android 启动过程 booting process

流程图

主要以下几个阶段

• Step 1: Power On and System Startup
• Step 2: Bootloader
• Step 3: Kernel
• Step 4: init process
• Step 5: Zygote and Dalvik
• Step 6: System service

step1: Power On and System Startup

When we press the power button, the Boot ROM code starts executing from a pre-defined location which is hardwired in ROM. It loads the Bootloader into RAM and starts executing.

Step 2: Bootloader

总的来说，bootloader是为了定位并加载linux kernal的。
Bootloader核心任务就是要初始化内存，把boot.img(kernel 和 ramdisk)从flash上copy到RAM上面，然后交出对CPU的控制权给kernel。当然，这个过程bootloader还会初始化一下LCM、串口、时钟神马的。什么是ramdisk(虚拟内存盘)

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个人认为，step 1 和 step 2比较像是某种黑盒子，我们不太需要深究它的细节，只要大概清除它的作用是什么就好了。

当机器/芯片启动时，芯片上的写死的 bootloader会解压linux kernel(kernal以压缩方式存储在闪存上)，把它加载到内存中，并把控制权交给linux

Step 4: init process

Init is the very first process, we can say it is a root process, or the grandfather of all processes. The init process has two responsibilities.

• 1- 挂载目录。Mounts directories like /sys , /dev or /proc 什么是挂载目录

• 2- 运行init.rc。Runs init.rc script

• The init process can be found at /init :: /system/core/init
• Init.rc file can be found at :: /system/core/rootdir/

Android has specific format and rules for init.rc files. More information about this rules can be found in: What is inside the init.rc and what is it used for.

At this stage, you can finally see the Android logo in your screen.

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参考文献
什么是挂载目录
Digging Into Android Startup(youtube视频)
Android白话启动篇（Android booting process）
Android 应用进程启动流程
The Android Booting process