3. Boot

The general boot chain and boot-time assets that don’t belong to a single loader stage: verified boot and image signing, the boot image (kernel/DTB/ ramdisk) and its recovery-image twin, the SoC register-level view of the hand-off below UEFI, the base device tree, USB/QUP transport, and the device-tree-overlay resources applied at boot. The two named loader stages have their own sections: XBL (PBL→XBL, XBL→ABL hand-off) and ABL (the application bootloader itself, fastboot, AVB). The boot splash shown during this whole sequence is documented next to the rest of the display pipeline, in Splash partition.

• 3.1. Boot chain and exception-level hand-off (PBL → XBL → ABL)
• 3.2. Secure boot and image signing
• 3.3. Boot image: kernel, DTB, ramdisk, config
• 3.4. Recovery ramdisk (recovery environment)
• 3.5. Physical keys: power, volume, and the boot-mode dispatcher
• 3.6. Below UEFI — execution state and hardware register surface
• 3.7. Base kernel device tree (the SoC boot contract)
• 3.8. USB transport — the DWC3 controller behind every off-device path
• 3.9. QUP/GENI serial-engine bus map (what is wired where)
• 3.10. DTBO overlays