Linux boot process (version 2)

Linux boot process (version 2)
Linux boot process

Firmware

Press the first start step of the power supply: Start the firmware → POST power-on self-check

Check the hardware loading status, such as the keyboard and memory.

It has nothing to do with the operating system. However, firmware varies with different hardware platforms;

When you add a new hard disk, the first step is to first enter the firmware mode and check whether the firmware has identified this hard disk!

The most important function of firmware is to read MBR->...

UDF

Read MBR [master boot record, master boot record]

[MBR is located in the disk's 0 cylinder, 0 head, 1 sector]

The purpose of this step is to read data from MBR [three types of data are saved in MBR]:

1. Boot Loader: self-initiated program. In Linux, It is GRUB.

2. Partition table: disk Partition table

3. Magic Number: indicates the ending sign.

Brief Analysis on/etc/grub. conf:

Kernel/vmlinuz-2.6.32-358.el6.i686 ro root = UUID = 6e428aef... used to specify the location of the kernel

2: motherboard ID

6: minor version number. If the odd number is the test version, the even number is the official version.

32: Last Version

From this we can see that GRUB is the most important role-> loading the kernel

Appendix-about kernel:

1. It is an executable file, not a pile of files.

2. z-indicates a compressed file, so it will be decompressed continuously at startup.

Load the kernel

During this period, the kernel only performs two tasks:

1. Driver hardware

2. Start the process init:

After init is started, read the configuration file inittab and execute the default running level to continue the boot.

In UNIX/Linux systems, the first process that can exist in init is always 1. [generally, the PID is randomly allocated, but init is special. You can run the following command: ps-el | more view] in Linux, init is essentially the parent process of all processes. Although the init PID is invariably 1, it must also be responsible for a more advanced function: the Kernel scheduler (Kernel scheduler) with a PID of 0 to obtain the CPU time.

The kernel scheduler is responsible for allocating system time and switching between processes.

Init reads the configuration file/etc/inittab

Linux running level,

0-halt: Shutdown

1-single user mode, no graphic interface

[Repair mode, similar to Windows security mode]

2-multi-user mode, but there is no graphical interface. 2 and 3 are the most widely used Linux servers.

3-like 2, NFS is added

4-unused, but can be customized by the user

5-default running level: x11 indicates a graphical multi-user environment.

6-Restart

Run-level switch:

1. view the current runlevel

N indicates that the running level has not been switched.

S indicates the running level before switching

Number indicates the current running level

2. Switch the running level

Init [0123456Ss]

Or telinit [0123456Ss]

Appendix-synchronous software and hardware clock: [the two time periods should be consistent]

Hwclock display hardware clock

Hwclock -- help: Describes the synchronization clock options in detail.

Change Time:

-S | -- hctosys set the system time from the hardware clock

-W | -- systohc set the hardware clock to the current system time

-- Set the rtc to the time given with -- date

E. g.

Date MMDDhhmm

Hwclock -- set -- date = "9/23/14 16:31:05"