Boot Sequence

1. ROM Code

• Runs directly after power up
• Provided by chip manufacturer
• Can usually only access SRAM (no DRAM)
• Capable of loading code (of limited size) from several predefined locations into SRAM:
  • NAND flash
  • SPI
  • MMC (eMMC or SD card)
  • from file called MLO located on first partition of MMC device
  • Ethernet, USB, UART
  • …

2. Secondary Program Loader (SPL)

• Used if bootloader too big to be loaded by ROM code
• Setup memory controller to be able to load Tertiary Program Loader (TPL)
• Other peripherals might need setup at that stage
• Might have support to read files from filesystems (e.g. u-boot.img)
• Often proprietary binary blobs provided by manufacturer

3. Tertiary Program Loader (TPL)

• Full bootloader (i.e. U-Boot)
• Usually has simple user interface (command-line)
• Used to load kernel into DRAM
• TPL is often removed from memory after kernel boot

4. Loading Kernel

Bootloader passes basic information to kernel:

• Machine number: on PPC and Arm without device tree support
• Hardware information detected so far:
  • location and size of RAM
  • CPU clock speed
• Kernel command line
• Location and size of device tree binary (if supported)
• Location and size of initramfs (optional)