example 1:
特理地址和虚拟地址一致
Physical addressing mode requires no page tables and the CPU does not attempt to perform any address translations in this mode. The Linux kernel is linked to run in physical address space.
example 2 :
内核代码在高端内存当中，用户代码在低端内存当中。
processor does not have a special physical addressing mode. Instead, it divides up the memory space into several areas and designates two of them as physically mapped addresses. This kernel
address space is known as KSEG address space and it encompasses all addresses upwards from 0xfffffc0000000000. In order to execute from code linked in KSEG (by definition, kernel code) or access
data there, the code must be executing in kernel mode. The Linux kernel is linked to execute from address 0xfffffc0000310000.