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。 10 segments, CS/DS/ES/FS/GS/SS user segments which hold software,data,and the stack and can be used by both application and system software. GDT/LDT/IDT/TR system segments contains data structures intialized and used only by system software.

segments registers contains a base address pointing to the start location of a segment, a limit defining the segment size, and the attribuits defining the segment-protection characteristics. although modern OS bypass the segmentation feature, but segmentation cannot be completedly disabled and an understanding of the segmentation mechanism is important to implementing long-mode system software.

4.1 Read Mode Segmentation

reset the first mode cpu enter is real mode, protected mode are enteredfrom real mode. real mode means addr are real(phsical) which is only 1M size. CS/DS/ES/FS/GS/SS aligned on 16-byte boundaries. POP MOV to operate on the CS/DS/ES/FS/GS/SS

4.2 virtual-8086 Mode segmentation

4.3 Protected Mode Segmented-Memory Models.

4.3.2 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？ ### 4.3.3 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 ### 4.4 Segmentation Data Structure and Registers
这是关键