simple manual linux

In Linux, the flow of control during a boot is from BIOS, to boot loader, to kernel. The kernel then starts the scheduler (to allow multi-tasking) and runs Init (which sets up the user environment and allows user interaction and login), at which point the kernel goes idle unless called externally.

In detail:

1. The BIOS performs hardware-platform specific startup tasks
2. Once the hardware is recognized and started correctly, the BIOS loads and executes the partition boot code from the designated boot device, which contains phase 1 of a Linux boot loader. Phase 1 loads phase 2 (the bulk of the boot loader code). Some loaders may use an intermediate stage to achieve this (known as phase 1.5) since modern large disks may not be fully readable without further code.
3. The boot loader often presents the user with a menu of possible boot options. It then loads the kernel, which decompresses into memory, and sets up system functions such as essential hardware and memory paging, before calling start_kernel().
4. start_kernel() then performs the majority of system setup (interrupts, the rest of memory management, device initialization, drivers, etc) before spawning separately, the idle process and scheduler, and the Init process (which is executed in user space).
5. The scheduler effectively takes control of the system management, as the kernel goes dormant .
6. The Init process executes scripts as needed that set up all non-kernel services and structures in order to allow a user environment to be created, and then presents the user with a login screen.

On shutdown, Init is called to close down all user space functionality in a controlled manner, again via scripted directions, following which Init terminates and the Kernel executes its own shutdown.