chip is akin to the board manufacturer carving words into the board when its made. These are now permanent and cant be altered by you. Theres no space for you to write with chalk, all of it is made of pre-carved information.
This was typically done in the factory by etching this data with a process called lithography onto the chip instead of having writable memory cells. Over time many different ways of making irreversible (or nearly so) data sets have been developed.
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