Arch boot process
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Boot Process under BIOS
- System switched on - Power-on self-test or POST process
- After POST BIOS initializes the necessary system hardware for booting (disk, keyboard controllers etc.)
- BIOS launches the first 440 bytes (Master Boot Record) of the first disk in the BIOS disk order
- The MBR boot code then takes control from BIOS and launches its next stage code (if any) (mostly boot loader code)
- The launched (2nd stage) code (actual boot loader) then reads its support and config files
- Based on the data in its config files, the boot loader loads the kernel and initramfs into system memory (RAM) and launches the kernel
The kernel is the core of an operating system. It functions on a low level (kernelspace) interacting between the hardware of the machine and the programs which use the hardware to run. To make efficient use of the CPU, the kernel uses a scheduler to arbitrate which tasks take priority at any given moment, creating the illusion of many tasks being executed simultaneously.
After the kernel is loaded, it unpacks the initramfs (initial RAM filesystem), which becomes the initial root filesystem. The kernel then executes
/init as the first process. The early userspace starts.
The purpose of the initramfs is to bootstrap the system to the point where it can access the root filesystem (see FHS for details). This means that any modules that are required for devices like IDE, SCSI, SATA, USB/FW (if booting from an external drive) must be loadable from the initramfs if not built into the kernel; once the proper modules are loaded (either explicitly via a program or script, or implicitly via udev), the boot process continues. For this reason, the initramfs only needs to contain the modules necessary to access the root filesystem; it does not need to contain every module one would ever want to use. The majority of modules will be loaded later on by udev, during the init process.
At the final stage of early userspace, the real root is mounted, and then replaces the initial root filesystem.
/sbin/init is executed, replacing the
/init process. Arch uses systemd as the init process.