![]() ![]() Init is going to be replaced by systemd in the future, as systemd offers a cleaner way of booting. If somehow the init daemon could not start, no process will be started and the system will enter Kernel Panic. Init is a daemon process which starts as soon as the computer starts and continues running till it is shutdown. The files for init are stored in /etc/init It also looks into the folder /etc/rc*and runs the startup scripts there. The kernel looks at the /etc/inittab file and decides the run level. Once the kernel has been decompressed, it begins the init function, which is the low level initialization of the kernel. Low-level initialization (init, Upstart and Systemd) Once the kernel has extracted itself, it loads systemd, which is the replacement for the old SysV init program, and turns control over to it.Īt this point, the Linux kernel and systemd (or in some systems, init) are running 5. After decompression, the expanded image overwrite the initial uncompressed image. ![]() Once the kernel is loaded into the main memory, it begins decompression. The kernel is a compressed image, and it is store in /boot along with an initial RAM disk image, and device maps of the hard drives. GRUB2 is the default boot loader for RHEL 7, Fedora, and Ubuntu. Nowadays GRUB2 is being used in place of GRUB. Once the user selects a kernel, the kernel image is then loaded into memory and control is passed to the kernel. The GRUB displays the available Kernels to the user. The GRUB is located 30 KB away from the MBR and before the first partition. If the CPU has a valid boot device, which has a valid MBR that contains the Primary boot loader code / Stage 1 Boot loader, we proceed to load the Stage 2 Boot loader, or the GRUB (Grand Unified Bootloader) into memory. It is used to recover the MBR when it gets corrupted. This number is use for validating the MBR. Since the Partition Table Information section is only 64 Bytes long, we can store at most 64/16=4 partitions on a hard disk Magic Number Each partition needs 16 Bytes to be represented. It includes information such as the start and end of the partition, and what type of partition it is. This information here stores the details of all the partitions in the hard disk. It is used to load the actual boot loader Partition Table Information This code provides information regarding the actual boot loader on the hard disk. Primary boot loader code / Stage 1 Boot loader (446 Bytes).The MBR consists of the following information The CPU will then read the first 512 bytes of the boot device to obtain the Master Boot Recorder (MBR). One the boot device has been identified, the BIOS passes control back to the CPU. Bios Loads Boot Sector from the Boot Device If BIOS do not find any available boot devices it will alert user stating "No boot device found" 2. If the first boot device cannot be found, it will continue to look for the next available boot device. It will select the first boot device and returns control to the CPU. These activities are outside the scope of Linux, as the Kernel is not even loaded in yet until later Selecting the First Boot DeviceĪfter running POST is completed, the BIOS will have a list of available devices. If no electrical pulses are sent back, it means the device is faulty or does not exists. The POST operation checks for hardware availability by sending electrical pulses to the devices. Run POST (Power On Self Test) operation. ![]() The first instruction it runs passes control to the BIOS (Basic Input/Output System). On startup, power is sent to the CPU and it starts executing instructions. Lets look at each of the stages, and see what they do 1.
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