At the beginning of SoC init, once the CPU has initialized itself, the internal SRAM controller pushes a jump instruction to the address of BootROM, which is a small program that will initialize the flash storage and do some basic UART setup. The UART interface is still not ready yet. BootROM will check storage and try to load a working Preloader from the flash storage. BootROM may also implement security to prevent unauthorized alteration to the storage.