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Such an index is called a segment selector (or selector). The selector must be loaded into a segment register to be used. Apart from the machine instructions which allow one to set/get the position of the GDT, and of the Interrupt Descriptor Table (IDT), in memory, every machine instruction referencing memory has an implicit segment register ...
A segment register called the task register (TR) holds a segment selector that points to a valid TSS segment descriptor which resides in the GDT (a TSS descriptor may not reside in the LDT). Therefore, to use a TSS the following must be done by the operating system kernel: Create a TSS descriptor entry in the GDT
Because of the way the segment address and offset are added, a single linear address can be mapped to up to 2 12 = 4096 distinct segment:offset pairs. For example, the linear address 08124h can have the segmented addresses 06EFh:1234h, 0812h:0004h, 0000h:8124h, etc.
There are two such tables, the Global Descriptor Table (GDT) and the Local Descriptor Table (LDT), each holding up to 8192 segment descriptors, each segment giving access to 64 KB of memory. In the 80286, a segment descriptor provides a 24-bit base address, and this base address is added to a 16-bit offset to create an absolute address. The ...
The most significant 16 bits of the segment selector defines the address of the segment descriptor, which is stored either in the Global Descriptor Table (GDT) or Local Descriptor Table (LDT). The description details mainly include the memory segment 's first byte in linear address (Base), size (Limit) and Type (Bovet & Cesati, 2000, p. 36 - 41).
In protected mode, the segment selector can be broken down into three parts: a 13-bit index, a Table Indicator bit that determines whether the entry is in the GDT or LDT and a 2-bit Requested Privilege Level; see x86 memory segmentation.
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In protected mode, the segment_part is replaced by a 16-bit selector, in which the 13 upper bits (bit 3 to bit 15) contain the index of an entry inside a descriptor table. The next bit (bit 2) specifies whether the operation is used with the GDT or the LDT.