1) Power on the Raspberry Pi to start
After the Raspberry Pi is powered on, the first boot program in the SoC is executed first, and its role is to mount the FAT32 partition on the SD card to load the next stage of the bootloader . This part of the program is solidified in SoC ROM and cannot be modified by the user.
4B adds an EEPROM inside the chip to avoid risks (due to the 4B hardware upgrade, the startup process is more complicated, if all the codes are written into ROM, it will increase the complexity and introduce possible BUG), 4B is powered on to run the SPI connection bootloader code in EEPROM.
2) Raspberry Pi executes bootloader
bootcode.binAt this stage, the Raspberry Pi will load the ( ) file on the SD card bootloaderas the second-stage boot program. bootloaderwill start the GPU and load the third-stage launcher start.elf.
Note that it is not used on 4B bootcode.binas it is EEPROM replaced by the boot code onboard.
3) Raspberry Pi boot kernel
start.elf Read the file storing the system configuration config.txt, set the CPU operating parameters and memory allocation according to its content, then load the user code into the memory, and start the CPU;
After the CPU starts, load the kernel to start.
The system will first search for the file specified config.txt in the parameter kernel=xxx as the next object to accept the control of the system. The Raspberry Pi defaults to the file as the linux kernel. We can replace it with a uboot new one bootloader and then start the linux kernel.
If config.txt no kernel parameter is specified, the default is to search for kernel8.img, kernel8-32.img, kernel7.img, kernel.imgrespectively, corresponding to ARMv8-aarch64, ARMv8-aarch32, ARMv7 and previous versions.