Professional Linux Kernel Architecture

Appendix D

System Startup

Like any other program, the kernel goes through a load and initialization phase before performing its normal tasks. Although this phase is not particularly interesting in the case of normal applications, the kernel—as the central system layer—has to address a number of specific problems. The boot phase is split into the following three parts:

• Kernel loading into RAM and the creation of a minimal runtime environment.
• Branching to the (platform-dependent) machine code of the kernel and system-specific initialization of the elementary system functions written in assembly language.
• Branching to the (platform-independent) part of the initialization code written in C, and complete initialization of all subsystems with a subsequent switch to normal operation.

As usual, a boot loader is responsible for the first phase. Its tasks depend largely on what the particular architecture is required to do. Because in-depth knowledge of specific processor features and problems is needed to understand all details of the first phase, the architecture-specific reference manual is a good source of information. The second phase is also very hardware-dependent. Consequently, this appendix describes only some key areas of the IA-32 architecture.

In the third, system-independent phase, the kernel is already resident in memory and (on some architectures) the processor has switched from boot mode to execution mode in which the kernel then runs. On IA-32 machines, it is necessary ...