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LDTs are the siblings of the Global Descriptor Table (GDT), and each define up to 8192 memory segments accessible to programs - note that unlike the GDT, the zeroeth entry is a valid entry, and can be used like any other LDT entry. Also note that unlike the GDT, the LDT cannot be used to store certain system entries: TSSs or LDTs. Call Gates ...
The descriptors may be either interrupt gates, trap gates or, for 32-bit protected mode only, task gates. Interrupt and trap gates point to a memory location containing code to execute by specifying both a segment (present in either the GDT or LDT) and an offset within that segment. The only difference between trap and interrupt gates is that ...
By the original design the GDT algorithm calculates 20 GDT scores, i.e. for each of 20 consecutive distance cutoffs (0.5 Å, 1.0 Å, 1.5 Å, ... 10.0 Å). [2] For structure similarity assessment it is intended to use the GDT scores from several cutoff distances, and scores generally increase with increasing cutoff.
The segment registers FS and GS can still have a nonzero base address. This allows operating systems to use these segments for special purposes. Unlike the global descriptor table mechanism used by legacy modes, the base address of these segments is stored in a model-specific register.
The next bit (bit 2) specifies whether the operation is used with the GDT or the LDT. The lowest two bits (bit 1 and bit 0) of the selector are combined to define the privilege of the request, where the values of 0 and 3 represent the highest and the lowest privilege, respectively.
If set, this is a code segment descriptor. If clear, this is a data/stack segment descriptor, which has "D" replaced by "B", "C" replaced by "E"and "R" replaced by "W". This is in fact a special case of the 2-bit type field, where the preceding bit 12 cleared as "0" refers to more internal system descriptors, for LDT, LSS, and gates. C=Conforming
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
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.