Android Boot Sequence

In this post we will discuss about the boot process of Android operating system. As the Android is based on Linux Kernel , so its boot process will give good idea about Linux boot process also. Most android based systems run on ARM processors.

First of all we will see the meaning of various terms that is used in the android boot sequence.

• BootROM- It is a hardwired code resided in CPU ASICS.
• Bootloader- It is a small program which runs before the Android OS starts running.
• Init process- It is the first process launched by Linux Kernel when it is finished with the setup.

Let's see what happens when a user presses power ON button--

Complete path the boot process takes when we press the power button on our android phones.

Step 1-System start-- Boot ROM code start execution from pre defined location. It load Bootloader into RAM and start execution.

• A. The Boot ROM code will detect the boot media using a system register that maps to some physical balls on the ASIC. This is to determine where to find the first stage of the boot loader.
• B. Once the boot media sequence is established the boot ROM will try to load the first stage boot loader to internal RAM. Once the boot loader is in place the boot ROM code will perform a jump and execution continues in the boot loader.

Step 2 : Bootloader--

• Bootloader perform execution in two stages, first stage It to detect external RAM and load program which helps in second stage, In second stage bootloader setup network, memory, etc. which requires to run kernel, bootloader is able to provide configuration parameters or inputs to the kernel for specific purpose.  
• Android bootloader can be found at 

<AndroidSource>\bootable\bootloader\legacy\usbloader

• Legacy loader contain two important files viz. init.S and main.c 1. init.s - Initializes stacks, zeros the BSS segments, call _main() in main.c
  2. main.c - Initializes hardware (clocks, board, keypad, console), creates Linux tags.
• In embedded Linux uBoot is often the boot loader of choice. Device manufacturers often use their own proprietary boot loaders.

• A. The first boot loader stage will detect and set up external RAM.
• B. Once external RAM is available and the system is ready the to run something more significant the first stage will load the main boot loader and place it in external RAM.
• C. The second stage of the boot loader is the first major program that will run. This may contain code to set up file systems, additional memory, network support and other things. On a mobile phone it may also be responsible for loading code for the modem CPU and setting up low level memory protections and security options.
• D. Once the boot loader is done with any special tasks it will look for a Linux kernel to boot. It will load this from the boot media (or some other source depending on system configuration) and place it in the RAM. It will also place some boot parameters in memory for the kernel to read when it starts up.
• E. Once the boot loader is done it will perform a jump to the Linux kernel, usually some decompression routine, and the kernel assumes system responsibility.

3. The Linux kernel

The Linux kernel starts up in a similar way on Android as on other systems. It will set up everything that is needed for the system to run. Initialize interrupt controllers, set up memory protections, caches and scheduling.

• A. Once the memory management units and caches have been initialized the system will be able to use virtual memory and launch user space processes.
• B. The kernel will look in the root file system for the init process (found under system/core/init in the Android open source tree) and launch it as the initial user space process.

4. The init process

init it very first process, we can say it is root process or grandmother of all processes. init process has two responsibilities 1. mount directories like /sys, /dev, /proc and 2. run init.rc script.

• init process can be found at init : <android source>/system/core/init
• init.rc file can be found in source tree at <android source>/system/core/rootdir/init.rc
•  This is a script that describes the system services, file system and other parameters that need to be set up. The init.rc script is placed in system/core/rootdir in the Android open source project.
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5. Zygote and Dalvik -

• In a Java, We know that separate Virtual Machine(VMs) instance will popup in memory for separate per app, In case of Android app should launch as quick as possible, If Android os launch different instance of Dalvik VM for every app then it consume lots of memory and time. so, to overcome this problem Android OS as system named “Zygote”. 
• Zygote enable shared code across Dalvik VM, lower memory footprint and minimal startup time. 
• Zygote is a VM process that starts at system boot time as we know in previous step.
• Zygote preloads and initialize core library classes.  
• Normally there core classes are read-only and part of Android SDK or Core frameworks. In Java VM each instance has it’s own copy of core library class files and heap objects.
• In short, The Zygote is launched by the init process and will basically just start executing and and initialize the Dalvik VM.

6. The system server-

• The system server is the first java component to run in the system. It will start all the Android services such as telephony manager and bluetooth.
•  Start up of each service is currently written directly into the run method of the system server. 
• The system server source can be found in the file frameworks/base/services/java/com/android/server/SystemServer.java in the open source project

Core Services:

1.     Starting Power Manager

2.     Creating Activity Manager

3.     Starting Telephony Registry

4.     Starting Package Manager

5.     Set Activity Manager Service as System Process

6.     Starting Context Manager

7.     Starting System Context Providers

8.     Starting Battery Service

9.     Starting Alarm Manager

10.   Starting Sensor Service

11.   Starting Window Manager

12.   Starting Bluetooth Service

13.   Starting Mount Service

Other services

1.    Starting Status Bar Service

2.     Starting Hardware Service

3.     Starting NetStat Service

4.     Starting Connectivity Service

5.     Starting Notification Manager

6.     Starting DeviceStorageMonitor Service

7.     Starting Location Manager

8.     Starting Search Service

9.     Starting Clipboard Service

10.   Starting Checkin Service

11.   Starting Wallpaper Service

12.   Starting Audio Service

13.   Starting HeadsetObserver

14.   Starting AdbSettingsObserver

 

7. Boot completed

Once System Services up and running in memory, Android has completed booting process, At this time “ACTION_BOOT_COMPLETED” standard broadcast action will fire. Ref: https://androidmaterial.blogspot.com/2013/03/in-depth-android-boot-sequence- process_16.html