How to understand rom?

read-only-memory[ROM] is a type of computer memory unlike RAM, it keeps contents even when computer or device is turned off.

Usually, ROM cannot be written to when the computer runs normally . ROM is used for BIOS which tells the computer how to start.or important programs like the firmware of certain devices.

there are 3 different ROM's : 1. masked ROM: this kind is written once,at the factory,it can't be changed at all later on . its main benefits is that it is really cheap to make.