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Detailed introduction of CentOS 6 startup process

1. POST (Power On Selt Test) Power-on self-Test:

Press the power key to first check whether the hardware devices are working properly, such as cpu, memory, hard disk and other hardware devices. Of course, POST is not detected by hardware (neither will hardware!) But rely on a software, this software is BIOS (Basic Input Output System) basic input and output system, it is installed on the CMOS chip, power up is to power up the CMOS, and then start the BIOS program, BIOS will read the hardware according to the hardware information recorded on the CMOS and check whether it is working properly, and then initialize the hardware device.

2. MBR (Master Boot Record) primary boot partition:

After initializing the hardware, BIOS will read MBR through the INT13 interrupt function of the hardware. Reading Boot Loader,Boot Loader in MBR is a software that provides menu function, directly loads kernel information and related control conversion. MBR only has 512bytes, in which the former 446bytes stores Boot Loader,46bytes storage partition table information, and 2bytes is the partition flag bit. 446bytes is not enough to install Boot Loader, in fact, 446bytes before MBR only installed the minimum main program of Boot Loader, that is, stage1, because Boot Loader is divided into stage1, stage1.5, stage2 three stages to execute, stage1.5 is stored in the subsequent sector of MBR, mainly used for interaction with the file system of the partition where stage2 is located, stage2 loads all files under the hard disk partition / boot directory, configuration files and environment parameter information.

3 、 Kernel 、 initrd/initramfs:

The / boot directory contains vmlinuz,initramfs and other kernel-related files. After Boot Loader reads all the files, configuration files and environment parameter information under the / boot directory, the kernel is decompressed and loaded into memory. According to the functions provided by the kernel, it starts to detect and load each device. This detection is different from the BIOS self-test, and the subsequent work is done by the kernel (because Boot Loader has loaded vmlinuz,initramfs into memory). At this time, the kernel has not loaded the disk, because there is no disk driver, the kernel simulates a rootfs (root file system) through the virtual file system in the / boot directory, that is, initramfs, loads the core modules needed during startup, releases rootfs (root file system) after loading, and mounts the real root file system read-only to start the normal boot process.

4 、 Init:

The first executor in user space. After the kernel, hardware and driver are loaded, the kernel executes the first executor / sbin/init,init program in user space. The main function is to prepare the software running environment, including system host name, network configuration, file system format and other service configuration.

Configuration file for Init:

CentOS 6: Upstart is adopted, which is characterized by the fact that the communication between daemons depends on D-Bus, so it can be basically implemented similar to parallel startup, configuration files: / etc/inittab, / etc/init/*.conf

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