When a computer is powered on, there is nothing running on the CPU. For a program to be set running, the binary image of the program must first be loaded into memory from a storage device. Many systems automatically start programs that reside in nonvolatile storage devices such as read-only memories (ROMs). Once the image of the program is loaded, the CPU must be directed to start execution at the first memory location of the loaded program. This process of bootstrapping a program into execution starts a program running on a CPU.
In this chapter, we examine how the FreeBSD kernel, or any other similar program, is bootstrapped. We then study the operation of the system during the initialization phase, which takes the system from a cold start to the point at which user-mode programs can be run. A final section examines topics that are related to the startup procedure. These topics include configuring the kernel load image, shutting down a running system, and debugging system failures.
