linux boot sequence

The following will introduce you to the Linux startup sequence. For more Linux usage tutorials, please visit Linux Video Tutorial to learn!

Linux startup sequence flow chart:

The first step of startup - loading BIOS

When you turn on the computer power, the computer will first load the BIOS information, BIOS The information is so important that the computer must find it in the first place. This is because the BIOS contains CPU-related information, device boot sequence information, hard disk information, memory information, clock information, PnP features, etc. After this, the computer has a mental map and knows which hardware device it should read.

Start the second step - read MBR

As we all know, the first sector of track 0 on the hard disk is called MBR. That is, the Master Boot Record, the main

boot record, its size is 512 bytes, although the space is small, but it stores pre-boot information and partition table information .

After the system finds the MBR of the hard disk specified by the BIOS, it will be copied to the physical memory at the 0x7c00 address.

In fact, the content copied to the physical memory is Boot Loader, and specifically for your computer, it is lilo or grub

.

The third step of starting--Boot Loader

Boot Loader is a small program that runs before the operating system kernel runs. Through this small program, we can

initialize the hardware device and establish a map of the memory space, thereby bringing the system's software and hardware environment to a suitable state for the final call. The operating system kernel is ready for everything. There are several types of Boot Loaders, among which Grub, Lilo and spfdisk are common Loaders.

Let’s take Grub as an example. After all, not many people use lilo and spfdisk.

The system reads the grub configuration information in the memory (usually menu.lst or grub.lst), and

starts different operating systems based on this configuration information.

Start the fourth step - load the kernel

According to the path of the kernel image set by grub, the system reads Get the memory image and decompress it. At this time, the screen

will generally output the prompt "Uncompressing Linux". When the decompression of the kernel is completed, the screen outputs "OK,

booting the kernel". The system places the decompressed kernel in the memory and calls the start_kernel() function to start a series of initialization functions

and initialize various devices to complete the establishment of the Linux core environment . At this point, the Linux kernel has been established, and programs based on Linux should be able to run normally.

Start the fifth step--User layer init Set the running level according to the inittab file

After the kernel is loaded, the first run The program is /sbin/init, which reads the /etc/inittab file and performs initialization based on this file.

In fact, the main function of the /etc/inittab file is to set the running level of Linux. The setting format is ":

id:5:initdefault:", which shows that Linux needs Runs at level 5. The running level settings of Linux are as follows:

0: Shutdown1: Single-user mode

2: Multi-user mode without network support

3 : Multi-user mode with network support

4: Reserved, not used

5: Multi-user mode with network support and X-Window support

6: Reboot System, that is, restart

Start the sixth step - the init process executes rc.sysinit

After setting the running level, the Linux system The first user-level file executed is the /etc/rc.d/rc.sysinit script program. It does a lot of work, including setting PATH and setting network configuration (/etc/sysconfig /network), enable swap partition, set /proc, etc. If you are interested, you can check the rc.sysinit file in /etc/rc.d.

Start the seventh step--Start the kernel module

According to the /etc/modules.conf file or /etc/modules.d directory to load kernel modules. Start the eighth step--Execute script programs at different run levels

According to different run levels, the system will run the corresponding script programs in rc0.d to rc6.d to complete Corresponding initialization work and starting corresponding services.

Start the ninth step - execute /etc/rc.d/rc.local

If you open this file, there is a sentence in it. After reading it, you will The function of this command will be clear at a glance

rc.local is the place where Linux leaves the user with personalization after all initialization work. You can put what you want to set up

and start here.

Start the tenth step - execute the /bin/login program and enter the login state

The above is the detailed content of linux boot sequence. For more information, please follow other related articles on the PHP Chinese website!