本系列博客用来记录自己对x86体系结构的学习总结，同时方便大家查阅，节省大家学习的时间。 除了最基本的virtual -> physical mapping和基本的protection，还有如下的用途 1 Shared mapping is typically used to allow access of shared-library routines by multiple applications. A read-only copy of the library routine is mapped to each application virtual-address space, but only a single copy of the library routine is present in physical memory.

2 Supervisor pages are only accessible by system software.

finally, we only maintains little page in memory while most of the unused pages are in hard drives, which is called demand-paged virtual memory.

5.1 Page Translation Overview

1. legacy is 32 to 32(legacy has special mode to 36 or 40bits)
2. amd64 is 64 to 52 如下是一个经典的page translation的示意图 size:
3. 4 Kbytes(long mode) (legacy mode)
4. 2 Mbytes(long mode) (legacy mode)
5. 4 Mbytes (legacy mode)
6. 1 Gbytes(long mode) Currently the AMD64 architecture defines a mechanism for translating 48-bit virtual addresses to 52-bit physical addresses.

   5.1.1 Page-Translation Options

7. Page-Translation Enable(CR0.PG)
8. Physical-Addr extensions(CR4.PAE)
9. Page-Size Extensions(CR4.PSE)
10. Long-Mode active page options 和模式交叉的关系如下表格

5.2 virtual-8086 Mode segmentation

5.3 Protected Mode Segmented-Memory Models.

5.3.2 flat-memory model

全部seg base全部设置成为0可以好理解，为何limit都必须为4G? 这个是不是说, flat-memory model是用来做legacy的32的model？ ### 5.3.3 Segmentation in 64-bit mode
disabled. 
exception: 1 CS-segment DPL D and L
exception: 2 FS/GS segments can be used as additional base regs in addr calc.
and those segments can have non-zero base-addresss values.

exception: 1 CS-segment DPL D and L ### 5.4 Segmentation Data Structure and Registers
这是关键