Understanding the Linux Kernel, Second Edition

Appendix A. System Startup

This appendix explains what happens right after users switch 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 his 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.

Prehistoric Age: The BIOS

The moment after a computer is powered on, it is practically useless because the RAM chips contain random data and no operating system is running. To begin the boot, a special hardware circuit raises the logical value of the RESET pin of the CPU. After RESET is asserted, some registers of the processor (including cs and eip) are set to fixed values, and the code found at physical address 0xfffffff0 is executed. This address is mapped by the hardware to a certain read-only, persistent memory chip that is often called Read-Only ...