In most architectures, on reset, processor is initialized in normal or physical address mode (also called real mode in x86) and begins executing the platform's firmware instructions found at the reset vector. These firmware instructions (which can be single binary or multi-stage binary) are programmed to carry out various operations, which include initialization of the memory controller, calibration of physical RAM, and loading the binary kernel image into a specific region of physical memory, among others.

When in real mode, processors do not support virtual addressing, and Linux, whichis designed and implemented for systems withprotected mode, requires virtual addressing to enable process protection and isolation, a crucial abstraction provided by the kernel (recall from Chapter 1, Comprehending Processes, Address Space, and Threads). This mandates the processor to be switched into protected mode and turn on virtual address support before the kernel kicks in and...