centos7 system startup process

The startup process is as follows:

Step one: POST power-on self-test

The main function is to detect each Whether the peripheral hardware devices exist and can run normally, what realizes this self-test function is the BIOS (Basic Input/Output System) program solidified on the ROM (mainly represented by CMOS) chip on the motherboard;

Second step: Boot Sequence (select the boot device to load the MBR)

The main function is to select the hardware device to be started. After selecting, you can read the MBR located on this device The bootloader inside is gone. The implementation of this step is as follows: According to the setting of the boot sequence in the BIOS, the BIOS itself will scan each boot device in turn, and then the first device that is scanned to have a bootloader will be used as the boot device to be started. .

The third step: Load bootloader (MBR)

There are many steps to implement this step. The previous BIOS reads and executes the MBR of the boot device. bootloader, and the function of bootloader is to provide a menu to the user, allowing the user to select the system or different kernel versions to be started, and then load the kernel version selected by the user into a specific space in RAM, and then decompress it in RAM. Expand and then transfer system control to the kernel.

(Recommended tutorial: centos tutorial)

Step 4: Kernel itself initialization

Kerenl is getting system control After that, it must first initialize itself, and the main functions of initialization are:

(1) Detect all identifiable hardware devices;

bootloader transfers system control to the kernel, such as After the later dynasty overthrows the previous dynasty and the ruler (core) takes power, he must first check what is left by the previous dynasty, such as what territory, manpower, financial resources, military strength are available, etc.

(2) Load the hardware driver, that is, load the driver of the device where the real root file system is located (the driver may be loaded with the help of ramdisk);

This is like the ruler (kernel) ) After learning about the manpower and financial resources that existed underneath, he began to recruit manpower that could be "used for me" under his command, and he followed his orders, and killed those who did not obey his orders;

(3) In read-only mode Mount the root file system;

If there is a temporary file system (virtual root) using ramdisk, root switching will be performed after this step; otherwise, root switching will not be performed.

(4) Run the first application in user space: /sbin/init.

The startup process of kernel space is over here, and the next step is for user space to complete the subsequent steps. System startup process.

Step 5: init manages the user space service process

Init can be understood as a messenger sent by the kernel to manage user space, just like an angel visiting the world on behalf of God. Same. init is an initialization program that performs a series of operations based on its configuration file. Although the init configuration files of CentOS 5, CentOS 6 and CentOS 7 are different, the overall startup process remains the same.

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