The

booting In computing, booting is the process of starting a computer as initiated via hardware such as a button or by a software command. After it is switched on, a computer's central processing unit (CPU) has no software in its main memory, so som ...

process of

Android Android may refer to: Science and technology * Android (robot), a humanoid robot or synthetic organism designed to imitate a human * Android (operating system), Google's mobile operating system ** Bugdroid, a Google mascot sometimes referred to ...

devices starts at the power-on of the SoC (

system on a chip A system on a chip or system-on-chip (SoC ; pl. ''SoCs'' ) is an integrated circuit that integrates most or all components of a computer or other electronic system. These components almost always include a central processing unit (CPU), memory ...

) and ends at the visibility of the home screen, or special modes like recovery and fastboot. The boot process of devices that run Android is influenced by the firmware design of the SoC manufacturers.

Background

As of 2018, 90% of the SoCs of the Android market are supplied by either

Qualcomm Qualcomm () is an American multinational corporation headquartered in San Diego, California, and incorporated in Delaware. It creates semiconductors, software, and services related to wireless technology. It owns patents critical to the 5G, 4 ...

, Samsung or MediaTek. Other vendors include Rockchip, Marvell, Nvidia and previously Texas Instruments.

History

Verified boot was introduced with Android KitKat.

Stages

Primary Bootloader

The Primary Bootloader (PBL), which is stored in the Boot ROM is the first stage of the boot process. This code is written by the chipset manufacturer. The PBL verifies the authenticly of the next stage. On Samsung smartphones, the Samsung Secure Boot Key (SSBK) is used by the boot ROM to verify the next stages. On SoCs from Qualcomm, it is possible to enter the Qualcomm Emergency Download Mode from the primary bootloader. If the verification of the secondary bootloader fails, it will enter EDL.

Secondary Bootloader

Because the space in the boot ROM is limited, a secondary bootloader on the eMMC or

eUFS Universal Flash Storage (UFS) is a flash storage specification for digital cameras, mobile phones and consumer electronic devices. It was designed to bring higher data transfer speed and increased reliability to flash memory storage, while reduc ...

is used. The secondary bootloader initializes TrustZone. On the Qualcomm MSM8960 for example, the Secondary Bootloader 1 loads the Secondary Bootloader 2. The Secondary Bootloader 2 loads TrustZone and the Secondary Bootloader 3. The SBL is now called XBL by Qualcomm and uses UEFI to be cross compatible for booting operating systems other than Android in the second stage.

Aboot

Qualcomm uses Little Kernel, MediaTek uses Das U-Boot. Little Kernel is a microkernel for embedded devices, which has been modified by Qualcomm to use it as an Android bootloader. The Android Bootloader (Aboot), which implements the fastboot interface (which is absent in Samsung devices). Aboot verifies the authenticity of the boot and recovery partitions. By pressing a specific key combination, devices can also boot in recovery mode. Aboot then transfers control to the Linux kernel.

Kernel and initramfs

The initramfs is a gzip'ed cpio archive that contains a small root file system. It contains init, which is executed. The Android kernel is a modified version of the Linux kernel. Init does mount the partitions. dm-verity verifies the integrity of the partitions that are specified in the fstab file. dm-verity is a Linux kernel module that was introduced by Google in Android since version 4.4. The stock implementation only supports block based verification, but Samsung has added support for files.

Zygote

Zygote is spawned by the init process, which is responsible for starting Android applications and service processes. It loads and initializes classes that are supposed to be used very often into the heap. For example, dex data structures of libraries. After Zygote has started, it listens for commands on a socket. When a new applications starts, a command is sent to Zygote which executes a fork() system call.

Partition layout

The Android system is divided across different partitions. The Qualcomm platform makes use of the GUID partition table. Although this specification is part of the UEFI specification, it does not depend on UEFI.

Explanatory notes

References

External links

Android.com - Boot Flow

Managing Boot Time

Qualcomm Bootloaders

Qualcomm's Chain of Trust

Secure Boot and Image Authentication

Secure boot on Snapdragon 410

msm8916-mainline/qhypstub

Android system init process startup and init.rc full analysisAndroid Init Language
{{Firmware and booting Android (operating system) Booting processes Boot loaders