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Concept
Booting process of Linux
Summary
The multi-stage booting process of Linux is in many ways similar to the BSD and other Unix-style boot processes, from which it derives.
Booting a Linux installation involves multiple stages and software components, including firmware initialization, execution of a boot loader, loading and startup of a Linux kernel image, and execution of various startup scripts and daemons. For each of these stages and components there are different variations and approaches; for example, GRUB, coreboot or Das U-Boot can be used as boot loaders (historical examples are LILO, SYSLINUX or Loadlin), while the startup scripts can be either traditional init-style, or the system configuration can be performed through modern alternatives such as systemd or Upstart.
Early stages of the Linux startup process depend very much on the computer architecture. IBM PC compatible hardware is one architecture Linux is commonly used on; on these systems, the BIOS plays an important role, which might not have exact analogs on other systems. In the following example, IBM PC compatible hardware is assumed:
The BIOS performs startup tasks like the Power-on self-test specific to the actual hardware platform. Once the hardware is enumerated and the hardware which is necessary for boot is initialized correctly, the BIOS loads and executes the boot code from the configured boot device.
The boot loader often presents the user with a menu of possible boot options and has a default option, which is selected after some time passes. Once the selection is made, the boot loader loads the kernel into memory, supplies it with some parameters and gives it control.
The kernel, if compressed, will decompress itself. It then sets up system functions such as essential hardware and memory paging, and calls start_kernel() which performs the majority of system setup (interrupts, the rest of memory management, device and driver initialization, etc.). It then starts up, separately, the idle process, scheduler, and the init process, which is executed in user space.
Official source
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