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An Android 101 : Boot and platform init.

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Over the past few months I have been dabbling with Android  SDK and a some amount  of programming.  Sharing what I learnt .


A Linux system:

Apart from the application framework, much of Android uses concepts from Linux directly. This includes file systems , partitions and init scripts (like the init.d). A typical Android device would have the following partitions :


Besides there may be some device specific partitions (/modem etc) that the OEMs may use for their own (non –android ) purpose.

Of these ,the recovery partition is particularly interesting. Normal android boot would start running the image in the  boot partition. However if you power on device in the ‘recovery mode’ , you could get started with an alternate boot image (the recovery image).

It was originally designed for OEMs to provide OTA upgrades. But custom ROM makes have come up with custom recovery images  that could flash other kernel images (like CynogenMod etc). A popular custom recovery image is the ClockworkMod Recovery.



When a normal boot happens, the Android platform starts executing code in the /boot partition .The bootloader then starts the kernel. The kernel does the basic initialization (of hardware ,memory subsystems) before mounting the root file system ( / ). Once / is mounted the ‘init’ process is started.

init.rc :
Like normal (desktop ) Linux systems ,Android init parses and runs the various commands as mentioned in the init.rc  script. The bulk of Android platform init happens here. The init.rc script mounts various partitions , It then starts  daemons like the adbd, service manager (responsible for IPC ).,rild, netd and others.  Finally the init.rc also invokes app_process which actually results in the process called zygote.

Zygote: Every new app is hosted on a dedicated VM. However each VM is not started afresh (not a cold start).  Zygote takes care of loading all system libraries beforehand . When a new app is started ,zygote forks itself to provide a new VM that has most components preloaded .
There is more detail on this here : http://elinux.org/Android_Zygote_Startup

SystemServer:   Once Zygote is fully launched, we have the VM initialized . The platform then starts the SystemServer.(frameworks/base/services/java/com/android/server/SystemServer.java)

SystemServer is the first Java component to run on the system . This module starts all ‘Android services’  like the PowerManager, ActivityManager etc. Once the SystemService completes, Android’s bootup is considered complete.

The platform sends a standard broadcast action called ‘ACTION_BOOT_COMPLETED ‘. Clients that want to do some action once boot up is complete could register for this module.

In the next post I will share about other components of Android , like its IPC mechanism (Binder)  and shared memory implementations (ASHMEM)

[Image courtesy: http://www.techvibes.com/ , http://hmtsay.blogspot.com/2010/10/android-startup.html ]

Written by sujai

November 7, 2011 at 7:15 pm

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