Android Boot Process

Power On

The Android boot process begins when the user presses the power buttonto turn on the device.

The power management unit (PMU) initializes, supplying power to the device's hardware components.

Bootloader

The bootloader is a piece of software that runs before the Android operating system starts up.

It is stored in a dedicated, read-only memory section.

It conducts basic hardware checks.

Then bootloader proceeds to load the kernel into memory (System OS).

Kernel

• Initializes:

  • Process scheduler

  • Memory management

  • Drivers (display, camera, USB, etc.)

• Mounts root filesystem (ramdisk)

Init Process

• Responsibilities:

  • Mount partitions: /system, /vendor, /data

  • Parse .rc scripts

• Set up SELinux policies

• Initialize the start Property Service (Initializes the properties within the system)

• Parse the init.rc configuration file and start Zygote process

Zygote

Zygote is a parent process that creates and manages all the application processes.

It starts a Dalvik/ART VM.

It will call runSelectLoop() method, which will wait for the request from the ActivityManagerService.

It creates App Process and System Server by fork mechanism.

fork() uses copy-on-write, meaning new apps share the same memory pages. This makes app startup much faster and lighter.

System Server

System Server is a parent process that creates and manages all the system services.

• It calls three methods:

  • startBootstrapServices()

  • startCoreServices()

  • startOtherServices()

1. Services started by startBootstrapServices() method are:

   • Installer

   • ActivityManagerService

   • PowerManagerService

   • LightsService

   • DisplayManagerService

   • SensorService

   • PackageManagerService

2. Services started by startCoreServices() method are:

   • BatteryService

   • UsageStatsService

   • GpuService

   • WebViewUpdateService

3. Services started by startOtherServices() method are:

   • CameraService

   • AlarmManagerService

   • InputManagerService

   • WindowManagerService

   • BluetoothService

   • NotificationManagerService

   • LocationManagerService

   • AudioService

System UI initialization

It initializes the System UI components.

System UI refers to any element displayed on the screen that is not part of an app.

• Status bar

• Navigation bar

• Recent apps screen

• PowerUI

• VolumeUI

• PipUI (Picture-in-Picture)

• Stack Divider (Split Screen)