Kernel Segmentation Brief Memo

 Segmentation Unit is the Unit which translates from logical address to linear address. (and Paging Unit translates it into physical address).

 Segment Selector is the segment identifier of the logical address of CPU. It is totally 16bit, and composed of index, Table Indexer(TI), and RPL.

TI is the field where the address belongs to GDT(Global Descriptor Table) or LDT(Local Descriptor Table). While GDT stays unchanged and always used once, LDT often called or changed during execution and it is plural.

RPL is Request Privilege Level, which is under the concept of rings, it describes whether is has permission of Kernel-Level or User-Level.

 GDT/LDT each is assigned in gdtr/ldtr(registers). The picture above shows how the logical address (at the bottom = Index + TI + offset) is translated into linear address.


For more details about GDT/LDT:
http://chunqili.blogspot.jp/2013/06/what-is-gdtldtidttss.html