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

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.