"In-depth understanding of the Linux kernel", chapter II memory addressing Notes (2014-06-28-12:38)

2.1 Memory Address

• Logical Address: Segment + offset
• Linear address (virtual address)
• Physical Address

2.2 Segmentation in Hardware

2.2.1 Segment selector and segment register

15 3 2) 1 0
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| | TI | RPL |
|    Index number |       | |
------------------------------------------------
TI = Table Indicator

RPL = requester Privilege level

The only purpose of the segment register is to hold the segment selector. These segment registers are called Cs,ss,ds,es,fs and GS.

2.2.2 Segment Descriptor

Each segment is represented by a 8-byte segment descriptor that describes the characteristics of the segment. The segment descriptor is placed in the Global Descriptor List (GDT) or the local Descriptor descriptor (LDT).

Typically only one GDT is defined, and each process can have its own LDT in addition to the segments that are stored in the GDT if additional segments need to be created. The address and size of the GDT in main memory are stored in the GDTR control register, and the LDT address and size currently being used are placed in the LDTR control register.