Is somehow locked physically during production, or through software?
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It used to be very common to use lasers on a microchip before it was packaged. You would test the microchip and based on the test results, pass coordinates to a laser telling it where to burn. This gave you the opportunity to fix bad chips or program customer specific information.
This is still done at a lot of companies that use older manufacturing processes.
The switch to EEPROMs and similar methodologies has happened in the last 20 years.
It used to be very common to use lasers on a microchip before it was packaged. You would test the microchip and based on the test results, pass coordinates to a laser telling it where to burn. This gave you the opportunity to fix bad chips or program customer specific information.
This is still done at a lot of companies that use older manufacturing processes.
The switch to EEPROMs and similar methodologies has happened in the last 20 years.
They are generally not able to be written to by the device that they’re meant to be in. This doesn’t mean they can never be written to ever. It means that the circuitry to read from them is different from what is needed to write it or that it can be written to once. Write once can literally have severing circuits that fuse off and can never be used again after writing.
They are generally not able to be written to by the device that they’re meant to be in. This doesn’t mean they can never be written to ever. It means that the circuitry to read from them is different from what is needed to write it or that it can be written to once. Write once can literally have severing circuits that fuse off and can never be used again after writing.
In ancient times, writing was done on soft clay. When done and to preseve it, the clay is heated until it hardens. That’s your ROM. It was totally possible to write to it initially, but now you cannot simply delete, add or change content. All you could do is destroy it and make a new one.
In ancient times, writing was done on soft clay. When done and to preseve it, the clay is heated until it hardens. That’s your ROM. It was totally possible to write to it initially, but now you cannot simply delete, add or change content. All you could do is destroy it and make a new one.
Have you ever heard of vinyl records?
Have you ever heard of vinyl records?
ROM usually *can* be written to, but only once. The way ROM works is you essentially have a series of wires that pass through fuses and connect to the 5V rail, so basically each wire is already set to a logic 1 by default. You “write” to ROM by passing enough current through the wires to blow some of the fuses. The wires with blown fuses then become logic 0.
This is a massive simplification of the process, and different types of ROM do work in different ways, but this is the easiest one to ELI5. Some types of ROM actually can be “reset” and the programmed again, but it’s usually difficult to do. And sometimes the “ROM” is actually just a conventional flash memory chip that was programmed before being soldered to the board, and simply has the “write mode” pin hardwired to 0.
ROM usually *can* be written to, but only once. The way ROM works is you essentially have a series of wires that pass through fuses and connect to the 5V rail, so basically each wire is already set to a logic 1 by default. You “write” to ROM by passing enough current through the wires to blow some of the fuses. The wires with blown fuses then become logic 0.
This is a massive simplification of the process, and different types of ROM do work in different ways, but this is the easiest one to ELI5. Some types of ROM actually can be “reset” and the programmed again, but it’s usually difficult to do. And sometimes the “ROM” is actually just a conventional flash memory chip that was programmed before being soldered to the board, and simply has the “write mode” pin hardwired to 0.
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