Google official provides a classic four-tier architecture diagram, from bottom to top divided into: Linux kernel, system libraries and Android runtime environment, framework layer, application layer, each layer contains a large number of sub-modules or sub- system.
Android system startup process from the bottom up:
Loader -> Kernel -> Native -> Framework -> App
1, Loader
Boot ROM: When the phone is in the off state, long press the Power button to boot, the boot chip starts to execute from the preset code solidified in the ROM, and then load the boot program to the RAM;
Boot Loader: This is the boot program before starting the Android system, mainly check the RAM, initialize the hardware parameters and other functions.
2, Kernel
It has only just begun to enter the Android system here.
Start Kernel 0 process: initialization process management, memory management, loading Display, Camera Driver, Binder Driver and other related work;
Start kthreadd process (pid = 2): Linux kernel process, will create a kernel worker thread kworkder, soft interrupt thread ksoftirqd, thermal and other kernel daemons. The kthreadd process is the originator of all kernel processes.
Android based on Linux provides core system services,
For example: security, memory management, process management, network stack, driver model. The Linux Kernel also acts as an abstraction layer between hardware and software. It hides specific hardware details and provides a unified service for upper layers. If you just do application development, you don't need to understand the Linux Kernel layer
3, Native
Start the init process (pid=1), which is the user process of the Linux system. The init process is the originator of all user processes.
The init process starts important services such as Media Server (multimedia service), service manager (binder service manager), and bootanim (boot animation);
The init process also spawns user daemons such as installd (for App installation), ueventd, adbd, and lkmd (for memory management);
The init process spawns the Zygote process. The Zygote process is the first Java process on the Android system. Zygote is the parent process of all Java processes. The Zygote process itself is hatched from the init process.
4, Framework
Zygote process is determined by the init process generated by the parsing init.rc file fork, Zygote process mainly includes: - Load ZygoteInit class, registered Zygote Socket server socket; - loading virtual machine; - PreloadClasses; -PreloadResouces.
The Zygote process fork out the System Server process. System Server is the first process in which Zygote hatches. The status is very important.
System Server process: responsible for starting and managing the entire Java framework, including ActivityManager, PowerManager and other services.
Media Server process: responsible for starting and managing the entire C ++ framework, contains AudioFlinger, Camera Service and other services.
5, App
The first app process spawned by the Zygote process is the Launcher. This is the desktop App that the user sees.
The Zygote process also creates application processes such as Browser, Phone, and Email. Each App runs on at least one process.
All App processes are generated by the Zygote process fork.
"Non-burning, nicotine for users, low tar content. As the heating temperature (below 500℃) is lower than the combustion temperature of traditional cigarettes (600-900℃), the harmful components produced by tobacco high-temperature combustion pyrolysis and thermal synthesis are reduced, and the release amount of side-flow smoke and environmental smoke (second-hand smoke) is also greatly reduced."
Heating non - combustion products are electronic devices containing tobacco. When you heat them, they produce a nicotine-containing vapor that you can inhale.
They are different from traditional cigarettes and work by heating tobacco to a very low temperature. Tobacco is heated to 350 ° C in a heat-incombustible device, while traditional cigarettes burn at up to 900 ° C.
Still, the temperature at which non-combustion products are heated is high enough to vaporize and inhale harmful chemicals.
Although both are electronic devices, heated non-combustible products are also different from e-cigarettes or steam devices. These usually use chemical liquids and do not necessarily contain nicotine. E-cigarettes tend to heat liquids to around 250 degrees Celsius to produce vapor.
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