This initial program, or bootstrap program, tends to be simple. Typically, it is stored within the computer hardware in read-only memory (ROM) or electrically erasable programmable read-only memory (EEPROM), known by the general term firmware. It initializes all aspects of the system, from CPU registers to device controllers to memory contents.
When you get a spare time, scrutinise this file: https://github.com/torvalds/linux/blob/master/init/main.c#L514
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