Understanding the Linux Kernel

Appendix A. System Startup

This appendix explains what happens right after users have switched on their computers, that is, how a Linux kernel image is copied into memory and executed. In short, we discuss how the kernel, and thus the whole system, is "bootstrapped."

Traditionally, the term bootstrap refers to a person who tries to stand up by pulling her own boots. In operating systems, the term denotes bringing at least a portion of the operating system into main memory and having the processor execute it. It also denotes the initialization of kernel data structures, the creation of some user processes, and the transfer of control to one of them.

Computer bootstrapping is a tedious, long task, since initially nearly every hardware device including the RAM is in a random, unpredictable state. Moreover, the bootstrap process is highly dependent on the computer architecture; as usual, we refer to IBM's PC architecture in this appendix.