Android 開機過程 boot sequence

請見The Android boot process from power on一文

全部有幾個階段:
1. Power on and boot ROM code execution
2. The boot loader
3. The Linux kernel
4. The init process
5. Zygote and Dalvik
6. The system server
7. Boot completed

在 kernel 之前還算跟一般的 embedded linux 一樣，但是在 init 之後就完全走它自己的路。先來看前面步驟的示意圖

1. Power on and boot ROM code 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.

2. The boot loader

• 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

• 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

• A. The init process in Android will look for a file called 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.
  
• B. The init process will parse the init script and launch the system service processes.
  

5. Zygote and Dalvik

The Zygote is launched by the init process and will basically just start executing 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.

7. Boot completed

Once the System Server is up and running and the system boot has completed there is a standard broadcast action called ACTION_BOOT_COMPLETED. To start your own service, register an alarm or otherwise make your application perform some action after boot you should register to receive this broadcast intent.